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Digestive System

Digestive system

The digestive system is a group of organs responsible for the conversion of food into nutrients and energy needed by the body. In humans, the digestive system consists of the mouth, esophagus, stomach, and small and large intestines. The digestive tube made up by these organs is known as the alimentary canal.

Several glandssalivary glands, liver, gall bladder, and pancreasalso play a part in digestion. These glands secrete digestive juices containing enzymes that break down the food chemically into smaller molecules that are more easily absorbed by the body. The digestive system also separates and disposes of waste products ingested with the food.

Ingestion

Food taken into the mouth is first broken down into smaller pieces by the teeth. The tongue then rolls these pieces into balls called boluses. Together, the sensations of sight, taste, and smell of the food cause the salivary glands, located in the mouth, to produce saliva. An enzyme in the saliva called amylase begins the breakdown of carbohydrates (starch) into simple sugars.

The bolus, which is now a battered, moistened, and partially digested ball of food, is swallowed, moving to the pharynx (throat) at the back of the mouth. In the pharynx, rings of muscles force the food into the esophagus, the first part of the upper digestive tube. The esophagus extends from the bottom part of the throat to the upper part of the stomach.

The esophagus does not take part in digestion. Its job is to move the bolus into the stomach. Food is moved through the esophagus (and other parts of the alimentary canal) by a wavelike muscular motion known as peristalsis (pronounced pear-i-STALL-sis). This motion consists of the alternate contraction and relaxation of the smooth muscles lining the tract.

Words to Know

Alimentary canal: Tube formed by the pharynx, esophagus, stomach, and intestines through which food passes.

Amylase: Digestive enzyme that breaks down carbohydrates to simple sugars.

Bile: Bitter, greenish liquid produced in the liver and stored in the gall bladder that dissolves fats.

Bolus: Battered, moistened, and partially digested ball of food that passes from the mouth to the stomach.

Carbohydrate: A compound consisting of carbon, hydrogen, and oxygen found in plants and used as a food by humans and other animals.

Chyme: Thick liquid of partially digested food passed from the stomach to the small intestine.

Enzyme: Any of numerous complex proteins that are produced by living cells and spark specific biochemical reactions.

Esophagitis: Commonly known as heartburn, an inflammation of the esophagus caused by gastric acids flowing back into the esophagus.

Gastric juice: Digestive juice produced by the stomach wall that contains hydrochloric acid and the enzyme pepsin.

Pepsin: Digestive enzyme that breaks down protein.

Peristalsis: Wavelike motion of the digestive system that moves food through the system.

Proteins: Large molecules that are essential to the structure and functioning of all living cells.

Ulcer: Inflamed sore or lesion on the skin or a mucous membrane of the body.

Villi: Fingerlike projections found in the small intestine that increase the absorption area of the intestine.

At the junction of the esophagus and stomach there is a powerful musclethe esophageal sphincterthat acts as a valve to keep food and stomach acids from flowing back into the esophagus and mouth.

Digestion in the stomach

Chemical digestion begins in the stomach. The stomach is a large, hollow, pouched-shaped muscular organ. Food in the stomach is broken down by the action of gastric juice, which contains hydrochloric acid and pepsin (an enzyme that digests protein). The stomach begins its production of gastric juice while food is still in the mouth. Nerves from the cheeks and tongue are stimulated and send messages to the brain. The brain in turn sends messages to nerves in the stomach wall, stimulating the secretion of gastric juice before the arrival of food. The second signal for gastric juice production occurs when food arrives in the stomach and touches the lining.

Gastric juice is secreted from the linings of the stomach walls, along with mucus that helps to protect the stomach lining from the action of the acid. Three layers of powerful stomach muscles churn food into a thick liquid called chyme (pronounced KIME). From time to time, chyme is passed through the pyloric sphincter, the opening between the stomach and the small intestine.

Digestion and absorption in the small intestine

The small intestine is a long, narrow tube running from the stomach to the large intestine. The small intestine is greatly coiled and twisted. Its full length is about 20 feet (6 meters). The small intestine is subdivided into three sections: the duodenum (pronounced do-o-DEE-num), the jejunum (pronounced je-JOO-num), and the ileum (pronounced ILL-ee-um).

The duodenum is about 10 inches (25 centimeters) long and connects with the lower portion of the stomach. When chyme reaches the duodenum, it is further broken down by intestinal juices and through the action of the pancreas and gall bladder. The pancreas is a large gland located below the stomach that secretes pancreatic juice into the duodenum through the pancreatic duct. There are three enzymes in pancreatic juice that break down carbohydrates, fats, and proteins. The gall bladder, located next to the liver, stores bile produced by the liver. While bile does not contain enzymes, it contains bile salts that help to dissolve fats. The gall bladder empties bile into the duodenum when chyme enters that portion of the intestine.

The jejunum is about 8.2 feet (2.5 meters) long. The digested carbohydrates, fats, proteins, and most of the vitamins, minerals, and iron are absorbed in this section. The inner lining of the small intestine is composed of up to five million tiny, fingerlike projections called villi. The villi increase the rate of absorption of nutrients into the bloodstream by greatly increasing the surface area of the small intestine.

The ileum, the last section of the small intestine, is the longest, measuring 11 feet (3.4 meters). Certain vitamins and other nutrients are absorbed here.

Absorption and elimination in the large intestine

The large intestine is wider and heavier than the small intestine. However, it is much shorteronly about 5 feet (1.5 meters) long. It rises up on the right side of the body (the ascending colon), crosses over to the other side underneath the stomach (the transverse colon), descends on the left side, (the descending colon), then forms an s-shape (the sigmoid colon) before reaching the rectum and anus. The muscular rectum, about 6 inches (16 centimeters) long, expels feces (stool) through the anus, which has a large muscular sphincter that controls the passage of waste matter.

The large intestine removes water from the waste products of digestion and returns some of it to the bloodstream. Fecal matter contains undigested food, bacteria, and cells from the walls of the digestive tract. Millions of bacteria in the large intestine help to produce certain B vitamins and vitamin K. These vitamins are absorbed into the bloodstream along with the water.

Disorders of the digestive system

Among the several disorders that affect the digestive system are esophagitis (heartburn) and ulcers. Esophagitis is an inflammation of the esophagus caused by gastric acids flowing back into the esophagus. Mild cases of this condition are usually treated with commercial antacids.

Stomach ulcers are sores that form in the lining of the stomach. They may vary in size from a small sore to a deep cavity. Ulcers that form in the lining of the stomach and the duodenum are called peptic ulcers because they need stomach acid and the enzyme pepsin to form. Duodenal ulcers are the most common type. They tend to be smaller than stomach ulcers and heal more quickly. Any ulcer that heals leaves a scar.

Until the early 1990s, the medical community generally believed that ulcers were caused by several factors, including stress and a poor diet. However, medical researchers soon came to believe that a certain bacterium that can live undetected in the mucous lining of the stomach was responsible. This bacterium irritated and weakened the lining, making it more susceptible to damage by stomach acids.

It is believed that about 80 percent of stomach ulcers may be caused by the bacterial infection. With this discovery, ulcer patients today are being treated with antibiotics and antacids rather than special diets or expensive medicines.

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Digestive System

Digestive System

The digestive systems of animals are quite diverse. Sponges, the simplest animals, do not have specialized digestive systems. Food particles filtered from the water are simply digested within individual cells (intracellular digestion). One of the first steps toward a complex digestive system in animals, and the processing of larger prey, is the evolution of a gastrovascular cavity, a digestive sac with a single opening to the external environment. The gastrovascular cavity serves as a protected space for extracellular digestion inside the animal, and at the same time allows distribution of the digested material to most cells of the body. Following extracellular digestion in the gastrovascular cavity, the digestion products from carbohydrates, proteins, and fats are taken up by cells lining the gastrovascular cavity, where digestion is completed intracellularly.

A One-Way Digestive Tract

Cnidarians and flatworms have a gastrovascular cavity. Cnidarians, such as the hydra, use their tentacles to move food through their mouth into their gastrovascular cavity. Then the cells lining this cavity excrete digestive enzymes that will start extracellular digestion and break the prey into smaller pieces. Any undigested remnants of the prey are expelled through the mouth opening. Like cnidarians, flatworms have a gastrovascular cavity with a single opening, but the cavity itself is highly folded. These folds greatly increase the surface area and extend throughout the body, bringing nutrients within the reach of all cells.

The gastrovascular cavity of cnidarians and flatworms allows them to digest larger prey than they could with intracellular digestion. However, the effectiveness of a gastrovascular cavity in supplying the animal with nutrients is limited. Because there is only one opening to the external environment through which prey is taken in and remnants are expelled, the animals have to complete digestion of the first prey and expel its remnants before taking in another prey. With the evolution of a second opening in the digestive system, the digestive system became a digestive tract, or alimentary canal, making it a one-way system between mouth and anus. Food could now be taken in and processed continuously, providing the animal with more nutrients. Most animalsincluding vertebrates, arthropods, mollusks, round worms and earthwormshave this form of digestive tract.

A one-way digestive tract is efficient because it allows the food to pass through a series of specialized regions. Such regions may be specialized for protein, fat, or carbohydrate digestion, making each step more efficient. Other regions may be used for food storage or for preparing the food for chemical digestion by physically grinding it into smaller pieces, which exposes more surface area to the action of digestive enzymes. These specialized regions eventually evolved into organs as parts of a complex digestive organ system. However, because nutrient dispersal, by the digestive system itself to all cells of the body, was no longer feasible with such a specialized digestive system (and animals became larger and bulkier) a separate cardiovascular system evolved to serve that function.

Simple animals such as earthworms suck soil into the mouth with the pharynx, pass it through the esophagus into the crop, where it is moistened and stored. From there it is moved into the gizzard, which contains small grains of sand that help grind down the food. The actual digestion and absorption of nutrients takes place in the intestine, and anything that remains is excreted through the anus. Insects also move food from the mouth through the esophagus into a crop (all parts of the foregut) for food storage and moistening. From there it is moved to a midgut where digestion and nutrient absorption through specialized extensions, or ceca, takes place. The hindgut functions mainly to reclaim water and ions from the gut content that would otherwise be lost in the feces.

Physical Digestion of Vertebrates

During the evolution of vertebrates, two trends can be recognized. First, animals became larger, requiring a more efficient digestive system to meet their nutritional needs. Second, some animals moved from living in water to living on land. This meant they required more energy for locomotion and a more efficient digestive system to provide that energy.

In vertebrates, the physical digestion of food begins in the mouth. Birds crunch seeds with their beaks, and mammals use their powerful jaws and specialized teeth to chew food into smaller pieces, increasing the surface area for digestive enzymes to work on. Salivary glands in the mouth coat the food with saliva to make it slippery for swallowing. After swallowing, the food is moved along the digestive tract with the help of involuntary smooth muscle contractions, called peristalsis. Sphincters regulate the passage of food from one chamber of the digestive tract into the next. First the food passes through the esophagus into the stomach. In the stomach, the food is stored and mixed with gastric juice. The gastric juice kills most swallowed bacteria, breaks down most food into individual cells (increasing the surface area for enzyme attack), and begins the digestion of proteins. Birds may store food in a crop without digesting it before passing it into the stomach. This allows parent birds to regurgitate food from their crops for their nestlings. Some birds move food from the stomach into a muscular gizzard containing swallowed stones that grind down seeds before digestion continues in the small intestine. The small intestine is the major site of digestion and absorption in vertebrates, and has three distinct regionsthe duodenum, the jejunum, and the ileum. Accessory glands such as the pancreas and liver secrete digestive enzymes and other products into the duodenum. The jejunum also releases digestive enzymes. These enzymes digest carbohydrates, proteins, nucleic acids, and fat, and the products of the digestion are absorbed by cells lining the small intestine, especially the ileum. The large intestine is connected to the small intestine. The major function of the large intestine is to reabsorb water that was added to the gut content in the small intestine, and to absorb inorganic ions from the digested food. As a result the feces become more solid. The large intestine also contains many bacteria, which may produce gases as byproduct of their metabolism, but also vitamins, such as vitamin K, that are absorbed into the blood. Feces are stored in the rectum until they can be eliminated through the anus.

Many grazing animals (e.g., deer, cattle, sheep, giraffes) who swallow grass hastily without chewing while watching out for predators, have a two-part stomach. In the first part of the stomach, the unchewed grass is fermented and predigested by bacteria before it is regurgitated back up into the mouth. There it can be chewed more thoroughly when the animal is in a safe place. After chewing, the food is swallowed again and passes into the second section where digestion takes place. Herbivores generally have a longer and more complex intestine than carnivores. This allows them to get as many nutrients as possible out of their more nutrient-poor food.

see also Digestion.

Kathrin F. Stanger-Hall

Bibliography

Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology, 5th ed. Menlo Park, CA: Addison Wesley Longman, Inc., 1999.

Eckert, Roger, David Randall, and George Augustine. Animal Physiology, 3rd ed. New York: W. H. Freeman and Company, 1988.

Kapit, Wynne, Robert I. Macey, and Esmail Meisami. The Physiology Coloring Book. Cambridge, MA: Harper Collins Publishers, 1987.

Purves, William K., Gordon Orian, Craig H. Heller. Life: The Science of Biology, 4th ed. Sunderland, MA: Sinauer Associates, and W. H. Freeman and Company, 1994.

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Digestive System

Digestive System

The human digestive system is responsible for food ingestion and digestion as well as the absorption of digested food molecules and the elimination of undigested molecules. It consists of a long tube called the gastrointestinal tract or GI tract (alimentary canal) and several accessory organs. The major components of the GI tract are the mouth, pharynx , esophagus , stomach, small intestine, and large intestine. The major accessory organs are the teeth, salivary glands, liver, gallbladder, and pancreas.

Mastication and Swallowing

Ingestion (the intake of food) occurs in the mouth where food is chewed and mixed with saliva. The teeth have different shapes to perform different tasks; the incisors (chisel-shaped anterior teeth) are used to cut into food, the canines (pointed teeth located lateral to the incisors) are used to tear or pierce food, and the premolars and molars (having broad surfaces) are used for crushing and grinding food. Chewing (mastication) of food is accompanied by mixing of the food with saliva. The mouth is normally kept moist by the continual production of small quantities of saliva by numerous tiny intrinsic salivary glands located in the inner lining of the mouth.

During chewing, much greater quantities of saliva are secreted by three pairs of extrinsic salivary glands, namely the parotid glands (located under the skin anterior to each earlobe), the submandibular glands (located under the base of the tongue), and the sublingual glands (located in the floor of the mouth). Saliva is a watery fluid containing several components including lysozyme, an enzyme that kills bacteria, and salivary amylase, an enzyme that begins the digestion of starch.

Once the food has been chewed into a soft, flexible mass called a bolus, it is swallowed for delivery to the stomach. On its journey, the bolus passes through the pharynx and then through the esophagus, a straight muscular tube that descends through the thoracic (chest) cavity, anterior to the spine. Each bolus of food is propelled through the esophagus by gravity, and by the process of peristalsis, a wave of muscular contraction that pushes the bolus downward. The lower end of the esophagus, which passes through a hole in the diaphragm to meet the stomach within the abdominal cavity, has a lower esophageal (or gastroesophageal or cardiac) sphincter which briefly relaxes to allow the bolus of food to enter the stomach.

Stomach and Intestines

The stomach is a muscular sac that is located in the upper left portion of the abdominal cavity. The inner lining of the stomach wall contains millions of tiny gastric glands that secrete gastric juice, which dissolves the food to form a thick liquid called chyme. Gastric juice contains several substances including hydrochloric acid, intrinsic factor (which is essential for the intestinal absorption of vitamin B12) and pepsinogen (an inactive protein -digesting enzyme). The hydrochloric acid has several functions including destroying ingested bacteria, and converting pepsinogen into its active form, pepsin, in order to initiate the digestion of protein.

At the lower end of the stomach is the pyloric sphincter, a valve through which chyme must flow to enter the small intestine. Most meals are gradually emptied into the small intestine after two to six hours due to peristaltic contractions that travel toward the lower end of the stomach. Most digestion and absorption occur within the small intestine. The small intestine consists of three segments named the duodenum, jejunum and ileum. The duodenum receives chyme from the stomach as well as pancreatic juice from the pancreas and bile from the liver (and stored in the gallbladder).

Pancreatic juice contains digestive enzymes capable of digesting proteins, carbohydrates , and lipids . Bile emulsifies lipids to increase the efficiency of lipid digestion and absorption. Once digestion has been completed, the digested nutrients are absorbed into blood vessels and lymphatic vessels within the wall of the small intestine.

Peristaltic contractions move chyme through the small intestine and into the large intestine. The large intestine consists of three major segments, the cecum (which receives chyme from the small intestine), the colon, and the rectum. As peristalsis moves chyme through the colon, water is absorbed to gradually convert the chyme into semisolid material called feces. The feces contain indigestible food molecules (primary cellulose ) and intestinal bacteria that live in the colon (primarily Escherichia coli ). Peristalsis delivers the feces into the rectum where they are stored until they are expelled through the anus by the process of defecation.

see also Digestion; Liver; Pancreas

Izak Paul

Bibliography

Saladin, Kenneth S. Anatomy & Physiology: The Unity of Form and Function, 2nd ed. New York: McGraw-Hill, 2001.

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digestive system

digestive system, in the animal kingdom, a group of organs functioning in digestion and assimilation of food and elimination of wastes. Virtually all animals have a digestive system. In the vertebrates (phylum Chordata, subphylum Vertebrata) the digestive system is very complex. It consists of the gastrointestinal tract (gut), an extensive tube extending from the mouth to the anus, through which the swallowing, digestion, and assimilation of food and the elimination of waste products are accomplished.

The Human Digestive System

In the digestive system, ingested food is converted into a form that can be absorbed into the circulatory system for distribution to and utilization by the various tissues of the body. This is accomplished both physically, by mastication in the mouth and churning of the stomach, and chemically, by secretions and enzymes of the gastrointestinal tract. Beginning at the mouth, all food passes through the alimentary canal (pharynx, esophagus, stomach, and intestines) before it reaches the anus, where undigested matter is eliminated as waste. The outer walls of the digestive tract are composed of layers of muscle and tissue that undergo waves of contraction (peristalsis), thereby pushing the food along its digestive path. The inner lining contains glands that secrete the acids and enzymes necessary to break down food into a form utilizable by the body.

Digestion begins in the mouth, where chewing reduces the food to fine texture, and saliva moistens it and begins the conversion of starch into simple sugars by means of an enzyme, salivary amylase. The food is then swallowed, passing through the pharynx and down the muscular esophagus, or gullet, to the expanded muscular pouchlike section of the gastrointestinal tract, the stomach. Specialized cells in the stomach secrete digestive enzymes and gastric juices, which act on the partially digested food. The stomach also physically churns and mixes the food. The stomach secretions include the enzyme pepsin, which acts on proteins; hydrochloric acid, essential for the action of pepsin; and an enzyme, gastric lipase, which begins the breakdown of fats. The gastric juices of young children contain, in addition to those just mentioned, the enzyme rennin, which acts on milk. Some foods, including simple sugars and alcohol, are absorbed directly through the stomach wall and do not remain in the stomach. Most food, however, is not absorbed in the stomach and passes into the duodenum (first section of the small intestine) in the form of a thick liquid called chyme.

Digestive enzymes from the pancreas and bile from the liver act on the chyme in the duodenum. These enzymes include pancreatic lipase, which breaks down fats into glycerol and fatty acids; pancreatic amylase, which continues the breakdown of starches and most other carbohydrates into disaccharides; and trypsin and erepsin, which break down whole and partially digested proteins (proteoses and peptones) into amino acids, the end products of protein digestion. Bile is essential for emulsifying large fat globules into smaller ones that are more easily digested by pancreatic lipase. In addition, intestinal juices are secreted by small glands in the intestinal wall called the crypts of Lieberkühn. Like the pancreatic juices, intestinal juices contain enzymes that continue the digestion of proteins and fats and also contain three enzymes that break down disaccharides into glucose, galactose, and fructose (simple sugars). The digested food is absorbed into the circulatory and lymphatic systems through small fingerlike projections of the intestinal wall, called villi. Undigested material passes into the large intestine, where most of the water is absorbed and the solid material, or feces, is excreted through the anus.

Bibliography

See J. E. Morton, Guts: The Form and Function of the Digestive System (1967); H. W. Davenport, Physiology of the Digestive Tract: An Introductory Text (3d ed. 1971).

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digestive system

digestive system (alimentary system) Group of organs of the body concerned with the digestion of foodstuffs. In humans, it begins with the mouth, and continues into the oesophagus, which carries food into the stomach. The stomach leads to the small intestine, which then opens into the colon. After food is swallowed, it is pushed through the digestive tract by peristalsis, contractions of the muscles. On its journey, food is transformed into small molecules that can be absorbed into the bloodstream and carried to the tissues. Carbohydrate is broken down to sugars, protein to amino acids, and fat to fatty acids and glycerol. Indigestible matter, mainly cellulose, passes into the rectum, and is eventually eliminated from the body (as faeces) through the anus.

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digestive system

digestive system The system of organs that are involved in the process of digestion. The digestive system of mammals is divided into the gastrointestinal tract (see alimentary canal) and accessory structures, such as teeth, tongue, liver, pancreas, and gall bladder.

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Digestive System

Digestive System

Definition

The digestive system is a group of organs and tissues responsible for the conversion of food into absorbable chemicals which are then used to provide energy for growth and repair.

Description

The digestive system is also known by a number of other names, including the gut, the digestive tube, the alimentary canal, the gastrointestinal (GI) tract, the intestinal tract, and the intestinal tube. The digestive system consists of the mouth, esophagus, stomach, and small and large intestines, along with several glands, such as the salivary glands, liver, gall bladder, and pancreas.

Function and role in human health

The glands in the digestive system secrete digestive juices containing enzymes that break down the food chemically into smaller, more absorbable molecules. In addition to providing the body with the nutrients and energy it needs to function, the digestive system also separates and disposes of waste products ingested with the food.

Food is moved through the alimentary canal by a wavelike muscular motion known as peristalsis, which consists of the alternate contraction and relaxation of the smooth muscles lining the tract. In this way, food is passed through the gut in much the same manner as toothpaste is squeezed from a tube. Churning is another type of movement that takes place in the stomach and small intestine, which mixes the food so that the digestive enzymes can break down the food molecules.

Food in the human diet consists of carbohydrates, proteins, fats, vitamins, and minerals. The remainder of the food is fiber and water. The majority of minerals and vitamins pass through to the bloodstream without the need for further digestive changes, but other nutrient molecules must be broken down to simpler substances before they can be absorbed and used.

Ingestion

Food taken into the mouth is first prepared for digestion in a two-step process known as mastication. In the first stage, the teeth tear and break down food into smaller pieces. In the second stage, the tongue rolls these pieces into balls (boluses). Sensory receptors on the tongue (taste buds) detect taste sensations of sweet, salt, bitter, and sour, or cause the rejection of bad-tasting food. The olfactory nerves contribute to the sensation of taste by picking up the aroma of the food and passing the sensation of smell on to the brain.

The sight of the food also stimulates the salivary glands. Altogether, the sensations of sight, taste, and smell cause the salivary glands, located in the mouth, to produce saliva, which then pours into the mouth to soften the food. An enzyme in the saliva called amylase begins the break down of carbohydrates (starch) into simple sugars, such as maltose. Ptyalin is one of the main amylase enzymes found in the mouth; ptyalin is also secreted by the pancreas.

The bolus of food, which is now a battered, moistened, and partially digested ball of food, is swallowed, moving to the throat at the back of the mouth (pharynx). In the throat, rings of muscles force the food into the esophagus, the first part of the upper digestive tube. The esophagus extends from the bottom part of the throat to the upper part of the stomach.

The esophagus does not take part in digestion. Its job is to get the bolus into the stomach. There is a powerful muscle (the esophageal sphincter), at the junction of the esophagus and stomach, which acts as a valve to keep food, stomach acids, and bile from flowing back into the esophagus and mouth.

Digestion in the stomach

Chemical digestion begins in the stomach. The stomach, a large, hollow, pouch-shaped muscular organ, is shaped like a lima bean. When empty, the stomach becomes elongated; when filled, it balloons out.

Food in the stomach is broken down by the action of the gastric juice containing hydrochloric acid and a protein-digesting enzyme called pepsin. Gastric juice is secreted from the lining of the stomach walls, along with mucus, which helps to protect the stomach lining from the action of the acid. The three layers of powerful stomach muscles churn the food into a fine semiliquid paste called chyme. The chyme is periodically passed through an opening (the pyloric sphincter), which controls the passage of chyme between the stomach and the beginning of the small intestine.

Gastric juice

There are several mechanisms responsible for the secretion of gastric juice in the stomach. The stomach begins its production of gastric juice while the food is still in the mouth. Nerves from the cheeks and tongue are stimulated and send messages to the brain. The brain in turn sends messages to nerves in the stomach wall, stimulating the secretion of gastric juice before the arrival of the food. The second signal for gastric juice production occurs when the food arrives in the stomach and touches the lining. This mechanism provides for only a moderate addition to the amount of gastric juice that was secreted when the food was in the mouth.

Gastric juice is needed mainly for the digestion of protein by pepsin. If a hamburger and bun reach the stomach, there is no need for extra gastric juice for the bun (carbohydrate), but the hamburger (protein) will require a much greater supply of gastric juice. The gastric juice already present will begin the break down of the large protein molecules of the hamburger into smaller molecules: polypeptides and peptides. These smaller molecules in turn stimulate the cells of the stomach lining to release the hormone gastrin into the bloodstream.

Gastrin then circulates throughout the body, and eventually reaches the stomach, where it stimulates the cells of the stomach lining to produce more gastric juice. The more protein there is in the stomach, the more gastrin will be produced, and the greater the production of gastric juice. The secretion of more gastric juice by the increased amount of protein in the stomach represents the third mechanism of gastric juice secretion.

Digestion and absorption in the small intestine

While digestion continues in the small intestine, it also becomes a major site for the process of absorption, that is, the passage of digested food into the bloodstream, and its transport to the rest of the body.

The small intestine is a long, narrow tube, about 20 ft (6 m) long, running from the stomach to the large intestine. The small intestine occupies the area of the abdomen between the diaphragm and hips, and is greatly coiled and twisted. The small intestine is lined with muscles that move the chyme toward the large intestine. The mucosa, which lines the entire small intestine, contains millions of glands that aid in the digestive and absorptive processes of the digestive system.

The small intestine, or small bowel, is subdivided into three sections, the duodenum, the jejunum, and the ileum. The duodenum is about 1 ft (0.3 m) long and connects with the lower portion of the stomach. When fluid food reaches the duodenum it undergoes further enzymatic digestion and is subjected to pancreatic juice, intestinal juice, and bile.

The pancreas is a large gland located below the stomach that secretes pancreatic juice into the duodenum via the pancreatic duct. There are three enzymes in pancreatic juice which digest carbohydrates, lipids, and proteins. Amylase, (the enzyme found in saliva) breaks down starch into simple sugars such as maltose. The enzyme maltase in intestinal juice completes the break down of maltose into glucose.

Lipases in pancreatic juice break down fats into fatty acids and glycerol, while proteinases continue the breakdown of proteins into amino acids. The gallbladder, located next to the liver, secretes bile into the duodenum. While bile does not contain enzymes, it contains bile salts and other substances that help to emulsify (dissolve) fats, which are otherwise insoluble in water. Breaking the fat down into small globules allows the lipase enzymes a greater surface area for their action.

Chyme passing from the duodenum next reaches the jejunum of the small intestine, which is about 3 ft (0.91 m) long. Here, in the jejunum, the digested breakdown products of carbohydrates, fats, proteins, and most of the vitamins, minerals, and iron are absorbed. The inner lining of the small intestine is composed of up to five million tiny, finger-like projections called villi. The villi increase the rate of absorption of the nutrients into the bloodstream by extending the surface of the small intestine to about five times that of the surface area of the skin.

There are two transport systems that pick up the nutrients from the small intestine. Simple sugars, amino acids, glycerol, and some vitamins and salts are conveyed to the liver in the bloodstream. Fatty acids and vitamins are absorbed and then transported through the lymphatic system, the network of vessels that carry lymph and white blood cells throughout the body. Lymph eventually drains back into the bloodstream and circulates throughout the body.

The last section of the small intestine is the ileum. It is smaller and thinner-walled than the jejunum, and it is the preferred site for vitamin B12 absorption and bile acids derived from the bile juice.

Absorption and elimination in the large intestine

The large intestine, or colon, is wider and heavier then the small intestine, but much shorter—only about 4 ft (1.2 m) long. It rises up on one side of the body (the ascending colon), crosses over to the other side (the transverse colon), descends (the descending colon), forms an s-shape (the sigmoid colon), reaches the rectum, and anus, from which the waste products of digestion (feces or stool), are passed out, along with gas. The muscular rectum, about 5 in (13 cm) long, expels the feces through the anus, which has a large muscular sphincter that controls the passage of waste matter.

The large intestine extracts water from the waste products of digestion and returns some of it to the bloodstream, along with some salts. Fecal matter contains undigested food, bacteria, and cells from the walls of the digestive tract. Certain types of bacteria of the large intestine help to synthesize the vitamins needed by the body. These vitamins find their way to the bloodstream along with the water absorbed from the colon, while excess fluids are passed out with the feces.

Liver

The liver is the largest organ in the body and plays a number of vital roles, including metabolizing the broken-down products of digestion, and detoxifying substances that are harmful to the body. The liver also provides a quick source of energy when the need arises and it produces new proteins. Along with the regulation of stored fats, the liver also stores vitamins, minerals, and sugars. The liver controls the excretion and production of cholesterol and metabolizes alcohol into a mild toxin. The liver also stores iron, maintains the hormone balance, produces immune factors to fight infections, regulates blood clotting, and produces bile.

Gallbladder

The gallbladder lies under the liver and is connected by various ducts to the liver and the duodenum. The gallbladder is a small hollow organ; its main function is to store bile until it is concentrated enough to be used by the small intestine. The gallbladder can store about 2 oz of bile. Bile consists of bile salts, bile acids, and bile pigments. In addition, bile contains cholesterol dissolved in the bile acids.

Appendix

The appendix is a hollow finger-like projection that hangs from the cecum at the junction between the small intestine and the large intestine. The appendix does not function in humans; however, in some animals, such as rabbits, the appendix is rather large and helps in the digestion of cellulose from bark and wood, which rabbits eat. The appendix in humans is therefore a vestigial organ, which may have had uses for earlier types of ancestral human digestive processes before the evolution of Homo sapiens.

Pancreas

When food reaches the small intestine, the pancreas secretes pancreatic juices. When there is no food in the small intestine, the pancreas does not secrete its juices.

Insulin is another important hormone secreted by a group of cells within the pancreas called the islets of Langerhans, which are part of the endocrine system, rather than the digestive system. Insulin released into the bloodstream targets liver and muscle cells, and allows them to take excess sugar from the blood and store it in the form of glycogen.

Common diseases and disorders

Several disorders of the esophagus are esophagitis, esophageal spasm, and esophageal cancer. Esophagitis (heartburn) is an inflammation of the esophagus usually caused by the reflux of gastric acids into the esophagus and is treated with antacid (alkalis). Esophageal spasm is also caused by acid reflux. Esophageal cancer can be caused by smoking and is generally fatal.

Disorders of the stomach include hiatal hernia, ulcers, and gastric cancer. A hiatal hernia occurs when a portion of the stomach extends upwards into the thorax through a large opening in the diaphragm. It is a condition that commonly occurs in people over the age of 50. Stomach ulcers are sores that form in the lining of the stomach. They may vary in size from a small sore to a deep cavity, surrounded by an inflamed area, sometimes called ulcer craters. Stomach ulcers and ulcers that form in the esophagus and in the lining of the duodenum are called peptic ulcers because they need stomach acid and the enzyme pepsin to form. Duodenal ulcers are the most common type. They tend to be smaller than stomach ulcers and heal more quickly. Ulcers that form in the stomach lining are called gastric ulcers. About 4 million people have ulcers, and 20% of those have gastric ulcers. Those people who are at most risk for ulcers are those who smoke, are middle-age and older men, are chronic users of alcohol, and those who take anti-inflammatory drugs, such as aspirin and ibuprofen. It is believed that about 80% of stomach ulcers may be caused by the bacterial infection, while about 20% may be from other causes, such as the use of anti-inflammatory medicines.

The most common liver disorder in the United States and other developed countries is cirrhosis of the liver. The main cause for this disease is alcoholism. Cirrhosis is characterized by the replacement of healthy liver cells by fibrous tissue. The replacement process is gradual and takes a period of two to 10 years to complete. There is no cure for the disease. Symptoms may not be noticed in its early development, but in its advanced stages there are a number of symptoms and the condition can lead to coma. Close medical attention is required to treat the disease.

Another common liver disorder is hepatitis. It is an inflammation of the liver caused by viruses. The most noticeable symptom of this disease is jaundice, that is, the skin, eyes, and urine turn yellow. The nine viruses known to cause hepatitis include hepatitis A, B, C, D, and E; the recently discovered F and G viruses; and two herpes viruses (Epstein-Barr and cytomegalovirus).

Gallstones may form in the gallbladder. If the amount of cholesterol in the bile acids increases or the amount of acid decreases, then some of the cholesterol will settle out of the acid to form gallstones that accumulate and block the ducts to the gallbladder. Infection in the gallbladder may lead to gallstones. Gallstones may be in the gallbladder for years without giving any signs of the condition, but when they obstruct the bile duct they cause considerable pain and inflammation. Infection and blockage of the bile flow may follow. Surgical removal of the gallbladder may be necessary to treat this condition. Since the liver both produces and stores sufficient amounts of bile, the loss of the gallbladder does not interfere with the digestive process provided fat intake in the diet is regulated. If the gallstones contain mainly cholesterol, drug treatment for stones may be possible. But if there is too much other material in the stones, surgery may be necessary. Even after being treated successfully by drugs and diet, the condition can return. The drug treatment takes years to dissolve gallstones.

If food gets trapped in the appendix, an irritation of its membranes may occur leading to swelling and inflammation, a condition known as appendicitis. If the condition becomes serious, removal of the appendix is necessary to avoid a life-threatening condition if it were to rupture.

When the pancreas does not produce sufficient insulin to store dietary sugar, the blood and urine levels of sugar reach dangerous levels. Diabetes mellitus is the resultant disease. Mild cases can be controlled by a properly regulated diet, but severe cases require the regular injection of insulin.

KEY TERMS

Amylase— A digestive enzyme found in saliva and the pancreas that breaks down carbohydrates to simple sugars.

Bile— Liquid produced in the liver and stored in the gall bladder that emulsifies fats.

Gastric juice— Digestive juice produced by the stomach wall that contains hydrochloric acid and the enzyme pepsin.

Gastrin— A hormone produced by the stomach lining in response to protein in the stomach that produces increased gastric juice.

Helicobacter pylori Recently discovered bacteria that live in gastric acids and are believed to be a major cause of most stomach ulcers.

Lower esophageal sphincter— A strong muscle ring between the esophagus and the stomach that keeps gastric juice, and even duodenal bile from flowing upwards out of the stomach.

Lymphatic system— The transport system linked to the cardiovascular system that contains the immune system and also carries metabolized fat and fat soluble vitamins throughout the body.

Mucosa— The digestive lining of the intestines.

Nutrients— Vitamins, minerals, proteins, lipids, and carbohydrates needed by the body.

Peristalsis— The wavelike motion of the digestive system that moves food through the digestive system.

Villi— Fingerlike projections found in the small intestine that add to the absorptive area for the passage of digested food to the bloodstream and lymphatic system.

Resources

BOOKS

Greenspan, Francis S., and David G. Gardner, eds. Basic & Clinical Endocrinology, 6th ed. Stamford, CT: Appleton & Lange, 2000.

Johnson, Leonard R., and Thomas A. Gerwin, eds. Gastrointestinal Physiology, 6th ed. London: Mosby, Inc.

Morrison, Ben. The Digestive System (Insider's Guide to the Body). New York: Rosen Publishing Group, 2001.

PERIODICALS

"Digestive Disorders." US News and World Report 131, no. 3 (July 23, 2001): 74.

Voelker, Rebecca. "Digestive Health in Children." Journal of the American Medical Association 284, no. 10 (Sept. 13, 2000): 1235.

OTHER

"Pathophysiology of the Digestive System." Colorado State University. 〈http://arbl.cvrnbs.colostate.edu/hbooks/pathphys/digestive〉.

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Digestive System

Digestive System

Ingestion

Digestion in the stomach

Gastric juice

Alexis St. Martins stomach

Digestion and absorption in the small intestine

Absorption and elimination in the large intestine

Liver

Gallbladder

Appendix

Pancreas

Disorders of the digestive system

Resources

The digestive system is a group of organs responsible for the conversion of food into absorbable chemicals that are then used to provide energy for growth and repair. A number of other names also describe the digestive system. Some of those names include the gut, digestive tube, alimentary canal, gastrointestinal (GI) tract, intestinal tract, and intestinal tube. The digestive system consists of the mouth, esophagus, stomach, and small and large intestines, along with several glands, such as the salivary glands, liver, gall bladder, and pancreas. These glands secrete digestive juices containing enzymes that break down the food chemically into smaller, more absorbable molecules. In addition to providing the body with the nutrients and energy it needs to function, the digestive system also separates and disposes of waste products ingested with the food.

Food is moved through the alimentary canal by a wavelike muscular motion known as peristalsis, which consists of the alternate contraction and relaxation of the smooth muscles lining the tract. In this way, food is passed through the gut in much the same manner as toothpaste is squeezed from a tube. Churning is another type of movement that takes place in the stomach and small intestine, which mixes the food so that the digestive enzymes can break down the food molecules.

Food in the human diet consists of carbohydrates, proteins, fats, vitamins, and minerals. The remainder of the food is fiber and water. The majority of minerals and vitamins pass through to the bloodstream without the need for further digestive changes, but other nutrient molecules must be broken down to simpler substances before they can be absorbed and used.

Ingestion

Food taken into the mouth is first prepared for digestion in a two step process known as mastication.

In the first stage, the teeth tear and break down food into smaller pieces. In the second stage, the tongue rolls these pieces into balls (boluses). Sensory receptors on the tongue (taste buds) detect taste sensations of sweet, salt, bitter, and sour, or cause the rejection of bad-testing food. The olfactory nerves contribute to the sensation of taste by picking up the aroma of the food and passing the sensation of smell on to the brain.

The sight of the food also stimulates the salivary glands. Altogether, the sensations of sight, taste, and smell cause the salivary glands, located in the mouth, to produce saliva, which then pours into the mouth to soften the food. An enzyme in the saliva called amylase begins the break down of carbohydrates (starch) into simple sugars, such as maltose. Ptyalin is one of the main amylase enzymes found in the mouth; ptyalin is also secreted by the pancreas.

The bolus of food, which is now a battered, moistened, and partially digested ball of food, is swallowed, moving to the throat at the back of the mouth (pharynx). In the throat, rings of muscles force the food into the esophagus, the first part of the upper digestive tube. The esophagus extends from the bottom part of the throat to the upper part of the stomach.

The esophagus does not take part in digestion. Its job is to get the bolus into the stomach. There is a powerful muscle (the esophageal sphincter), at the junction of the esophagus and stomach, which acts as a valve to keep food, stomach acids, and bile from flowing back into the esophagus and mouth.

Digestion in the stomach

Chemical digestion begins in the stomach. The stomach, a large, hollow, pouched-shaped muscular organ, is shaped like a lima bean. When empty, the stomach becomes elongated; when filled, it balloons out.

Food in the stomach is broken down by the action of the gastric juice containing hydrochloric acid and a protein-digesting enzyme called pepsin. Gastric juice is secreted from the linings of the stomach walls, along with mucus, which helps to protect the stomach lining from the action of the acid. The three layers of powerful stomach muscles churn the food into a fine semi-liquid paste called chyme. From time to time, the chyme is passed through an opening (the pyloric sphincter), which controls the passage of chyme between the stomach and the beginning of the small intestine.

Gastric juice

There are several mechanisms responsible for the secretion of gastric juice in the stomach. The stomach begins its production of gastric juice while the food is still in the mouth. Nerves from the cheeks and tongue are stimulated and send messages to the brain. The brain in turn sends messages to nerves in the stomach wall, stimulating the secretion of gastric juice before the arrival of the food. The second signal for gastric juice production occurs when the food arrives in the stomach and touches the lining. This mechanism provides for only a moderate addition to the amount of gastric juice that was secreted when the food was in the mouth.

Gastric juice is needed mainly for the digestion of protein by pepsin. If a hamburger and bun reach the stomach, there is no need for extra gastric juice for the bun (carbohydrate), but the hamburger (protein) will require a much greater supply of gastric juice. The gastric juice already present will begin the break down of the large protein molecules of the hamburger into smaller molecules: polypeptides and peptides. These smaller molecules in turn stimulate the cells of the stomach lining to release the hormone gastrin into the bloodstream.

Gastrin then circulates throughout the body, and eventually reaches the stomach, where it stimulates the cells of the stomach lining to produce more gastric juice. The more protein there is in the stomach, the more gastrin will be produced, and the greater the production of gastric juice. The secretion of more gastric juice by the increased amount of protein in the stomach represents the third mechanism of gastric juice secretion.

Alexis St. Martins stomach

An understanding of the complex mechanisms of gastric juice secretion began with American army doctor William Beaumont (17851853). He was able to directly observe the process of digestion in the stomach from the wound of a soldier named Alexis St. Martin.

In 1822, Beaumont treated the soldier for an accidental gunshot wound. This wound left a large hole in the left side of St. Martins body, tearing away parts of the ribs, muscles, and stomach wall. When the wound healed, the stomach wall had grown to the outer body wall, leaving a permanent hole from the outer body to the interior of the stomach. When St.Martin ate, bandages were needed to keep the food in place. For the first time in medical history, a physician was able to study the inner workings of the stomach. Beaumonts observations and experiments on St. Martins stomach extended over 11 years.

In that time, he observed the secretion of gastric juice and placed the fluid from St. Martins stomach on a piece of meat. There he could observe the

digestion of protein. He was also able to observe the churning movements of the stomach when food entered it. Beaumonts investigation of St. Martins stomach laid the groundwork for later investigations into the complexities of the digestive process.

Digestion and absorption in the small intestine

While digestion continues in the small intestine, it also becomes a major site for the process of absorption, that is, the passage of digested food into the bloodstream, and its transport to the rest of the body.

The small intestine is a long, narrow tube, about 20 ft (6 m) long, running from the stomach to the large intestine. The small intestine occupies the area of the abdomen between the diaphragm and hips, and is greatly coiled and twisted. The small intestine is lined with muscles that move the chyme toward the large intestine. The mucosa, which lines the entire small intestine, contains millions of glands that aid in the digestive and absorptive processes of the digestive system.

The small intestine, or small bowel, is sub-divided by anatomists into three sections, the duodenum, the jejunum, and the ileum. The duodenum is about 1 ft (0.3 m) long and connects with the lower portion of the stomach. When fluid food reaches the duodenum it undergoes further enzymatic digestion and is subjected to pancreatic juice, intestinal juice, and bile.

The pancreas is a large gland located below the stomach that secretes pancreatic juice into the duodenum via the pancreatic duct. Three enzymes in pancreatic juice digest carbohydrates, lipids, and proteins. Amylase (the enzyme found in saliva) breaks down starch into simple sugars such as maltose. The enzyme maltase in intestinal juice completes the break down of maltose into glucose.

Lipases in pancreatic juice break down fats into fatty acids and glycerol, while proteinases continue the break down of proteins into amino acids. The gall bladder, located next to the liver, secretes bile into the duodenum. While bile does not contain enzymes, it contains bile salts and other substances that help to emulsify (dissolve) fats, which are otherwise insoluble in water. Breaking the fat down into small globules allows the lipase enzymes a greater surface area for their action.

Chyme passing from the duodenum next reaches the jejunum of the small intestine, which is about 3 ft (0.91 m) long. Here, in the jejunum, the digested breakdown products of carbohydrates, fats, proteins, and most of the vitamins, minerals, and iron are absorbed. The inner lining of the small intestine is composed of up to five million tiny, fingerlike projections called villi. The villi increase the rate of absorption of the nutrients into the bloodstream by extending the surface of the small intestine to about five times that of the surface area of the skin.

There are two transport systems that pick up the nutrients from the small intestine. Simple sugars, amino acids, glycerol, and some vitamins and salts are conveyed to the liver in the bloodstream. Fatty acids and vitamins are absorbed and then transported through the lymphatic system, the network of vessels that carry lymph and white blood cells throughout the body. Lymph eventually drains back into the bloodstream and circulates throughout the body.

The last section of the small intestine is the ileum. It is smaller and thinner-walled than the jejunum, and it is the preferred site for vitamin B12 absorption and bile acids derived from the bile juice.

Absorption and elimination in the large intestine

The large intestine, or colon, is wider and heavier then the small intestine, but much shorteronly about 4 ft (1.2 m) long. It rises up on one side of the body (the ascending colon), crosses over to the other side (the transverse colon), descends (the descending colon), forms an s-shape (the sigmoid colon), reaches the rectum, and anus, from which the waste products of digestion (feces or stool), are passed out, along with gas. The muscular rectum, about 5 in (13 cm) long, expels the feces through the anus, which has a large muscular sphincter that controls the passage of waste matter.

The large intestine extracts water from the waste products of digestion and returns some of it to the bloodstream, along with some salts. Fecal matter contains undigested food, bacteria, and cells from the walls of the digestive tract. Certain types of bacteria of the large intestine help to synthesize the vitamins needed by the body. These vitamins find their way to the bloodstream along with the water absorbed from the colon, while excess fluids are passed out with the feces.

Liver

The liver is the largest organ in the body and plays a number of vital roles, including metabolizing the breakdown products of digestion, and detoxifying substances that are harmful to the body. The liver also provides a quick source of energy when the need arises and it produces new proteins. Along with the regulation of stored fats, the liver also stores vitamins, minerals, and sugars. The liver controls the excretion and production of cholesterol and metabolizes alcohol into a mild toxin. The liver also stores iron, maintains the hormone balance, produces immune factors to fight infections, regulates blood clotting, and produces bile.

The most common liver disorder in the United States and other developed countries is cirrhosis of the liver. The main cause for this disease is alcoholism. Cirrhosis is characterized by the replacement of healthy liver cells by fibrous tissue. The replacement process is gradual and extends over a period of two to ten years to complete. There is no cure for the disease. Symptoms may not be noticed in its early development, but in its advanced stages there are a number of symptoms and the condition can lead to coma. Close medical attention is required to treat the disease.

Another common liver disorder is hepatitis. It is an inflammation of the liver caused by viruses. The most noticeable symptom of this disease is jaundice, that is, the skin, eyes, and urine turn yellow. The nine viruses known to cause hepatitis include Hepatitis A, B, C, D, and E; the recently discovered F and G viruses; and two herpes viruses (Epstein-Barr and cytomegalovirus).

Gallbladder

The gallbladder lies under the liver and is connected by various ducts to the liver and the duodenum. The gallbladder is a small hollow organ resembling a money pouch. Its main function is to store bile until it is concentrated enough to be used by the small intestine. The gall bladder can store about 2 oz (57 g) of bile. Bile consists of bile salts, bile acids, and bile pigments. In addition, bile contains cholesterol dissolved in the bile acids. If the amount of cholesterol in the bile acids increases or the amount of acid decreases, then some of the cholesterol will settle out of the acid to form gallstones that accumulate and block the ducts to the gallbladder.

Infection in the gallbladder can be another cause for gallstones. Gallstones may be in the gallbladder for years without giving any signs of the condition, but when they obstruct the bile duct they cause considerable pain and inflammation. Infection and blockage of the bile flow may follow. Surgical removal of the gallbladder may be necessary to treat this condition. Since the liver both produces and stores sufficient amounts of bile, the loss of the gallbladder does not interfere with the digestive process provided fat intake in the diet is regulated.

If the gallstones contain mainly cholesterol, drug treatment for gallstones may be possible. Nevertheless, if there is too much other material in the gallstones, surgery may still be necessary. Even after drugs and diet have treated the condition successfully, the condition can return. The drug treatment takes years to dissolve the gallstones.

Appendix

The appendix is a hollow finger-like projection that hangs from the occum at the junction between the small intestine and the large intestine. The appendix does not function in humans; however, in some animals, such as rabbits, the appendix is rather large and helps in the digestion of cellulose from bark and wood, which rabbits eat. The appendix in humans is therefore a vestigial organ, which may have had uses for earlier types of ancestral human digestive processes before the evolution of Homo sapiens.

If food gets trapped in the appendix, an irritation of its membranes may occur leading to swelling and inflammation, a condition known as appendicitis. If the condition becomes serious, removal of the appendix is necessary to avoid a life-threatening condition if it were to rupture.

Pancreas

When food reaches the small intestine, the pancreas secretes pancreatic juices. When there is no food in the small intestine, the pancreas does not secrete its juices. The economy of this process puzzled researchers who wondered what the mechanism for this control might be. In 1902, Sir William Bayliss (18601924) and Ernest Starling (18661927), two British physiologists, conducted experiments to find the answer. They reasoned that the same mechanism that initiated gastric juices when food first enters the mouth might be the same mechanism for releasing the flow of pancreatic juices.

These researchers made an extract from the lining of the small intestine and injected it into an experimental animal. The extract caused the animal to secrete large amounts of pancreatic juice. They concluded that the extract from the intestinal lining must have some substance responsible for the flow, which they named secretin. The experiment gave the first real proof for the existence of hormones, substances secreted by one group of cells that travel around the body which target other groups of cells.

KEY TERMS

Amylase A digestive enzyme found in saliva and the pancreas that breaks down carbohydrates to simple sugars.

Bile A greenish yellow liquid secreted by the liver and stored in the gall bladder that aids in the digestion of fats and oils in the body.

Gastric juice Digestive juice produced by the stomach wall that contains hydrochloric acid and the enzyme pepsin.

Gastrin A hormone produced by the stomach lining in response to protein in the stomach that produces increased gastric juice.

Helicobacter pylori Recently discovered bacteria that live in gastric acids and are believed to be a major cause of most stomach ulcers.

Lower esophageal sphincter A strong muscle ring between the esophagus and the stomach that keeps gastric juice, and even duodenal bile from flowing upwards out of the stomach.

Lymphatic system The transport system linked to the cardiovascular system that contains the immune system and also carries metabolized fat and fat soluble vitamins throughout the body.

Mucosa The digestive lining of the intestines.

Nutrients Vitamins, minerals, proteins, lipids, and carbohydrates needed by the body.

Peristalsis The wavelike motion of the digestive system that moves food through the digestive system.

Villi Finger-like projections found in the small intestine that add to the absorptive area for the passage of digested food to the bloodstream and lymphatic system.

Insulin is another important hormone secreted by a group of cells within the pancreas called the islets of Langerhans, which are part of the endocrine system rather than the digestive system. Insulin released into the bloodstream targets liver and muscle cells. It allows them to take excess sugar from the blood and store it in the form of glycogen. When the pancreas does not produce sufficient insulin to store dietary sugar, the blood and urine levels of sugar reach dangerous levels. Diabetes mellitus is the resultant disease. Mild cases can be controlled by a properly regulated diet, but severe cases require the regular injection of insulin.

Disorders of the digestive system

Several disorders of the esophagus are esophagitis, esophageal spasm, and esophageal cancer. Esophagitis (heartburn) is an inflammation of the esophagus usually caused by the reflux of gastric acids into the esophagus and is treated with (alkalis) antacid. Esophageal spasm is also caused by acid reflux and is sometimes treated with nitroglycerine placed under the tongue. Esophageal cancer can be caused by smoking and is generally fatal.

Disorders of the stomach include hiatal hernia, ulcers, and gastric cancer. A hiatal hernia occurs when a portion of the stomach extends upwards into the thorax through a large opening in the diaphragm. People over the age of 50 years often contract the illness. Stomach ulcers are sores that form in the lining of the stomach. They may vary in size from a small sore to a deep cavity, surrounded by an inflamed area, sometimes called ulcer craters. Stomach ulcers and ulcers that form in the esophagus and in the lining of the duodenum are called peptic ulcers because they need stomach acid and the enzyme pepsin to form. Duodenal ulcers are the most common type. They tend to be smaller than stomach ulcers and heal more quickly. Ulcers that form in the stomach lining are called gastric ulcers. As of 2005, about six million people in the United States have ulcers and 20% of those have gastric ulcers. About 10% of all adults in the United States will have an ulcer at some point in their lives. Those people who are at most risk for ulcers are those who smoke, middle-age and older men, chronic users of alcohol, and those who take anti-inflammatory drugs, such as aspirin and ibuprofen.

Until 1993, the general belief in the medical community concerning the cause of stomach ulcers was that there were multiple factors responsible for their development. By 1993 there was mounting evidence that an S-shaped bacterium, Helicobacter pylori, could be one of the factors causing ulcers. Helicobacter pylori live in the mucous lining of the stomach near the surface cells and may go undetected for years. Researchers argued that irritation to the stomach caused by the bacteria weakened the lining, making it more susceptible to damage by acid and resulting in the formation of ulcers.

Barry James Marshall (1951), an Australian gastroenterologist, was the chief proponent of the theory that stomach ulcers are caused by H. pylori infections, rather than a multiple factor explanation, such as stress or poor diet. Although Marshall was discouraged by his colleagues from pursuing this line of research, he demonstrated his hypothesis by swallowing a mixture containing H. pylori. Marshall soon developed gastritis, which is the precursor condition to ulcers.

The treatment of ulcers has undergone a radical change with Marshalls discovery that stomach ulcers are caused by H. pylori infections. Ulcer patients today are being treated with antibiotics and antacids rather than special diets or expensive medicines. It is believed that about 80% of stomach ulcers may be caused by the bacterial infection, while about 20% may be from other causes, such as the use of anti-inflammatory medicines.

Resources

BOOKS

NIH (National Institutes of Health) publication. Your Digestive System and How it Works. National Institute of Diabetes and Digestive and Kidney Diseases, 2004.

Sullivan, Robert James. Digestion and Nutrition. Philadelphia, PA: Chelsea House, 2004.

Jordan P. Richman

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Digestive System

Digestive system

Definition

The digestive system is a group of organs and tissues responsible for the conversion of food into absorbable chemicals which are then used to provide energy for growth and repair.

Description

The digestive system is also known by a number of other names, including the gut, the digestive tube, the alimentary canal, the gastrointestinal (GI) tract, the intestinal tract, and the intestinal tube. The digestive system consists of the mouth, esophagus, stomach , and small and large intestines, along with several glands, such as the salivary glands, liver , gall bladder, and pancreas .

Function and role in human health

The glands in the digestive system secrete digestive juices containing enzymes that break down the food chemically into smaller, more absorbable molecules. In addition to providing the body with the nutrients and energy it needs to function, the digestive system also separates and disposes of waste products ingested with the food.

Food is moved through the alimentary canal by a wavelike muscular motion known as peristalsis, which consists of the alternate contraction and relaxation of the smooth muscles lining the tract. In this way, food is passed through the gut in much the same manner as toothpaste is squeezed from a tube. Churning is another type of movement that takes place in the stomach and small intestine , which mixes the food so that the digestive enzymes can break down the food molecules.

Food in the human diet consists of carbohydrates , proteins , fats , vitamins , and minerals . The remainder of the food is fiber and water. The majority of minerals and vitamins pass through to the bloodstream without the need for further digestive changes, but other nutrient molecules must be broken down to simpler substances before they can be absorbed and used.

Ingestion

Food taken into the mouth is first prepared for digestion in a two-step process known as mastication. In the first stage, the teeth tear and break down food into smaller pieces. In the second stage, the tongue rolls these pieces into balls (boluses). Sensory receptors on the tongue (taste buds) detect taste sensations of sweet, salt, bitter, and sour, or cause the rejection of bad-testing food. The olfactory nerves contribute to the sensation of taste by picking up the aroma of the food and passing the sensation of smell on to the brain .

The sight of the food also stimulates the salivary glands. Altogether, the sensations of sight, taste, and smell cause the salivary glands, located in the mouth, to produce saliva, which then pours into the mouth to soften the food. An enzyme in the saliva called amylase begins the break down of carbohydrates (starch) into simple sugars, such as maltose. Ptyalin is one of the main amylase enzymes found in the mouth; ptyalin is also secreted by the pancreas.

The bolus of food, which is now a battered, moistened, and partially digested ball of food, is swallowed, moving to the throat at the back of the mouth (pharynx). In the throat, rings of muscles force the food into the esophagus, the first part of the upper digestive tube. The esophagus extends from the bottom part of the throat to the upper part of the stomach.

The esophagus does not take part in digestion. Its job is to get the bolus into the stomach. There is a powerful muscle (the esophageal sphincter), at the junction of the esophagus and stomach, which acts as a valve to keep food, stomach acids, and bile from flowing back into the esophagus and mouth.

Digestion in the stomach

Chemical digestion begins in the stomach. The stomach, a large, hollow, pouched-shaped muscular organ, is shaped like a lima bean. When empty, the stomach becomes elongated; when filled, it balloons out.

Food in the stomach is broken down by the action of the gastric juice containing hydrochloric acid and a protein-digesting enzyme called pepsin. Gastric juice is secreted from the lining of the stomach walls, along with mucus, which helps to protect the stomach lining from the action of the acid. The three layers of powerful stomach muscles churn the food into a fine semiliquid paste called chyme. The chyme (pronounced "kime") is periodically passed through an opening (the pyloric sphincter), which controls the passage of chyme between the stomach and the beginning of the small intestine.

Gastric juice

There are several mechanisms responsible for the secretion of gastric juice in the stomach. The stomach begins its production of gastric juice while the food is still in the mouth. Nerves from the cheeks and tongue are stimulated and send messages to the brain. The brain in turn sends messages to nerves in the stomach wall, stimulating the secretion of gastric juice before the arrival of the food. The second signal for gastric juice production occurs when the food arrives in the stomach and touches the lining. This mechanism provides for only a moderate addition to the amount of gastric juice that was secreted when the food was in the mouth.

Gastric juice is needed mainly for the digestion of protein by pepsin. If a hamburger and bun reach the stomach, there is no need for extra gastric juice for the bun (carbohydrate), but the hamburger (protein) will require a much greater supply of gastric juice. The gastric juice already present will begin the break down of the large protein molecules of the hamburger into smaller molecules: polypeptides and peptides. These smaller molecules in turn stimulate the cells of the stomach lining to release the hormone gastrin into the bloodstream.

Gastrin then circulates throughout the body, and eventually reaches the stomach, where it stimulates the cells of the stomach lining to produce more gastric juice. The more protein there is in the stomach, the more gastrin will be produced, and the greater the production of gastric juice. The secretion of more gastric juice by the increased amount of protein in the stomach represents the third mechanism of gastric juice secretion.

Digestion and absorption in the small intestine

While digestion continues in the small intestine, it also becomes a major site for the process of absorption, that is, the passage of digested food into the bloodstream, and its transport to the rest of the body.

The small intestine is a long, narrow tube, about 20 ft (6 m) long, running from the stomach to the large intestine . The small intestine occupies the area of the abdomen between the diaphragm and hips, and is greatly coiled and twisted. The small intestine is lined with muscles that move the chyme toward the large intestine. The mucosa, which lines the entire small intestine, contains millions of glands that aid in the digestive and absorptive processes of the digestive system.

The small intestine, or small bowel, is sub-divided into three sections, the duodenum, the jejunum, and the ileum. The duodenum is about 1 ft (0.3 m) long and connects with the lower portion of the stomach. When fluid food reaches the duodenum it undergoes further enzymatic digestion and is subjected to pancreatic juice, intestinal juice, and bile.

The pancreas is a large gland located below the stomach that secretes pancreatic juice into the duodenum via the pancreatic duct. There are three enzymes in pancreatic juice which digest carbohydrates, lipids , and proteins. Amylase, (the enzyme found in saliva) breaks down starch into simple sugars such as maltose. The enzyme maltase in intestinal juice completes the break down of maltose into glucose.

Lipases in pancreatic juice break down fats into fatty acids and glycerol, while proteinases continue the breakdown of proteins into amino acids. The gallbladder, located next to the liver, secretes bile into the duodenum. While bile does not contain enzymes, it contains bile salts and other substances that help to emulsify (dissolve) fats, which are otherwise insoluble in water. Breaking the fat down into small globules allows the lipase enzymes a greater surface area for their action.

Chyme passing from the duodenum next reaches the jejunum of the small intestine, which is about 3 ft (0.91m) long. Here, in the jejunum, the digested breakdown products of carbohydrates, fats, proteins, and most of the vitamins, minerals, and iron are absorbed. The inner lining of the small intestine is composed of up to five million tiny, finger-like projections called villi. The villi increase the rate of absorption of the nutrients into the bloodstream by extending the surface of the small intestine to about five times that of the surface area of the skin.

There are two transport systems that pick up the nutrients from the small intestine. Simple sugars, amino acids, glycerol, and some vitamins and salts are conveyed to the liver in the bloodstream. Fatty acids and vitamins are absorbed and then transported through the lymphatic system , the network of vessels that carry lymph and white blood cells throughout the body. Lymph eventually drains back into the bloodstream and circulates throughout the body.

The last section of the small intestine is the ileum. It is smaller and thinner-walled than the jejunum, and it is the preferred site for vitamin B 12 absorption and bile acids derived from the bile juice.

Absorption and elimination in the large intestine

The large intestine, or colon, is wider and heavier then the small intestine, but much shorter—only about 4 ft (1.2 m) long. It rises up on one side of the body (the ascending colon), crosses over to the other side (the transverse colon), descends (the descending colon), forms an s-shape (the sigmoid colon), reaches the rectum, and anus, from which the waste products of digestion (feces or stool), are passed out, along with gas. The muscular rectum, about 5 in (13 cm) long, expels the feces through the anus, which has a large muscular sphincter that controls the passage of waste matter.

The large intestine extracts water from the waste products of digestion and returns some of it to the bloodstream, along with some salts. Fecal matter contains undigested food, bacteria , and cells from the walls of the digestive tract. Certain types of bacteria of the large intestine help to synthesize the vitamins needed by the body. These vitamins find their way to the bloodstream along with the water absorbed from the colon, while excess fluids are passed out with the feces.

Liver

The liver is the largest organ in the body and plays a number of vital roles, including metabolizing the broken-down products of digestion, and detoxifying substances that are harmful to the body. The liver also provides a quick source of energy when the need arises and it produces new proteins. Along with the regulation of stored fats, the liver also stores vitamins, minerals, and sugars. The liver controls the excretion and production of cholesterol and metabolizes alcohol into a mild toxin. The liver also stores iron, maintains the hormone balance, produces immune factors to fight infections, regulates blood clotting, and produces bile.

Gallbladder

The gallbladder lies under the liver and is connected by various ducts to the liver and the duodenum. The gallbladder is a small hollow organ; its main function is to store bile until it is concentrated enough to be used by the small intestine. The gallbladder can store about 2 oz of bile. Bile consists of bile salts, bile acids, and bile pigments. In addition, bile contains cholesterol dissolved in the bile acids.

Appendix

The appendix is a hollow finger-like projection that hangs from the cecum at the junction between the small intestine and the large intestine. The appendix does not function in humans; however, in some animals, such as rabbits, the appendix is rather large and helps in the digestion of cellulose from bark and wood, which rabbits eat. The appendix in humans is therefore a vestigial organ, which may have had uses for earlier types of ancestral human digestive processes before the evolution of Homo sapiens.

Pancreas

When food reaches the small intestine, the pancreas secretes pancreatic juices. When there is no food in the small intestine, the pancreas does not secrete its juices.

Insulin is another important hormone secreted by a group of cells within the pancreas called the islets of Langerhans, which are part of the endocrine system , rather than the digestive system. Insulin released into the bloodstream targets liver and muscle cells, and allows them to take excess sugar from the blood and store it in the form of glycogen.


KEY TERMS


Amylase —A digestive enzyme found in saliva and the pancreas that breaks down carbohydrates to simple sugars.

Bile —Liquid produced in the liver and stored in the gallbladder that emulsifies fats.

Gastric juice —Digestive juice produced by the stomach wall that contains hydrochloric acid and the enzyme pepsin.

Gastrin —A hormone produced by the stomach lining in response to protein in the stomach that produces increased gastric juice.

Helicobacter pylori —Recently discovered bacteria that live in gastric acids and are believed to be a major cause of most stomach ulcers.

Lower esophageal sphincter —A strong muscle ring between the esophagus and the stomach that keeps gastric juice and even duodenal bile from flowing upwards out of the stomach.

Lymphatic system —The system that produces, transports, and filters lymph throughout the body. It also transports fats, proteins, and some vitamins to the blood system.

Mucosa —The digestive lining of the intestines.

Nutrients —Vitamins, minerals, proteins, lipids, and carbohydrates needed by the body.

Peristalsis —The wavelike motion of the digestive system that moves food through the digestive system.

Villi —Fingerlike projections found in the small intestine that add to the absorptive area for the passage of digested food to the bloodstream and lymphatic system.


Common diseases and disorders

Several disorders of the esophagus are esophagitis, esophageal spasm, and esophageal cancer . Esophagitis (heartburn) is an inflammation of the esophagus usually caused by the reflux of gastric acids into the esophagus and is treated with antacid (alkalis). Esophageal spasm is also caused by acid reflux. Esophageal cancer can be caused by smoking and is generally fatal.

Disorders of the stomach include hiatal hernia, ulcers, and gastric cancer. A hiatal hernia occurs when a portion of the stomach extends upwards into the thorax through a large opening in the diaphragm. It is a condition that commonly occurs in people over the age of 50. Stomach ulcers are sores that form in the lining of the stomach. They may vary in size from a small sore to a deep cavity, surrounded by an inflamed area, sometimes called ulcer craters. Stomach ulcers and ulcers that form in the esophagus and in the lining of the duodenum are called peptic ulcers because they need stomach acid and the enzyme pepsin to form. Duodenal ulcers are the most common type. They tend to be smaller than stomach ulcers and heal more quickly. Ulcers that form in the stomach lining are called gastric ulcers. About 4 million people have ulcers, and 20% of those have gastric ulcers. Those people who are at most risk for ulcers are those who smoke, are middle-age and older men, are chronic users of alcohol, and those who take anti-inflammatory drugs, such as aspirin and ibuprofen. It is believed that about 80% of stomach ulcers may be caused by the bacterial infection , while about 20% may be from other causes, such as the use of anti-inflammatory medicines.

The most common liver disorder in the United States and other developed countries is cirrhosis of the liver. The main cause for this disease is alcoholism . Cirrhosis is characterized by the replacement of healthy liver cells by fibrous tissue. The replacement process is gradual and takes a period of two to 10 years to complete. There is no cure for the disease. Symptoms may not be noticed in its early development, but in its advanced stages there are a number of symptoms and the condition can lead to coma . Close medical attention is required to treat the disease.

Another common liver disorder is hepatitis. It is an inflammation of the liver caused by viruses . The most noticeable symptom of this disease is jaundice , that is, the skin, eyes, and urine turn yellow. The nine viruses known to cause hepatitis include hepatitis A, B, C, D, and E; the recently discovered F and G viruses; and two herpes viruses (Epstine-Barr and cytomegalovirus).

Gallstones may form in the gallbladder. If the amount of cholesterol in the bile acids increases or the amount of acid decreases, then some of the cholesterol will settle out of the acid to form gallstones that accumulate and block the ducts to the gallbladder. Infection in the gallbladder may lead to gallstones. Gallstones may be in the gallbladder for years without giving any signs of the condition, but when they obstruct the bile duct they cause considerable pain and inflammation. Infection and blockage of the bile flow may follow. Surgical removal of the gallbladder may be necessary to treat this condition. Since the liver both produces and stores sufficient amounts of bile, the loss of the gallbladder does not interfere with the digestive process provided fat intake in the diet is regulated. If the gallstones contain mainly cholesterol, drug treatment for the stones may be possible. But if there is too much other material in the gallstones, surgery may still be necessary. Even after being treated successfully by drugs and diet, the condition can return. The drug treatment takes years to dissolve gallstones.

If food gets trapped in the appendix, an irritation of its membranes may occur leading to swelling and inflammation, a condition known as appendicitis . If the condition becomes serious, removal of the appendix is necessary to avoid a life-threatening condition if it were to rupture.

When the pancreas does not produce sufficient insulin to store dietary sugar, the blood and urine levels of sugar reach dangerous levels. Diabetes mellitus is the resultant disease. Mild cases can be controlled by a properly regulated diet, but severe cases require the regular injection of insulin.

Resources

BOOKS

Greenspan, Francis S., and David G. Gardner, eds. Basic & Clinical Endocrinology, 6th ed. Stamford, CT: Appleton & Lange, 2000.

Johnson, Leonard R., and Thomas A. Gerwin, eds. Gastrointestinal Physiology, 6th ed. London: Mosby, Inc.

Morrison, Ben. The Digestive System (Insider's Guide to the Body). New York: Rosen Publishing Group, 2001.

PERIODICALS

"Digestive Disorders." US News and World Report 131, no. 3 (July 23, 2001): 74.

Voelker, Rebecca. "Digestive Health in Children." Journal of the American Medical Association 284, no. 10 (Sept. 13,2000): 1235.

OTHER

"Pathophysiology of the Digestive System." Colorado State University. <http://arbl.cvrnbs.colostate.edu/hbooks/pathphys/digestive>.

Crystal Heather Kaczkowski, MSc.

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Digestive System

Digestive system

The digestive system is a group of organs responsible for the conversion of food into absorbable chemicals which are then used to provide energy for growth and repair. The digestive system is also known by a number of other names, including the gut, the digestive tube, the alimentary canal, the gastrointestinal (GI) tract, the intestinal tract, and the intestinal tube. The digestive system consists of the mouth, esophagus, stomach, and small and large intestines, along with several glands , such as the salivary glands, liver, gall bladder, and pancreas. These glands secrete digestive juices containing enzymes that break down the food chemically into smaller, more absorbable molecules. In addition to providing the body with the nutrients and energy it needs to function, the digestive system also separates and disposes of waste products ingested with the food.

Food is moved through the alimentary canal by a wavelike muscular motion known as peristalsis, which consists of the alternate contraction and relaxation of the smooth muscles lining the tract. In this way, food is passed through the gut in much the same manner as toothpaste is squeezed from a tube. Churning is another type of movement that takes place in the stomach and small intestine, which mixes the food so that the digestive enzymes can break down the food molecules.

Food in the human diet consists of carbohydrates, proteins , fats, vitamins, and minerals . The remainder of the food is fiber and water . The majority of minerals and vitamins pass through to the bloodstream without the need for further digestive changes, but other nutrient molecules must be broken down to simpler substances before they can be absorbed and used.


Ingestion

Food taken into the mouth is first prepared for digestion in a two step process known as mastication. In the first stage, the teeth tear and break down food into smaller pieces. In the second stage, the tongue rolls these pieces into balls (boluses). Sensory receptors on the tongue (taste buds) detect taste sensations of sweet, salt , bitter, and sour, or cause the rejection of bad-testing food. The olfactory nerves contribute to the sensation of taste by picking up the aroma of the food and passing the sensation of smell on to the brain .

The sight of the food also stimulates the salivary glands. Altogether, the sensations of sight, taste, and smell cause the salivary glands, located in the mouth, to produce saliva, which then pours into the mouth to soften the food. An enzyme in the saliva called amylase begins the break down of carbohydrates (starch) into simple sugars, such as maltose. Ptyalin is one of the main amylase enzymes found in the mouth; ptyalin is also secreted by the pancreas.

The bolus of food, which is now a battered, moistened, and partially digested ball of food, is swallowed, moving to the throat at the back of the mouth (pharynx). In the throat, rings of muscles force the food into the esophagus, the first part of the upper digestive tube. The esophagus extends from the bottom part of the throat to the upper part of the stomach.

The esophagus does not take part in digestion. Its job is to get the bolus into the stomach. There is a powerful muscle (the esophageal sphincter), at the junction of the esophagus and stomach, which acts as a valve to keep food, stomach acids, and bile from flowing back into the esophagus and mouth.


Digestion in the stomach

Chemical digestion begins in the stomach. The stomach, a large, hollow, pouched-shaped muscular organ , is shaped like a lima bean. When empty, the stomach becomes elongated; when filled, it balloons out.

Food in the stomach is broken down by the action of the gastric juice containing hydrochloric acid and a protein-digesting enzyme called pepsin. Gastric juice is secreted from the linings of the stomach walls, along with mucus, which helps to protect the stomach lining from the action of the acid. The three layers of powerful stomach muscles churn the food into a fine semiliquid paste called chyme. From time to time, the chyme is passed through an opening (the pyloric sphincter), which controls the passage of chyme between the stomach and the beginning of the small intestine.


Gastric juice

There are several mechanisms responsible for the secretion of gastric juice in the stomach. The stomach begins its production of gastric juice while the food is still in the mouth. Nerves from the cheeks and tongue are stimulated and send messages to the brain. The brain in turn sends messages to nerves in the stomach wall, stimulating the secretion of gastric juice before the arrival of the food. The second signal for gastric juice production occurs when the food arrives in the stomach and touches the lining. This mechanism provides for only a moderate addition to the amount of gastric juice that was secreted when the food was in the mouth.

Gastric juice is needed mainly for the digestion of protein by pepsin. If a hamburger and bun reach the stomach, there is no need for extra gastric juice for the bun (carbohydrate ), but the hamburger (protein) will require a much greater supply of gastric juice. The gastric juice already present will begin the break down of the large protein molecules of the hamburger into smaller molecules: polypeptides and peptides. These smaller molecules in turn stimulate the cells of the stomach lining to release the hormone gastrin into the bloodstream.

Gastrin then circulates throughout the body, and eventually reaches the stomach, where it stimulates the cells of the stomach lining to produce more gastric juice. The more protein there is in the stomach, the more gastrin will be produced, and the greater the production of gastric juice. The secretion of more gastric juice by the increased amount of protein in the stomach represents the third mechanism of gastric juice secretion.


Alexis St. Martin's stomach

An understanding of the complex mechanisms of gastric juice secretion began with an American army doctor, William Beaumont (1785-1853). He was able to directly observe the process of digestion in the stomach from the wound of a soldier named Alexis St. Martin.

In 1822, Beaumont treated the soldier for an accidental gunshot wound. This wound left a large hole in the left side of St. Martin's body, tearing away parts of the ribs, muscles, and stomach wall. When the wound healed, the stomach wall had grown to the outer body wall, leaving a permanent hole from the outer body to the interior of the stomach. When St. Martin ate, bandages were needed to keep the food in place. For the first time in medical history, a physician was able to study the inner workings of the stomach. Beaumont's observations and experiments on St. Martin's stomach extended over 11 years.

In that time, he observed the secretion of gastric juice and placed the fluid from St. Martin's stomach on a piece of meat. There he could observe the digestion of protein. He was also able to observe the churning movements of the stomach when food entered it. Beaumont's investigation of St. Martin's stomach laid the groundwork for later investigations into the complexities of the digestive process.


Digestion and absorption in the small intestine

While digestion continues in the small intestine, it also becomes a major site for the process of absorption, that is, the passage of digested food into the bloodstream, and its transport to the rest of the body.

The small intestine is a long, narrow tube, about 20 ft (6 m) long, running from the stomach to the large intestine. The small intestine occupies the area of the abdomen between the diaphragm and hips, and is greatly coiled and twisted. The small intestine is lined with muscles that move the chyme toward the large intestine. The mucosa, which lines the entire small intestine, contains millions of glands that aid in the digestive and absorptive processes of the digestive system.

The small intestine, or small bowel, is sub-divided by anatomists into three sections, the duodenum, the jejunum, and the ileum. The duodenum is about 1 ft (0.3 m) long and connects with the lower portion of the stomach. When fluid food reaches the duodenum it undergoes further enzymatic digestion and is subjected to pancreatic juice, intestinal juice, and bile.

The pancreas is a large gland located below the stomach that secretes pancreatic juice into the duodenum via the pancreatic duct. There are three enzymes in pancreatic juice which digest carbohydrates, lipids, and proteins. Amylase, (the enzyme found in saliva) breaks down starch into simple sugars such as maltose. The enzyme maltase in intestinal juice completes the break down of maltose into glucose.

Lipases in pancreatic juice break down fats into fatty acids and glycerol , while proteinases continue the break down of proteins into amino acids. The gall bladder, located next to the liver, secretes bile into the duodenum. While bile does not contain enzymes; it contains bile salts and other substances that help to emulsify (dissolve) fats, which are otherwise insoluble in water. Breaking the fat down into small globules allows the lipase enzymes a greater surface area for their action.

Chyme passing from the duodenum next reaches the jejunum of the small intestine, which is about 3 ft (0.91 m) long. Here, in the jejunum, the digested breakdown products of carbohydrates, fats, proteins, and most of the vitamins, minerals, and iron are absorbed. The inner lining of the small intestine is composed of up to five million tiny, finger-like projections called villi. The villi increase the rate of absorption of the nutrients into the bloodstream by extending the surface of the small intestine to about five times that of the surface area of the skin.

There are two transport systems that pick up the nutrients from the small intestine. Simple sugars, amino acids, glycerol, and some vitamins and salts are conveyed to the liver in the bloodstream. Fatty acids and vitamins are absorbed and then transported through the lymphatic system , the network of vessels that carry lymph and white blood cells throughout the body. Lymph eventually drains back into the bloodstream and circulates throughout the body.

The last section of the small intestine is the ileum. It is smaller and thinner-walled than the jejunum, and it is the preferred site for vitamin B12 absorption and bile acids derived from the bile juice.



Absorption and elimination in the large intestine

The large intestine, or colon, is wider and heavier then the small intestine, but much shorter—only about 4 ft (1.2 m) long. It rises up on one side of the body (the ascending colon), crosses over to the other side (the transverse colon), descends (the descending colon), forms an s-shape (the sigmoid colon), reaches the rectum, and anus, from which the waste products of digestion (feces or stool), are passed out, along with gas. The muscular rectum, about 5 in (13 cm) long, expels the feces through the anus, which has a large muscular sphincter that controls the passage of waste matter.

The large intestine extracts water from the waste products of digestion and returns some of it to the bloodstream, along with some salts. Fecal matter contains undigested food, bacteria , and cells from the walls of the digestive tract. Certain types of bacteria of the large intestine help to synthesize the vitamins needed by the body. These vitamins find their way to the bloodstream along with the water absorbed from the colon, while excess fluids are passed out with the feces.



Liver

The liver is the largest organ in the body and plays a number of vital roles, including metabolizing the breakdown products of digestion, and detoxifying substances that are harmful to the body. The liver also provides a quick source of energy when the need arises and it produces new proteins. Along with the regulation of stored fats, the liver also stores vitamins, minerals, and sugars. The liver controls the excretion and production of cholesterol and metabolizes alcohol into a mild toxin. The liver also stores iron, maintains the hormone balance, produces immune factors to fight infections, regulates blood clotting, and produces bile.

The most common liver disorder in the United States and other developed countries is cirrhosis of the liver. The main cause for this disease is alcoholism . Cirrhosis is characterized by the replacement of healthy liver cells by fibrous tissue . The replacement process is gradual and extends over a period of 2-10 years to complete. There is no cure for the disease. Symptoms may not be noticed in its early development, but in its advanced stages there are a number of symptoms and the condition can lead to coma . Close medical attention is required to treat the disease.

Another common liver disorder is hepatitis . It is an inflammation of the liver caused by viruses. The most noticeable symptom of this disease is jaundice , that is, the skin, eyes, and urine turn yellow. The nine viruses known to cause hepatitis include Hepatitis A, B, C, D, and E; the recently discovered F and G viruses; and two herpes viruses (Epstein-Barr and cytomegalovirus).


Gallbladder

The gallbladder lies under the liver and is connected by various ducts to the liver and the duodenum. The gall-bladder is a small hollow organ resembling a money pouch. Its main function is to store bile until it is concentrated enough to be used by the small intestine. The gall bladder can store about 2 oz (57 g) of bile. Bile consists of bile salts, bile acids, and bile pigments. In addition, bile contains cholesterol dissolved in the bile acids. If the amount of cholesterol in the bile acids increases or the amount of acid decreases, then some of the cholesterol will settle out of the acid to form gallstones that accumulate and block the ducts to the gallbladder.

Infection in the gallbladder can be another cause for gallstones. Gallstones may be in the gallbladder for years without giving any signs of the condition, but when they obstruct the bile duct they cause considerable pain and inflammation. Infection and blockage of the bile flow may follow. Surgical removal of the gallbladder may be necessary to treat this condition. Since the liver both produces and stores sufficient amounts of bile, the loss of the gallbladder does not interfere with the digestive process provided fat intake in the diet is regulated.

If the gallstones contain mainly cholesterol, drug treatment for gallstones may be possible. But if there is too much other material in the gallstones, surgery may still be necessary. Even after the condition has been treated successfully by drugs and diet, the condition can return. The drug treatment takes years to dissolve the gallstones.


Appendix

The appendix is a hollow finger-like projection that hangs from the occum at the junction between the small intestine and the large intestine. The appendix does not function in humans; however, in some animals, such as rabbits, the appendix is rather large and helps in the digestion of cellulose from bark and wood , which rabbits eat. The appendix in humans is therefore a vestigial organ, which may have had uses for earlier types of ancestral human digestive processes before the evolution of Homo sapiens.

If food gets trapped in the appendix, an irritation of its membranes may occur leading to swelling and inflammation, a condition known as appendicitis. If the condition becomes serious, removal of the appendix is necessary to avoid a life-threatening condition if it were to rupture.


Pancreas

When food reaches the small intestine, the pancreas secretes pancreatic juices. When there is no food in the small intestine, the pancreas does not secrete its juices. The economy of this process puzzled researchers who wondered what the mechanism for this control might be. In 1902, William Bayliss and Ernest Starling, two British physiologists, conducted experiments to find the answer. They reasoned that the same mechanism that initiated gastric juices when food first enters the mouth might be the same mechanism for releasing the flow of pancreatic juices.

These researchers made an extract from the lining of the small intestine and injected it into an experimental animal . The extract caused the animal to secrete large amounts of pancreatic juice. They concluded that the extract from the intestinal lining must have some substance responsible for the flow, which they named secretin. The experiment gave the first real proof for the existence of hormones , substances secreted by one group of cells that travel around the body which target other groups of cells.

Insulin is another important hormone secreted by a group of cells within the pancreas called the islets of Langerhans, which are part of the endocrine system rather than the digestive system. Insulin released into the bloodstream targets liver and muscle cells, and allows them to take excess sugar from the blood and store it in the form of glycogen. When the pancreas does not produce sufficient insulin to store dietary sugar, the blood and urine levels of sugar reach dangerous levels. Diabetes mellitus is the resultant disease. Mild cases can be controlled by a properly regulated diet, but severe cases require the regular injection of insulin.


Disorders of the digestive system

Several disorders of the esophagus are esophagitis, esophageal spasm, and esophageal cancer . Esophagitis (heartburn) is an inflammation of the esophagus usually caused by the reflux of gastric acids into the esophagus and is treated with (alkalis) antacid. Esophageal spasm is also caused by acid reflux and is sometimes treated with nitroglycerine placed under the tongue. Esophageal cancer can be caused by smoking and is generally fatal.

Disorders of the stomach include hiatal hernia , ulcers , and gastric cancer. A hiatal hernia occurs when a portion of the stomach extends upwards into the thorax through a large opening in the diaphragm. It is a condition that commonly occurs to people over the age of 50. Stomach ulcers are sores that form in the lining of the stomach. They may vary in size from a small sore to a deep cavity, surrounded by an inflamed area, sometimes called ulcer craters. Stomach ulcers and ulcers that form in the esophagus and in the lining of the duodenum are called peptic ulcers because they need stomach acid and the enzyme pepsin to form. Duodenal ulcers are the most common type. They tend to be smaller than stomach ulcers and heal more quickly. Ulcers that form in the stomach lining are called gastric ulcers. About four million people have ulcers and 20% of those have gastric ulcers. Those people who are at most risk for ulcers are those who smoke, middle-age and older men, chronic users of alcohol, and those who take anti-inflammatory drugs, such as aspirin and ibuprofen.

Until 1993, the general belief in the medical community concerning the cause of stomach ulcers was that there were multiple factors responsible for their development. By 1993 there was mounting evidence that an S-shaped bacterium, Helicobacter pylori, could be one of the factors causing ulcers. Helicobacter pylori live in the mucous lining of the stomach near the surface cells and may go undetected for years. Researchers argued that irritation to the stomach caused by the bacteria weakened the lining, making it more susceptible to damage by acid and resulting in the formation of ulcers.

Barry Marshall, an Australian gastroenterologist, was the chief proponent of the theory that stomach ulcers are caused by H. pylori infections, rather than a multiple factor explanation, such as stress or poor diet. Although Marshall was discouraged by his colleagues from pursuing this line of research, he demonstrated his hypothesis by swallowing a mixture containing H. pylori. Marshall soon developed gastritis, which is the precursor condition to ulcers.

The treatment of ulcers has undergone a radical change with Marshall's discovery that stomach ulcers are caused by H. pylori infections. Ulcer patients today are being treated with antibiotics and antacids rather than special diets or expensive medicines. It is believed that about 80% of stomach ulcers may be caused by the bacterial infection, while about 20% may be from other causes, such as the use of anti-inflammatory medicines.


Resources

books

Maryon-Davis, Alan, and Steven Parker. Food and digestion. London; New York: F. Watts, 1990.

Peikin, Steven R. Gastrointestinal Health. New York: Harper-Collins, 1991.


Jordan P. Richman

KEY TERMS


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Amylase

—A digestive enzyme found in saliva and the pancreas that breaks down carbohydrates to simple sugars.

Bile

—A greenish yellow liquid secreted by the liver and stored in the gall bladder that aids in the digestion of fats and oils in the body.

Gastric juice

—Digestive juice in produced by stomach wall that contains hydrochloric acid and the enzyme pepsin.

Gastrin

—A hormone produced by the stomach lining in response to protein in the stomach that produces increased gastric juice.

Helicobacter pylori

—Recently discovered bacteria that live in gastric acids and are believed to be a major cause of most stomach ulcers.

Lower esophageal sphincter

—A strong muscle ring between the esophagus and the stomach that keeps gastric juice, and even duodenal bile from flowing upwards out of the stomach.

Lymphatic system

—The transport system linked to the cardiovascular system that contains the immune system and also carries metabolized fat and fat soluble vitamins throughout the body.

Mucosa

—The digestive lining of the intestines.

Nutrients

—Vitamins, minerals, proteins, lipids, and carbohydrates needed by the body.

Peristalsis

—The wavelike motion of the digestive system that moves food through the digestive system.

Villi

—Finger-like projections found in the small intestine that add to the absorptive area for the passage of digested food to the bloodstream and lymphatic system.

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Digestive System

Digestive System


A digestive system is a system that allows an organism to take in food, break it down, absorb its nutrients, and excrete what is not usable. All organisms that cannot internally make their own food (as plants do) must ingest or eat it, and therefore must have a digestive system in some form. Different types of animals have different digestive systems according to their main diet and the amount they eat.

All living things need food in order to continue to live, grow, and reproduce. Except for green plants and some algae that can make their own food using the Sun's energy, all other living things get their energy from eating other living things, such as plants or animals. However, these plants and animals used as food are made up of large molecules that cannot be used by an organism's cells unless they are changed into smaller molecules that can be absorbed. The entire process by which food is converted into a form the body can use is called digestion. This process of digestion is carried out by the organism's digestive system. Digestive systems range from the very simple, primitive systems of one-celled organisms to the complex, many-organ systems used by vertebrates (animals with backbones).

INTRACELLULAR DIGESTION

Very simple, single-celled organisms practice what is called intracellular digestion in which they engulf or surround a food particle with their outer membrane. During this type of digestion, these organisms literally bring the food particle inside the cell. Strong enzymes (proteins that control the rate of chemical changes) break the food particle down into its usable components, which are then absorbed into the cell's cytoplasm (the jelly-like fluid inside a cell). Waste products are packaged up and passed back out through the cell membrane.

EXTRACELLULAR DIGESTION

Other slightly more complex organisms like a sponge may have a mouth that leads to a large, open body cavity. Organisms that have only one opening through which passes both their food and their waste are said to have an incomplete digestive system. Flatworms and hydras have this type of digestive system, sometimes called a blind gut. Food enters its mouth and is partially digested by chemicals released into its gut. This is called extracellular digestion because it occurs inside the gut cavity and not inside a cell. Once the food has been broken down, the smaller bits can be absorbed by the cells that line the gut. Waste products are passed back out through the mouth. Because of this two-way traffic, the organism's cavity cannot be subdivided into specialized compartments.

More complex organisms have more complicated digestive systems. These are called complete digestive systems. Higher up the evolutionary ladder, the blind gut develops a separate opening for waste removal, called the anus. This is seen in earthworms, clams, crabs, spiders, and starfish, among others, who have the simplest form of a complete digestive system. Food enters the mouth, is broken down, and passes in one

direction through a straight digestive tube where it is absorbed. Waste passes out of the body through an anus at the organism's other end. Earthworm waste called casts are deposited on the soil's surface, adding needed nutrients.

WILLIAM BEAUMONT

American surgeon William Beaumont (1785–1853) conducted pioneering studies on how the human stomach works. His highly accurate and well-documented firsthand observations of the stomach proved that digestion is basically a chemical process.

If U.S. Army surgeon William Beaumont had not been in the right place at the right time, he would never have become a pioneer of science. Until the event occurred that changed his life, Beaumont had done nothing remarkable. Born in Lebanon, Connecticut, on a farm, he was able to study medicine at St. Albans in Vermont. By 1812, he became an assistant surgeon in the U.S. Army. Three years later, he left the army and tried private practice in Plattsburgh, New York. However, in 1820 he returned to the army and was eventually transferred to the frontier post of Fort Mackinac in Michigan. While there, a bizarre medical event occurred on June 6, 1822. That day, Beaumont was called upon to assist a patient who had been accidentally shot at close range with a shotgun. What Beaumont found was a young French-Canadian trapper named Alexis St. Martin who had been struck by the blast on his left side. The shotgun had taken a deep chunk out of his side and no one expected the young man to live through the night. Beaumont tended him with care and skill nonetheless, and to everyone's surprise he remained alive. Beaumont continued his care, changing his bandages every day for a year, and in time, St. Martin was fully recovered.

However, despite the fact that he had regained his full strength and seemed normal, St. Martin was not literally whole. His wound never fully closed, and he was left with an inch-wide opening through which Beaumont could insert his finger all the way into St. Martin's stomach. The trapper had been left with what is called a permanent traumatic fistula, meaning he had a hole in his side that led directly to his stomach. About a year later when St. Martin needed to take some medicine from Beaumont, the surgeon decided to try an experiment and administered the medicine directly into the stomach rather than orally as he would have done normally. Beaumont soon found that the medicine worked on St. Martin exactly as it would have if it had been administered the regular way. This led Beaumont to realize that he had a unique research opportunity at hand, and he soon began a series of experiments and observations on his subject. First Beaumont attached small chunks of food to a string and inserted them directly into the St. Martin's stomach. Beaumont would then withdraw the string and observe the results of digestion hour by hour. Later, by using a hand lens, he began actually looking into his patient's stomach to see how it behaved. He was also able to extract and analyze samples of gastric juice and stomach contents, establishing that digestion is indeed a chemical process. He could also observe the muscular movements of the stomach. Over the next few years, Beaumont conducted more than two hundred carefully detailed experiments and, in 1833, published his findings in Experiments and Observations on the Gastric Juice and the Physiology of Digestion. As the first, well-documented and accurate observation of the digestive processes of a living human being, Beaumont's book was a one-of-kind source on the process of digestion. It also suggested to some scientists the possibility of using artificial fistulas on their research animals as a way of learning more about their bodies.

The human side of this story should also be noted. By about 1835, St. Martin had had enough, not only of Beaumont's experiments, but also of the bullying Beaumont himself. Poor St. Martin refused to cooperate anymore and eventually just ran away from Beaumont for good and returned to his native Canada. In his own way, the wounded trapper who lived to the ripe old age of eighty-two, had made his own contributions toward understanding more about the human body.

HOW THE DIGESTIVE SYSTEM WORKS

As organisms became more complex and evolved into what are called the higher animals, their digestive systems developed an alimentary tract or alimentary canal with specialized structures and compartments. Among vertebrates, a basic digestive scheme came about that resulted in structures responsible for: receiving food, conducting and storing food, breaking food down and absorbing its useful nutrients, and absorbing water while eliminating wastes. The alimentary canal in vertebrates is able to move food from the mouth to the anus entirely on its own. It does this by contracting its circular muscles and pushing food further along. This happens in a rhythmic wave called peristalsis. The canal itself is constantly lubricated by mucus, making the food move easily when the circular muscles contract. Automatically, these important muscles are regularly contracting and relaxing, pushing food and waste along as if a fist were closing on a tube, squeezing its contents in a certain direction.

In all vertebrates, food encounters four major areas in the alimentary canal where it is received and ingested (swallowed), broken down, absorbed and eliminated. The mouth is responsible for receiving food, and it is usually designed with the animal's diet in mind. The mouth may have teeth which, helped by the tongue, mechanically break food down into smaller pieces. Some species, like snakes, smell with their tongue, while others, such as frogs, capture their food with it. Since mammals chew their food in their mouth, they also have salivary glands that secrete enzymes that moisten their food and begin the chemical breakdown process. The esophagus connects to the stomach and moves the food downward after it is swallowed. Unlike most vertebrates, however, birds have an enlarged esophagus called a crop that is used to store food before it is sent to the stomach. It also can produce a "milky" substance that a parent bird can regurgitate (spit back up) and feed its young.

In most vertebrates, the real grinding and digestion of food is carried out in the stomach. This is a muscular pouch that works to mix the food with a highly acidic combination of chemicals that break it down or dissolve it further. Powerful stomach muscles churn up the food while certain cells lining the stomach produce gastric juice (mainly hydrochloric acid) that turn the broken-down food into a milky substance known as chyme. The stomach itself is protected from the strong acid by a thick layer of mucus. As the stomach fills with chyme, it gradually releases small amounts into the duodenum, the top part of the small intestine. The stomach of some vertebrates, like birds, who swallow their food without chewing (like birds), are called gizzards and often contain pebbles that mechanically break up the food for them as their muscle walls contract. Some plant-eating mammals called ruminants, like cows and horses, have special stomach chambers that help break down their difficult-to-digest diet. They also must have certain microorganisms in their gut in order to break down their food since mammals do not produce the digestive enzymes needed to break down cellulose (tough plant walls).

Once the now-liquid food passes into the duodenum, it begins its final trip to becoming completely digested and prepared for absorption. At this point, the liver and the pancreas come into play as both have ducts leading into the duodenum. The pancreas secretes enzymes or pancreatic juice that is alkaline and counteracts the strong acid made by the stomach. It also breaks down food molecules in the duodenum. The liver produces a fluid called bile that is essential if the body is to digest fats. Bile emulsifies (breaks down) large fat globules so they can be absorbed. Without bile, which is stored in a sac called the gall bladder, most of the fat would pass through the digestive system undigested. From the duodenum, the digested food flows into the small intestine, called the ileum in humans. The small intestine has densely folded structures called villi that look like tiny tongues or fingers. These villi absorb the molecules of nutrients and then rapidly send them into the bloodstream. The parts of food that have not been digested or absorbed continue to pass down into the large intestine (called the colon in humans) in the chyme. The large intestine's job is to absorb nearly all its water, and what is left arrives at the rectum where it is stored as feces. When a sufficient quantity has accumulated, the feces are expelled through the anus. This is called elimination.

Although mammals all have the same basic digestive system, there are differences that reflect their diet. Carnivores, or meat-eaters who have special teeth and live by hunting, have fairly short digestive systems since meat is mostly protein and requires very little hard work to digest. Some carnivores swallow their prey whole and thus do not need to have salivary glands. Herbivores (plant eaters) have to consume huge amounts of vegetative matter that is also hard to break down. As a consequence, their digestive systems are much more complicated than that of a carnivore. For example, a cow has four stomachs and an extremely long and coiled intestine. Some animals, like rodents and rabbits, even reingest their waste pellets so that their food passes through their systems twice. Since their digestion is basically incomplete, they must take in their partially digested fiber and have it pass through their digestive system again to benefit from it fully.

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