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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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