The Digestive System
The Digestive System
The foods we eat—apples, pepperoni pizzas, leafy green salads—taste good to us, but cannot be used by the body as they are. The nutrition the cells of the body need to keeping growing and working must be in a simple form: amino acids, simple sugars, and fatty acids. It is the job of the digestive system to take the complex organic molecules of the foods we ingest—proteins, carbohydrates, and fats—and break them down into their simple building blocks. This process is called digestion. Once digestion has occurred, the simple molecules (nutrients) are absorbed from the digestion system by the cardiovascular and lymphatic systems and transported to cells throughout the body.
DESIGN: PARTS OF THE DIGESTIVE SYSTEM
The digestive system may be broken into two parts: a long, winding, muscular tube accompanied by accessory digestive organs and glands. That open-ended tube, known as the alimentary canal or digestive tract, is composed of various organs. These organs are, in order, the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The rectum and anus form the end of the large intestine. The accessory digestive organs and glands that help in the digestive process include the tongue, teeth, salivary glands, pancreas, liver, and gall bladder.
The walls of the alimentary canal from the esophagus through the large intestine are made up of four tissue layers. The innermost layer is the mucosa, coated with mucus. This protects the alimentary canal from chemicals and enzymes (proteins that speed up the rate of chemical reactions) that break down food and from germs and parasites that might be in that food. Around the mucosa is the submucosa, which contains blood vessels, nerves, and lymph vessels. Wrapped around the submucosa are two layers of muscles that help move food along the canal. The outermost layer, the serosa, is moist, fibrous tissue that protects the alimentary canal and helps it move against the surrounding organs in the body.
Food enters the body through the mouth, or oral cavity. The lips form and protect the opening of the mouth, the cheeks form its sides, the tongue forms its floor, and the hard and soft palates form its roof. The hard palate is at the front; the soft palate is in the rear. Attached to the soft palate is a fleshy, fingerlike projection called the uvula (from the Latin word meaning "little grape"). Two U-shaped rows of teeth line the mouth—one above and one below. Three pair of salivary glands open at various points into the mouth.
The Digestive System: Words to Know
- Alimentary canal (al-i-MEN-tah-ree ka-NAL):
- Also known as the digestive tract, the series of muscular structures through which food passes while being converted to nutrients and waste products; includes the oral cavity, pharynx, esophagus, stomach, large intestine, and small intestine.
- Amylase (am-i-LACE):
- Any of various digestive enzymes that convert starches to sugars.
- Appendix (ah-PEN-dix):
- Small, apparently useless organ extending from the cecum.
- Greenish yellow liquid produced by the liver that neutralizes acids and emulsifies fats in the duodenum.
- Bolus (BO-lus):
- Rounded mass of food prepared by the mouth for swallowing.
- Cecum (SEE-kum):
- Blind pouch at the beginning of the large intestine.
- Chyle (KILE):
- Thick, whitish liquid consisting of lymph and tiny fat globules absorbed from the small intestine during digestion.
- Chyme (KIME):
- Soupylike mixture of partially digested food and stomach secretions.
- Colon (KOH-lun):
- Largest region of the large intestine, divided into four sections: ascending, transverse, descending, and sigmoid (colon is sometimes used to describe the entire large intestine).
- Colostomy (kuh-LAS-tuh-mee):
- Surgical procedure where a portion of the large intestine is brought through the abdominal wall and attached to a bag to collect feces.
- Defecation (def-e-KAY-shun):
- Elimination of feces from the large intestine through the anus.
- Dentin (DEN-tin):
- Bonelike material underneath the enamel of teeth, forming the main part.
- Duodenum (doo-o-DEE-num or doo-AH-de-num):
- First section of the small intestine.
- Emulsify (e-MULL-si-fie):
- To break down large fat globules into smaller droplets that stay suspended in water.
- Enamel (e-NAM-el):
- Whitish, hard, glossy outer layer of teeth.
- Enzymes (EN-zimes):
- Proteins that speed up the rate of chemical reactions.
- Epiglottis (ep-i-GLAH-tis):
- Flaplike piece of tissue at the top of the larynx that covers its opening when swallowing is occurring.
- Esophagus (e-SOF-ah-gus):
- Muscular tube connecting the pharynx and stomach.
- Feces (FEE-seez):
- Solid body wastes formed in the large intestine.
- Flatus (FLAY-tus):
- Gas generated by bacteria in the large intestine.
- Gastric juice (GAS-trick JOOSE):
- Secretion of the gastric glands of the stomach, containing hydrochloric acid, pepsin, and mucus.
- Ileocecal valve (ill-ee-oh-SEE-kal VALV):
- Sphincter or ring of muscule that controls the flow of chyme from the ileum to the large intestine.
- Ileum (ILL-ee-um):
- Final section of the small intestine.
- Jejunum (je-JOO-num):
- Middle section of the small intestine.
- Lacteals (LAK-tee-als):
- Specialized lymph capillaries in the villi of the small intestine.
- Larynx (LAR-ingks):
- Organ between the pharynx and trachea that contains the vocal cords.
- Lipase (LIE-pace):
- Digestive enzyme that converts lipids (fats) into fatty acids.
- Lower esophageal sphincter (LOW-er i-sof-ah-GEE-alSFINGK-ter):
- Strong ring of muscle at the base of the esophagus that contracts to prevent stomach contents from moving back into the esophagus.
- Palate (PAL-uht):
- Roof of the mouth, divided into hard and soft portions, that separates the mouth from the nasal cavities.
- Papillae (pah-PILL-ee):
- Small projections on the upper surface of the tongue that contain taste buds.
- Peristalsis (per-i-STALL-sis):
- Series of wavelike muscular contractions that move material in one direction through a hollow organ.
- Pharynx (FAR-inks):
- Short, muscular tube extending from the mouth and nasal cavities to the trachea and esophagus.
- Plaque (PLACK):
- Sticky, whitish film on teeth formed by a protein in saliva and sugary substances in the mouth.
- Pyloric sphincter (pie-LOR-ick SFINGK-ter):
- Strong ring of muscle at the junction of the stomach and the small intestine that regulates the flow of material between them.
- Rugae (ROO-jee):
- Folds of the inner mucous membrane of organs, such as the stomach, that allow those organs to expand.
- Trypsin (TRIP-sin):
- Digestive enzyme that converts proteins into amino acids; inactive form is trypsinogen.
- Uvula (U-vue-lah):
- Fleshy projection hanging from the soft palate that raises to close off the nasal passages during swallowing.
- Vestigial organ (ves-TIJ-ee-al OR-gan):
- Organ that is reduced in size and function when compared with that of evolutionary ancestors.
- Villi (VILL-eye):
- Tiny, fingerlike projections on the inner lining of the small intestine that increase the rate of nutrient absorption by greatly increasing the intestine's surface area.
THE TONGUE. The muscular tongue is attached to the base of the mouth by a fold of mucous membrane. On the upper surface of the tongue are small projections called papillae, many of which contain taste buds (for a discussion of taste, see chapter 12). Most of the tongue lies within the mouth, but its base extends into the pharynx. Located at the base of the tongue are the lingual tonsils, small masses of lymphatic tissue that serve to prevent infection.
TEETH. Humans have two sets of teeth: deciduous and permanent. The deciduous teeth (also known as baby or milk teeth) start to erupt through the gums in the mouth when a child is about six months old. By the age of two, the full set of twenty teeth has developed. Between the ages of six and twelve, the roots of these teeth are reabsorbed into the body and the teeth begin to fall out. They are quickly replaced by the thirty-two permanent adult teeth. (The third molars, the wisdom teeth, may not erupt because of inadequate space in the jaw. In such cases, they become impacted or embedded in the jawbone and must be removed surgically.)
Teeth are classified according to shape and function. Incisors, the chisel-shaped front teeth, are used for cutting. Cuspids or canines, the pointed teeth next to the incisors, are used for tearing or piercing. Bicuspids (or premolars) and molars, the back teeth with flattened tops and rounded, raised tips, are used for grinding.
Each tooth consists of two major portions: the crown and the root. The crown is the exposed part of the tooth above the gum line; the root is enclosed in a socket in the jaw. The outermost layer of the crown is the whitish enamel. Made mainly of calcium, enamel is the hardest substance in the body.
Underneath the enamel is a yellowish, bonelike material called dentin. It forms the bulk of the tooth. Within the dentin is the pulp cavity, which receives blood vessels and nerves through a narrow root canal at the base of the tooth.
THE SALIVARY GLANDS. Three pair of salivary glands produce saliva on a continuous basis to keep the mouth and throat moist. The largest pair, the parotid glands, are located just below and in front of the ears. The next largest pair, the submaxillary or submandibular glands, are located in the lower jaw. The smallest pair, the sublingual glands, are located under the tongue.
PAVLOV AND HIS SALIVATING DOG
Ivan Petrovich Pavlov (1849–1936) was a Russian physiologist (a person who studies the physical and chemical processes of living organisms) who conducted pioneering research into the digestive activities of mammals. His now-famous experiments with a dog ("Pavlov's dog") to show how the central nervous system affects digestion earned him the Nobel Prize for Medicine or Physiology in 1904.
Interested in the actions of digestion and gland secretion, Pavlov set up an ingenious experiment. In a laboratory, he severed a dog's throat (Pavlov was a skillful surgeon and the animal was unharmed). When the dog ate food, the food dropped out of the animal's throat before reaching its stomach. Through this simulated feeding, Pavlov discovered that the sight, smell, and swallowing of food was enough to cause the secretion of gastric juice. He demonstrated that the stimulation of the vagus nerve (one of the major nerves of the brain) influences the actions of the gastric glands.
In another famous study, Pavlov set out to determine whether he could turn unconditioned (naturally occurring) reflexes or responses of the central nervous system into conditioned (learned) reflexes. He had noticed that laboratory dogs would sometimes salivate merely at the approach of lab assistants who fed them. Pavlov then decided to ring a bell each time a dog was given food. After a while, he rang the bell without feeding the dog. He discovered that the dog salivated at the sound of the bell, even though food was not present. Through this experiment, Pavlov demonstrated that unconditioned reflexes (salivation and gastric activity) could become conditioned reflexes that were triggered by a stimulus (the bell) that previously had no connection with the event (eating).
Ducts or tiny tubes carry saliva from these glands into the mouth. Ducts from the parotid glands open into the upper portion of the mouth; ducts from the submaxillary and sublingual glands open into the mouth beneath the tongue.
The salivary glands are controlled by the autonomic nervous system, a division of the nervous system that functions involuntarily (meaning the processes it controls occur without conscious effort on the part of an individual). The glands produce between 1.1 and 1.6 quarts (1 and 1.5 liters) of saliva each day. Although the flow is continuous, the amount varies. Food (or anything else) in the mouth increases the amount produced. Even the sight or smell of food will increase the flow.
Saliva is mostly water (about 99 percent), with waste products, antibodies, and enzymes making up the small remaining portion. At mealtimes, saliva contains large quantities of digestive enzymes that help break down food. Saliva also controls the temperature of food (cooling it down or warming it up), cleans surfaces in the mouth, and kills certain bacteria present in the mouth.
The pharynx, or throat, is a short, muscular tube extending about 5 inches (12.7 centimeters) from the mouth and nasal cavities to the esophagus and trachea (windpipe). It serves two separate systems: the digestive system (by allowing the passage of solid food and liquids) and the respiratory system (by allowing the passage of air).
The esophagus, sometimes referred to as the gullet, is the muscular tube connecting the pharynx and stomach. It is approximately 10 inches (25 centimeters) in length and 1 inch (2.5 centimeters) in diameter. In the thorax (area of the body between the neck and the abdomen), the esophagus lies behind the trachea. At the base of the esophagus, where it connects with the stomach, is a strong ring of muscle called the lower esophageal sphincter. Normally, this circular muscle is contracted, preventing contents in the stomach from moving back into the esophagus.
The stomach is located on the left side of the abdominal cavity just under the diaphragm (a membrane of muscle separating the chest cavity from the abdominal cavity). When empty, the stomach is shaped like the letter J and its inner walls are drawn up into long, soft folds called rugae. When the stomach expands, the rugae flatten out and disappear. This allows the average adult stomach to hold as much as 1.6 quarts (1.5 liters) of material.
The dome-shaped portion of the stomach to the left of the lower esophageal sphincter is the fundus. The large central portion of the stomach is the body. The part of the stomach connected to the small intestine (the curve of the J) is the pylorus. The pyloric sphincter is a muscular ring that regulates the flow of material from the stomach into the small intestine by variously opening and contracting. That material, a soupylike mixture of partially digested food and stomach secretions, is called chyme.
The stomach wall contains three layers of smooth muscle. These layers contract in a regular rhythm—usually three contractions per minute—to mix and churn stomach contents. Mucous membrane lines the stomach. Mucus, the thick, gooey liquid produced by the cells of that membrane, helps protect the stomach from its own secretions. Those secretions—acids and enzymes—enter the stomach through millions of shallow pits that open onto the surface of the inner stomach. Called gastric pits, these openings lead to gastric glands, which secrete about 1.6 quarts (1.5 liters) of gastric juice each day.
Gastric juice contains hydrochloric acid and pepsin. Pepsin is an enzyme that breaks down proteins; hydrochloric acid kills microorganisms and breaks down cell walls and connective tissue in food. The acid is strong enough to burn a hole in carpet, yet the mucus produced by the mucous membrane prevents it from dissolving the lining of the stomach. Even so, the cells of the mucous membrane wear out quickly: the entire stomach lining is replaced every three days. Mucus also aids in digestion by keeping food moist.
A VIEW OF THE STOMACH
William Beaumont (1785–1853) was an American surgeon who served as an army surgeon during the War of 1812 (1812–15) and at various posts after the war. It was at one of these posts that he saw what perhaps no one before him had seen: the inner workings of the stomach.
In 1882, while serving at Fort Mackinac in northern Michigan, Beaumont was presented with a patient named Alexis St. Martin. The French Canadian trapper, only nineteen at the time, has been accidently shot in the stomach. The bullet had torn a deep chunk out of the left side of St. Martin's lower chest. At first, no one thought he would survive, but amazingly he did. However, his wound never completely healed, leaving a 1 inch-wide (2.5 centimeter-wide) opening. This opening allowed Beaumont to put his finger all the way into St. Martin's stomach.
Beaumont decided to take advantage of opening into St. Martin's side to study human digestion. He started by taking small chunks of food, tying them to a string, then inserting them directly into the young man's stomach. At irregular intervals, he pulled the food out to observe the varying actions of digestion. Later, using a hand-held lens, Beaumont peered into St. Martin's stomach. He observed how the human stomach behaved at various stages of digestion and under differing circumstances.
Beaumont conducted almost 240 experiments on St. Martin. In 1833, he published his findings in Experiments and Observations on the Gastric Juice and the Physiology of Digestion, a book that provided invaluable information on the digestive process.
The small intestine
The small intestine is the body's major digestive organ. Looped and coiled within the abdominal cavity, it extends about 20 feet (6 meters) from the stomach to the large intestine. At its junction with the stomach, it measures about 1.5 inches (4 centimeters) in diameter. By the time it meets the large intestine, its diameter has been reduced to 1 inch (2.5 centimeters). Although much longer than the large intestine, the small intestine is called "small" because its overall diameter is smaller.
The small intestine is divided into three regions or sections. The first section, the duodenum, is the initial 10 inches (25 centimeters) closest to the stomach. Chyme from the stomach and secretions from the pancreas and liver empty into this section. The middle section, the jejunum, measures about 8.2 feet (2.5 meters) in length. Digestion and the absorption of nutrients occurs mainly in the jejunum. The final section, the ileum, is also the longest, measuring about 11 feet (3.4 meters) in length. The ileum ends at the ileocecal valve, a sphincter that controls the flow of chyme from the ileum to the large intestine.
The inner lining of the small intestine is covered with tiny, fingerlike projections called villi (giving it an appearance much like the nap of a plush, soft towel). The villi greatly increase the intestinal surface area available for absorbing digested material. Within each villus (singular for villi) are blood capillaries and a lymph capillary called a lacteal. Digested food molecules are absorbed through the walls of the villus into both the capillaries and the lacteal. At the bases of the villi are openings of intestinal glands, which secrete a watery intestinal juice. This juice contains digestive enzymes that convert food materials into simple nutrients the body can readily use. On average, about 2 quarts (1.8 liters) of intestinal juice are secreted into the small intestine each day.
As with the lining of the stomach, a coating of mucus helps protect the lining of the small intestine. Yet again, the digestive enzymes prove too strong for the delicate cells of that lining. They wear out and are replaced about every two days.
The large intestine
Extending from the end of the small intestine to the anus, the large intestine measures about 5 feet (1.5 meters) in length and 3 inches (7.5 centimeters) in diameter. It almost completely frames the small intestine. The large intestine is divided into three major regions: the cecum, colon, and rectum.
Cecum comes from the Latin word caecum, meaning "blind." Shaped like a rounded pouch, the cecum lies immediately below the area where the ileum empties into the large intestine. Attached to the cecum is the slender, fingerlike appendix, which measures about 3.5 inches (9 centimeters) in the average adult. Composed of lymphatic tissue, the appendix seems to have no function in present-day humans. For that reason, scientists refer to it as a vestigial organ (an organ that is reduced in size and function when compared with that of evolutionary ancestors).
Sometimes used to describe the entire large intestine, the colon is actually the organ's main part. It is divided into four sections: ascending, transverse, descending, and sigmoid. The ascending colon travels from the cecum up the right side of the abdominal cavity until it reaches the liver. It then makes a turn, becoming the transverse colon, which travels horizontally across the abdominal cavity. Near the spleen on the left side, it turns down to form the descending colon. At about where it enters the pelvis, it becomes the S-shaped sigmoid colon.
After curving and recurving, the sigmoid colon empties into the rectum, a fairly straight, 6-inch (15-centimeter) tube ending at the anus, the opening to the outside. Two sphincters (rings of muscle) control the opening and closing of the anus.
Roughly 1.6 quarts (1.5 liters) of watery material enters the large intestine each day. No digestion takes place in the large intestine, only the reabsorption or recovery of water. Mucus produced by the cells in the lining of the large intestine help move the waste material along. As more and more water is removed from that material, it becomes compacted into soft masses called feces. Feces are composed of water, cellulose and other indigestible material, and dead and living bacteria. The remnants of worn red blood cells gives feces their brown color. Only about 3 to 7 ounces (85 to 200 grams) of solid fecal material remains after the large intestine has recovered most of the water. That material is then eliminated through the anus, a process called defecation.
The pancreas is a soft, pink, triangular-shaped gland that measures about 6 inches (15 centimeters) in length. It lies behind the stomach, extending from the curve of the duodenum to the spleen. While a part of the digestive system, the pancreas is also a part of the endocrine system, producing the hormones insulin and glucagon (for a further discussion of this process, see chapter 3).
Primarily a digestive organ, the pancreas produces pancreatic juice that helps break down all three types of complex food molecules in the small intestine. The enzymes contained in that juice include pancreatic amylase, pancreatic lipase, and trypsinogen. Amylase breaks down starches into simple sugars, such as maltose (malt sugar). Lipase breaks down fats into simpler fatty acids and glycerol (an alcohol). Trypsinogen is the inactive form of the enzyme trypsin, which breaks down proteins into amino acids. Trypsin is so powerful that if produced in the pancreas, it would digest the organ itself. To prevent this, the pancreas produces trypsinogen, which is then changed in the duodenum to its active form.
Pancreatic juice is collected from all parts of the pancreas through microscopic ducts. These ducts merge to form larger ducts, which eventually combine to form the main pancreatic duct. This duct, which runs the length of the pancreas, then transports pancreatic juice to the duodenum of the small intestine.
The largest glandular organ in the body, the liver weighs between 3 and 4 pounds (1.4 and 1.8 kilograms). It lies on the right side of the abdominal cavity just beneath the diaphragm. In this position, it overlies and almost completely covers the stomach. Deep reddish brown in color, the liver is divided into four unequal lobes: two large right and left lobes and two smaller lobes visible only from the back.
The liver is an extremely important organ. Scientists have discovered that it performs over 200 different functions in the body. Among its many functions are processing nutrients, making plasma proteins and blood-clotting chemicals, detoxifying (transforming into less harmful substances) alcohol and drugs, storing vitamins and iron, and producing cholesterol.
One of the liver's main digestive functions is the production of bile. A watery, greenish yellow liquid, bile consists mostly of water, bile salts, cholesterol, and assorted lipids or fats. Liver cells produce roughly 1 quart (1 liter) of bile each day. Bile leaves the liver through the common hepatic duct. This duct unites with the cystic duct from the gall bladder to form the common bile duct, which delivers bile to the duodenum.
In the small intestine, bile salts emulsify fats, breaking them down from large globules into smaller droplets that stay suspended in the watery fluid in the small intestine. Bile salts are not enzymes and, therefore, do not digest fats. By breaking down the fats in to smaller units, bile salts aid the fatdigesting enzymes present in the small intestine.
The gall bladder
The gall bladder is a small, pouchlike, green organ located on the undersurface of the right lobe of the liver. It measures 3 to 4 inches (7.6 to 10 centimeters) in length. The gall bladder's function is to store bile, of which it can hold about 1.2 to 1.7 ounces (35 to 50 milliliters).
The liver continuously produces bile. When digestion is not occurring, bile backs up the cystic duct and enters the gall bladder. While holding the bile, the gall bladder removes water from it, making it more concentrated. When fatty food enters the duodenum once again, the gall bladder is stimulated to contract and spurt out the stored bile.
WORKINGS: HOW THE DIGESTIVE SYSTEM FUNCTIONS
The digestive system breaks down food into a useful form through mechanical and chemical means. Mechanical digestion is the physical breaking up food into small pieces, such as by chewing. The smaller pieces are then acted upon by digestive enzymes, which change complex chemical molecules into much simpler molecules the body can easily utilize. This process involving enzymes is called chemical digestion.
Digestive activities in the mouth
Food taken into the mouth is broken down by both mechanical and chemical means. Through the process of mastication or chewing, teeth physically break down the tough tissues of meats and fibers of plants into smaller particles. The tongue helps move the food around the mouth, allowing the different sets of teeth variously to cut, tear, or grind the food. The jaw muscles, perhaps some of the strongest muscles in the body, help the teeth break down food in seconds.
Stimulated by the presence of something in the mouth, salivary glands secrete an increased amount of saliva (the sensations of sight, taste, and smell also increase saliva flow). As saliva is mixed with the food, salivary amylase (an enzyme in saliva) begins the chemical digestion of carbohydrates or starches, changing them into the simple sugar maltose.
As the food is broken down into pieces by the teeth and mixed with saliva, the tongue rolls these pieces into a battered, moistened, soft mass or ball called a bolus. Only after food has been compacted into a bolus of proper texture and consistency can swallowing occur.
Swallowing is both a voluntary and involuntary action. Once food has been properly chewed and mixed with saliva to create a bolus, the tongue forces the bolus toward the back of the mouth and into the pharynx. This a voluntary action; the individual has total control over moving the bolus while it is in the mouth. When the bolus presses against the soft palate, the soft palate and the uvula rise to close off the nasal passages to prevent the bolus from entering them.
Once the bolus enters the pharynx, swallowing becomes an automatic reflex action and cannot be stopped. The larynx, the upper part of the trachea that contains the vocal cords, rises. As it does so, a flaplike piece of tissue at the top of the larynx, the epiglottis, folds down to cover its opening. This prevents the bolus from passing into the trachea.
Sometimes, when a person laughs or talks while eating or drinking, the uvula and the epiglottis may not cover their openings quickly enough. If the uvula does not rise in time, bits of food or liquid may squirt up into the nose. If the epiglottis does not fold down, bits of food or liquid may enter the trachea, causing the person to cough (a protective reflex) until the food or liquid is expelled from the trachea.
Once material reaches the esophagus, the circular muscles in the walls of the esophagus begin alternately to contract and relax in a wavelike manner, pushing the bolus farther and farther down. This series of wavelike muscular motions is known as peristalsis (or peristaltic contractions). Material is pushed down the esophagus regardless what position a person is in: standing up, sitting, lying down, or upside down. Gravity helps move the bolus along, but peristalsis occurs even in the zero gravity of space.
A typical moist bolus takes about 9 seconds to travel through the esophagus. Drier boluses take longer. Liquids often pass through this muscular tube in just seconds, faster than the accompanying peristaltic waves. When the bolus or liquid reaches the lower esophageal sphincter, it presses against the sphincter, causing it to open. The material then passes into the stomach.
Digestive activities in the stomach
Gastric juices begin to flood the stomach even before food arrives. The sight, smell, taste, or even thought of food triggers the central nervous system to send nerve impulses to the gastric glands, which respond by secreting gastric juice. Once food does arrive in the stomach and touches its lining, cells in the lining release gastrin, a hormone. Gastrin, in turn, stimulates the production of even greater amounts of gastric juice.
As food fills the stomach, its wall begin to stretch. This initiates mechanical digestion in the stomach. The muscles in the walls being to contract, compressing and pummeling the food, breaking it apart physically. At the same time, the food is being mixed with gastric juices, and chemical digestion begins. Pepsin, the protein-digesting enzyme in gastric juice, starts to break down complex proteins. Little digestion of carbohydrates or fats takes place in the stomach. Water, alcohol, and drugs such as aspirin, however, are absorbed through the walls of the stomach into the bloodstream.
Once the food has been well mixed and broken down into chyme, peristalsis begins in the lower portion of the stomach. Chyme is moved downward into the pylorus. With each contraction of the stomach walls, the pyloric sphincter opens just a little, allowing a bit of chyme to squirt into the duodenum of the small intestine. When the duodenum is filled and its wall stretched, a nerve impulse is sent to the stomach to slow down its activity. It takes about four hours for the stomach to empty completely after receiving a well-balanced meal. If that meal contains much fat, then the process could take six or more hours.
Digestive activity in the small intestine
When chyme from the stomach enters the small intestine, it contains proteins and carbohydrates that have been only partially digested. Fats have been hardly digested. Over a three-to six-hour period, as chyme moves through the twists and coils of the small intestine, chemical digestion increases. By the time chyme reaches the end of the small intestine, eighty percent of all digestion in the body has taken place.
The presence of chyme in the duodenum stimulates the secretion of intestinal juice. Cells in the lining of the duodenum are also stimulated to produce hormones that, in turn, stimulate the pancreas to produce pancreatic juice and the liver to produce bile (the gall bladder is also stimulated to release its store of concentrated bile). Both enter the duodenum and combine with intestinal juice to digest or break down proteins, carbohydrates, and fats.
Peristalsis occurs in the small intestine, mixing the chyme with the intestinal juices and moving it through the organ. Although water and nutrients are absorbed all along the length of the small intestine, most absorption occurs in the jejunum. In this section, digested carbohydrates, fats, proteins, and most of the vitamins, minerals, and iron are absorbed. These nutrients pass through the walls of the villi into the blood capillaries and lacteals (lymph capillaries). The blood capillaries eventually drain into veins that connect with the portal vein, which transports the nutrient-rich blood to the liver. The lacteals, carrying fat nutrients, eventually drain into larger lymph vessels that connect with the venous system.
By the time material enters the ileum, the last section of the small intestine, all that remains is some water, indigestible food matter (such as plant fibers), and bacteria. This material then enters the large intestine through the ileocecal valve, which closes to prevent material from flowing backward.
Digestive activities in the large intestine
The large intestine does not produce any digestive enzymes; therefore, no digestion takes place. It functions mainly to absorb water and a few minerals from the waste products of digestion. Peristalsis in the large intestine occurs very slowly: material takes between twelve and twenty-four hours to pass through.
Millions of bacteria living in the large intestine feed on the waste products. In doing so, they produce vitamin K and some B vitamins that are absorbed through the wall of the large intestine into the bloodstream and then transported to the liver. The bacteria also produce intestinal gas—methane and hydrogen sulfide—that gives feces their characteristic odor. The amount of that gas, properly known as flatus, may increase if certain foods rich in carbohydrates (such as beans) are eaten.
DIGESTIVE SYSTEM DISORDERS
Anorexia nervosa (an-ah-REK-seeah ner-VO-sa): Eating disorder usually occurring in young women that is characterized by an abnormal fear of becoming obese, a persistent aversion to food, and severe weight loss.
Appendicitis (ah-pen-di-SIGH-tis): Inflammation of the appendix.
Biliary atresia (BILL-ee-a-ree ah-TREE-zee-ah): Condition in which ducts to transport bile from the liver to the duodenum fail to develop in a fetus.
Bulimia (boo-LEE-me-ah): Eating disorder characterized by eating binges followed by selfinduced vomiting or laxative abuse.
Cirrhosis (si-ROW-sis): Chronic disease of the liver in which normal liver cells are damaged and then replaced by scar tissue.
Crohn's disease (CRONES di-ZEEZ): Disorder that causes inflammation and ulceration of all the layers of the intestinal wall, particularly in the small intestine.
Diverticulosis (di-ver-ti-cue-LOW-sis): Condition in which the inner lining of the large intestine bulges out through its muscular wall; if the bulges become infected, the condition is called diverticulitis.
Gallstones (GAUL-stones): Solid crystal deposits that form in the gall bladder.
Hepatitis (hep-a-TIE-tis): Inflammation of the liver, caused mainly by a virus.
Lactose intolerance (LAK-tose in-TOL-er-ance): Inability of the body to digest significant amounts of lactose, the predominant sugar in milk.
Ulcer (digestive) (UL-sir): Any sore that develops in the lining of the lower esophagus, stomach, or duodenum.
Ulcerative colitis (UL-sir-a-tiv ko-LIE-tis): Disorder that causes inflammation and ulceration of the inner lining of the large intestine and rectum.
When powerful peristaltic contractions force the feces or compacted waste products from the sigmoid colon into the rectum, the wall of the rectum stretches. This triggers the defecation reflex. Signals from the spinal cord cause the walls of the sigmoid colon and rectum to contract and the anal sphincters to relax. Feces are then eliminated through the anus. The outer sphincter muscle can be controlled voluntarily, allowing an individual to delay defecation when necessary.
AILMENTS: WHAT CAN GO WRONG WITH THE DIGESTIVE SYSTEM
Many ailments or maladies can afflict the digestive system. Although it is remarkably resistant to abuse, it is still vulnerable and can break down. Some ailments are relatively minor, such as canker sores in the mouth. Others are severe and life-threatening, such as cancers that can target almost every part of the digestive system.
As an individual ages, the activity of the digestive system slows down. Fewer digestive juices are produced and secreted. Peristalsis slows. The sensations of taste and smell wane, and eating becomes less appealing. When less food is ingested, the body receives fewer nutrients. All body systems then weaken and become susceptible to disease.
The following are just a few of the legion of ailments that can beset the many parts of the digestive system.
Appendicitis is an inflammation of the appendix. It is the most common abdominal emergency found in children and young adults. Because of the appendix's position at the bottom of the cecum, scientists believe one of the main causes of appendicitis is an invasion of bacteria. When infected with bacteria, the appendix may become swollen and filled with pus. It may then eventually rupture. Symptoms of the condition include pain that begins above or around the navel. The pain, which may be severe or only achy, then moves into the right corner of the abdomen. In this position, the pain often becomes more steady and severe. If left untreated, appendicitis is fatal. The treatment for the condition is an immediate appendectomy or surgical removal of the inflamed and ruptured appendix.
Biliary atresia is a condition in which ducts to transport bile from the liver to the duodenum fail to develop in a fetus. The condition is the most common fatal liver disease in children. Half of all liver transplants are done for this reason. Scientists have yet to find a convincing cause for this birth defect.
In a child with this condition, bile begins to back up into the liver and eventually into the rest of the body. The child becomes jaundiced (skin turns yellow). The abdomen then begins to swell and the child becomes progressively more ill. If left untreated, liver failure and death will occur within two years.
Surgery is the only treatment for biliary atresia. The surgeon must find a way to create an adequate duct or pathway for the bile to drain from the liver into the intestine. Even if the surgery is successful, persistent disease in the liver will gradually destroy the organ. A liver transplant currently offers the best hope for this condition.
A dental cavity or tooth decay is the destruction of the enamel or outer surface of a tooth. It is a common health problem, second only to the common cold. It results from the action of bacteria that live in plaque. Plaque is a sticky, whitish film that forms on teeth, composed of a protein in saliva, sugars from foods, and bacteria. The bacteria use the sugars and starches from food particles in the mouth to produce acid that dissolves tooth enamel, creating cavities or holes. If the decay reaches the dentin, the tooth becomes sensitive to temperature and touch. If the decay reaches the pulp cavity, inflammation and pain (toothache) develop.
If left untreated, the decay can eventually destroy the entire tooth. Usually, a dentist is able to treat most cases of tooth decay by removing all decayed parts of the tooth and then filling the cavity with a hard material. If the decay has attacked the pulp, the dentist may perform a root canal treatment, removing the pulp and filling the inside chamber. If a majority of a tooth has to be removed, the dentist covers the tooth with a crown.
Cirrhosis is a chronic (long-term) disease in which cells of the liver are damaged and then replaced by scar tissue. The disease obviously affects the liver's ability to perform its many functions. The condition worsens over time and may lead to death. Twice as common in men as in women, cirrhosis is the seventh leading cause of disease-related death in the United States.
Long-term alcoholism is the primary cause of cirrhosis in the United States. Throughout the digestive system, alcohol interferes with the absorption of nutrients. Alcohol provides calories but no nourishment to the body. It also robs the body of vitamins and minerals necessary to maintain proper cell function. Because alcohol is detoxified within the liver, a constant level of alcohol in the organ severely affects it. Cirrhosis may also be brought about by viral infections like hepatitis B and hepatitis C (see page 56).
During the early stages of cirrhosis, the liver enlarges. The palms of the hands then turn red. Other symptoms include constipation or diarrhea, dull abdominal pain, fatigue, loss of appetite, nausea, vomiting, weakness, and weight loss. If left untreated, the symptoms increase and worsen, leading to liver failure and death.
The primary treatment for cirrhosis is to reduce the condition causing it. A person suffering from cirrhosis must not consume alcohol. A balanced diet, which helps regenerate healthy liver cells, must be followed. In patients with advanced cirrhosis, a liver transplant may be necessary.
Diverticulosis and diverticulitis
Diverticulosis is a condition in which the inner layer of the large intestine bulges out through the outer, muscular layer. These bulges are called diverticula. When they become infected and inflamed, the resulting condition is known as diverticulitis.
Diverticula occur most frequently in individuals whose diets are low in fiber. Since the amount of fecal matter produced is low, the large intestine must narrow itself and contract forcefully to move the smaller feces along to the rectum. Over time, this weakens the muscular wall, allowing diverticula to develop.
Diverticulitis occurs when a hardened piece of stool, undigested food or bacteria becomes lodged in the diverticula. Blood supply to the area is disrupted and infection sets in. Symptoms of diverticulitis include pain in the lower left side of the abdomen and fever. Abscesses (walled-off pockets of infection) may develop within the wall of the intestine.
Diverticulitis is quite treatable. Usually, the intestine is "rested" by preventing the individual from eating or drinking anything by mouth. Medications to fight the infection are also given. Once the condition is brought under control, the individual must adhere to a high-fiber diet.
Anorexia nervosa is an eating disorder that usually occurs in young women. Anorexia comes from the Greek word anorektos, meaning "without appetite." The problem for anorectics (individuals with anorexia) is not that they lack an appetite. The problem is self-image. Anorectics do not eat because they fear gaining weight. They see themselves as "fat" even if they are severely underweight.
Some anorectics refuse to eat at all; others eat only small portions of fruit or vegetables. A few live only on diet drinks. In addition to fasting, anorectics may exercise strenuously to keep their weight low.
The body is severely affected. Skin becomes dry and flaky. Muscles begin to waste away. Bones stop growing and become brittle. The heart weakens. Because the body has almost no fat to keep it warm, downy hair grows on the face, back, and arms. The heart weakens. Muscle cramps, dizziness, tiredness, and even brain damage and kidney and heart failure may occur. An estimated 10 to 20 percent of anorectics die, either from starvation or by committing suicide.
Medical researchers believe anorexia is caused by a combination of biological, psychological, and social factors. Low self-esteem, fear of losing control, and fear of growing up are common characteristics of anorectics. The emphasis on thinness in American culture is believed to contribute to the disorder.
Hospitalization, combined with psychotherapy and family counseling, is often needed to control anorexia. About 70 percent of anorectics who are treated for about six months return to normal body weight.
If the condition is severe and surgery is needed to remove a portion of the intestine filled with abscesses, a colostomy is performed. This involves pulling the end of remaining intestine through the abdominal wall and attaching it to a bag on the outside. Because the intestine no longer connects with the rectum, the individual's feces pass out of the intestine into the bag. The colostomy may be temporary (until healing has occurred) or it may be permanent.
Gallstones are solid crystal deposits that form in the gall bladder. They can vary in size from as small as a grain of sand to as large as a golf ball. Eighty percent of all gallstones are composed of cholesterol, a fatlike substance produced by the liver. Gallstones form when the liver produces more cholesterol than intestinal juices can liquefy.
Gallstones are the fifth most common reason adults are hospitalized in the United States. They usually develop in adults between the ages of twenty and fifty. The condition of developing gallstones tends to run in families. In addition, high levels of estrogen (female hormones), insulin (hormone that regulates sugar levels), and cholesterol in the body increase the risk of developing gallstones. A diet high in fat and low in fiber, heavy drinking, and smoking may also play a part.
Gallstones may block the common bile duct, preventing bile from flowing into the duodenum. A gallstone in the cystic duct may cause the gall bladder to become inflamed. Symptoms of a gallbladder attack include pain that begins in the abdomen and moves to the chest and back, chills and sweating, nausea and vomiting, and gas and belching.
Bulimia is an eating disorder that occurs chiefly in women in their teens and twenties. Bulimia comes from the Greek word boulimos, meaning "great hunger." Individuals who are bulimic go on eating binges (often gorging on junk food), then purge their bodies of the food by making themselves vomit or by taking large amounts of laxatives (medicines or foods that stimulate bowel movements).
During an eating binge, bulimics favor high-carbohydrate foods: candy, donuts, cookies, cakes, cereal, bread, soft drinks, and ice cream. At one sitting, bulimics consume more calories than they normally would in an entire day. They usually eat quickly and messily during a binge, stuffing the food in their mouths and gulping it down.
The self-induced vomiting after a binge can cause damage to the stomach and esophagus. Acid in the vomit from the stomach can irritate the throat and erode tooth enamel. Blood vessels in the eyes can burst. The overuse of laxatives can cause muscle cramps, stomach pains, dehydration, and even poisoning. Over time, bulimia causes vitamin deficiencies and an imbalance of critical body fluids. Seizures and kidney failure can ultimately result.
Bulimics know that their eating habits are abnormal. They often suffer from depression, especially after a binge. Bulimics may also suffer from anxiety and low self-esteem. Some medical researchers believe bulimia is related to an imbalance in the brain chemical serotonin, which influences mood. Most research on bulimia, however, focuses on psychological factors. Treatment for bulimia generally involves psychotherapy and, sometimes, the use of antidepressant medications.
Gallstones of a small size may pass out of the body through the urine. So they may more easily pass out, doctors may use high-frequency sound waves to break up the gallstones. To treat painful, severe cases, doctors may surgically remove the gall bladder and gallstones.
Heartburn is a burning sensation in the chest that can extend to the neck, throat, and face. It is caused by a backflow of the stomach's acids through the lower esophageal sphincter into the esophagus, leading to inflammation. More than one third of the population suffers from this disorder, commonly known as acid reflux.
Normally, the lower esophageal sphincter is tightly closed and opens only to allow food to pass from the esophagus into the stomach. However, many different factors may cause the sphincter to open inappropriately or fail to close completely. Fatty or greasy foods, cigarettes, alcohol, chocolate, caffeine, and certain medications can relax the sphincter, increasing reflux. A large meal, obesity, and pregnancy increase pressures with the abdomen, pushing the contents of the stomach into the esophagus.
Heartburn can be prevented by avoiding those foods, drugs, or conditions that cause the disorder. Mild cases of heartburn can be treated with over-the-counter antacids, which decrease the acidity in the stomach. Surgery to correct a defective or damaged sphincter may be necessary in severe cases.
Hepatitis is an often fatal disease that causes inflammation of the liver. There are various types of hepatitis, most of which are caused by a virus. The viral forms include hepatitis A, B, C, D, E, and G. The assorted symptoms marking hepatitis include jaundice (yellowing of the skin), nausea, vomiting, fever, weakness, loss of appetite, abdominal and joint pain, and cirrhosis (scarring of the liver).
Alcoholic hepatitis is a noninfectious type of hepatitis. Alcohol, a poison if taken in more than modest amounts, is detoxified in the liver. Too much alcohol causes the liver to become inflamed. The liver cannot function properly and eventually turns to useless fat. If the poisoning continues, cirrhosis develops.
WHAT IS LACTOSE INTOLERANCE?
Lactose is the primary carbohydrate or sugar in milk. Normally, the enzyme lactase (produced by the cells in the lining of the small intestine) breaks down lactose into simple sugars that can be absorbed into the bloodstream. In individuals who are lactose intolerant, however, the cells have stopped producing lactase. This usually occurs to an individual during adolescence.
When lactose intolerant individuals drink milk or eat small amounts of other dairy products, they can suffer discomfort from nausea, gas, cramps, bloating, and diarrhea. Undigested lactose brings about these symptoms because it provides a source of energy for the bacteria that inhabit the large intestine. The symptoms are a result of the bacteria's activities.
Roughly 30 to 50 million Americans are lactose intolerant. Certain ethnic and racial groups are more widely affected than others. As many as 75 percent of all African Americans and Native Americans and 90 percent of Asian Americans are lactose intolerant.
Two viral forms of hepatitis are most common: A and B. Hepatitis A (commonly known as infectious hepatitis) is spread through direct contact with contaminated feces, food, or water. Once infected, an individual usually recovers within two months. Hepatitis B (commonly called serum hepatitis) is much more severe. It is transmitted by sexual activity, blood transfusions, and the use of shared syringes by drug uses. Hepatitis B may destroy the liver through cirrhosis or it may lead to cancer of the liver.
Hepatitis C causes acute (rapidly developing) and chronic (long-term) disease. It is spread mainly through blood transfusions. Medical researchers believe hepatitis C may be caused by several viruses. As with hepatitis B, hepatitis C may lead to cirrhosis of the liver and, eventually, liver cancer. Vaccines have been developed for both hepatitis A and B, but no vaccine has yet been developed to prevent hepatitis C.
Inflammatory bowel disease
Inflammatory bowel disease (IBD) is a disorder that causes inflammation and ulceration (development of ulcers) in the small and large intestine. The two main forms of IBD are ulcerative colitis and Crohn's disease.
Ulcerative colitis, which occurs mainly in people between the ages of fifteen and forty, affects the inner lining of the large intestine and rectum. Inflammation usually begins in the rectum and spreads upward into the entire large intestine. Diarrhea, cramping or abdominal pain, fever, and weight loss
result. Ulcers (tiny open sores) develop in the intestinal lining, and blood and pus appear in the feces. If the disorder becomes widespread through the large intestine, the risk of cancer increases. The cause of ulcerative colitis is unknown, and the only cure for advanced cases is the surgical removal of the large intestine.
Crohn's disease is a life-long illness that may recur frequently over a person's lifetime. It causes the inflammation of all the layers of the intestinal wall, particularly in the small intestine. In turn, the inflammation brings about ulcerations in the intestinal wall. Crohn's disease can also affect the large intestine, mouth, esophagus, and stomach. The disease is marked by diarrhea, abdominal pain, weight loss, and fever. The cause of Crohn's disease has yet to be found, as well as a cure. Medications to control diarrhea, abdominal pain, and inflammation are the main forms of treatment. Surgery to remove or repair a section of the intestine may be required.
A digestive ulcer is any sore that develops in the lining of the stomach or duodenum (sores in the lower esophagus occur less frequently). Because these sores form in areas where gastric juice is present, they are generally referred to as peptic ulcers (pepsin is an enzyme in gastric juice). Peptic ulcers found in the stomach are more specifically called gastric ulcers. Those in the duodenum are called duodenal ulcers. Of the two, duodenal ulcers are the most common type, accounting for about 80 percent of all digestive ulcers. They tend to be smaller than gastric ulcers and heal more quickly. Any ulcer that heals leaves a scar.
The symptoms for gastric ulcers include feelings of heartburn, nausea, weight loss, and stomach pain. That pain is described as gnawing, dull, aching, or resembling hunger pangs. About one-third of those individuals suffering from gastric ulcers are awakened by pain at night.
The symptoms for duodenal ulcers differ slightly. They include heart-burn, stomach pain that is relieved by eating or antacids, and a burning sensation at the back of the throat. Pain is most often felt two to four hours after a meal. Citrus juices, coffee, and aspirin bring on pain more quickly. About 50 percent of individuals suffering from duodenal ulcers are awakened by pain at night.
Before the 1980s, physicians believed ulcers were caused by several factors—including stress and a poor diet—that resulted in excess stomach acid. Medical research has since shown that a certain bacterium that can live undetected in the mucous membrane of the digestive tract is the culprit. This bacterium irritates and weakens the lining, making it more susceptible to
damage by gastric juice. About 95 percent of duodenal ulcers and 70 percent of gastric ulcers are caused by this bacterium.
Treatment for peptic ulcers includes antibiotics to eliminate the bacterium and other drugs to reduce the amount of gastric juice secreted in the stomach. Very few ulcers fail to respond to the medications that are currently used to treat them.
TAKING CARE: KEEPING THE DIGESTIVE SYSTEM HEALTHY
The body is wholly dependent on the digestive system to provide it with the nutrients—fluids, carbohydrates, proteins, lipids, vitamins, and minerals—it needs to continue to function. If the digestive system fails to do this because it is not functioning properly, the entire body suffers.
A healthy lifestyle will keep the digestive system healthy. This includes following a proper diet, exercising regularly, maintaining a healthy weight, not smoking, drinking alcohol only in moderation, and reducing stress.
WORMS IN THE BODY
Tapeworms are parasites (organisms that live in or on other kinds of organisms) that live in the intestinal tracts of some animals. There are three major species of tapeworms that can infect humans. They are typically acquired from eating raw or undercooked beef, pork, or fish.
Tapeworm eggs are passed along in feces. When improperly treated human sewage is used to fertilize pastures or crops, pigs and cattle can become infected with the eggs. They can also become infected from drinking contaminated water. Freshwater fish become infected when human feces contaminates their water source.
The tapeworm eggs develop into larvae in the infected animals and fish. When humans eat the meat from those animals and fish without properly cooking it, they become infected. The tapeworm travels to the intestine, attaching itself to the inner lining by hooks on its head. If not treated, the tapeworm may stay in the intestine for years, absorbing nutrients through its outer covering. It may grow up to 30 feet (9 meters) in length.
Most individuals infected with a tapeworm have no symptoms. Some, however, may experience pain in the upper abdomen, diarrhea, unexplained weight loss, and weakness. A tapeworm's eggs or worn body parts that appear in an individual's feces are often the only sign of an infection.
Tapeworms are easily treated with medication. Practicing good hygiene and avoiding raw or undercooked meat or fish are important steps in preventing a tapeworm infection.
The "Food Guide" Pyramid developed by the U.S. Departments of Agriculture and Health and Human Services provides easy-to-follow guidelines for a proper diet. In general, foods that are low in fat (especially saturated fat), low in cholesterol, and high in fiber should be eaten. Fiber is especially important in maintaining the workings of the intestines. Fat should make up no more than 30 percent of a person's total daily calorie intake. Breads, cereals, pastas, fruits, and vegetables should form the bulk of a person's diet; meat, fish, nuts, and cheese and other dairy products should make up a lesser portion. Drinking fluids, especially water, helps move material through the digestive system.
Long-term ingestion of cigarette fumes, excessive alcohol, and spicy foods can cause serious damage to the digestive system. Toxins that are taken into the body by way of the mouth are absorbed by the digestive tract and transported to the liver, which can suffer permanent damage. Many medications can injure the lining of the esophagus, stomach, and intestines. Medicines in pill or capsule form should always be taken with plenty of water.
Teeth are an important, yet often overlooked, part of the digestive system. They begin the entire process of digestion. Oral hygiene is, therefore, a primary concern. The best way to prevent tooth decay is to brush the teeth at least twice a day, preferably after every meal and snack. The teeth should also be flossed daily to help prevent gum disease.
Minor irritations of the digestive system are common. Occasional diarrhea, constipation, or excessive gas is to be expected. Often, they are treated with nonprescription drugs. However, if these or any digestive problems persist, they should not be ignored and medical attention should be sought.
FOR MORE INFORMATION
Avraham, Regina. The Digestive System. New York: Chelsea House, 1989.
Ballard, Carol. The Stomach and Digestive System. Austin, TX: Raintree/Steck-Vaughn, 1997.
Bryan, Jenny. Digestion. Englewood Cliffs, NJ: Silver Burdett Press, 1993.
Epstein, Rachel. Eating Habits and Disorders. New York: Chelsea House, 1990.
Parker, Steve. Digestion. Brookfield, CT: Copper Beech Books, 1997.
Parramon, Merce. The Digestive System. New York: Chelsea House, 1994.
Silverstein, Alvin, Virginia B. Silverstein, and Robert A. Silverstein. The Digestive System. New York: Twenty-First Century Books, 1994.
American Digestive Health Foundation
Homepage of a national organization that advances digestive health through the financial support of research and education in the cause, prevention, diagnosis, treatment, and cure of digestive diseases.
Site includes a large image of the human digestive system with each part/organ linked to a paragraph explanation of its structure and function.
The Digestive System—How It Works
Site from the Jefferson Health System, a collective of member hospitals and healthcare organizations in the greater Philadelphia region. Information on the digestive system includes a discussion of how it works and the numerous disorders that can afflict it.
MUSC Digestive Disease Center
Site of the Medical University of South Carolina focusing on digestive diseases. Includes an overview of the digestive organs, an extensive list of diseases and their symptoms, and links to other sites.
Virtual Anatomy Textbook: The Digestive System
Site provides extensive coverage of the parts of the digestive system, mixing images and text. System parts are grouped into three classes: early digestion, middle digestion, and late digestion.
"The Digestive System." UXL Complete Health Resource. 2000. Encyclopedia.com. (August 28, 2016). http://www.encyclopedia.com/doc/1G2-3437000013.html
"The Digestive System." UXL Complete Health Resource. 2000. Retrieved August 28, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3437000013.html