What is the gastrointestinal system?

Structure and Functions

The gastrointestinal system or alimentary canal exists as a tube that runs through the body from mouth to anus. The wall of the tube is composed of four layers of tissue. The outermost layer, the serosa, is part of a large tissue called the peritoneum, which covers internal organs and lines body cavities. Extensions of the peritoneum called mesenteries anchor the organs of digestion to the body wall. Fatty, apronlike structures that hang in front of the abdominal organs are also modifications of the peritoneum. They are called the lesser and the greater omentum. The muscular layer, composed of circular and longitudinal muscles, makes up the bulk of the wall of the tube. The contractions of this layer aid in moving materials through the tube. Nerves, blood vessels, and lymph vessels are found in the third layer, the submucosa. The innermost or mucous layer has glands for secretion and modifications for absorption.

The tube is compartmentalized, and each section is equipped to accomplish some part of the digestive process. The mechanical phase of digestion involves the physical reduction of food to a semiliquid state; this is accomplished by tearing, chewing, and churning the food. Chemical digestion utilizes enzymes to reduce food to simple molecules that can be absorbed and used by the body to provide energy and to build and repair tissue.

The mouth (also called the buccal or oral cavity) marks the beginning of the gastrointestinal system and the digestive process. The mouth is divided into two areas. The vestibule is the space between the lips, cheeks, gums, and teeth. Lips, or labia, are the fleshy folds that surround the opening to the mouth. The skin covers the outside, while the inside is lined with mucous membrane. The colored part of the lips, called the vermilion, is a juncture of these two tissues. Because the tissue at this point is unclouded, underlying blood vessels can be seen. A membrane called the labial frenulum attaches each lip to the gum, or gingivalum.

The oral cavity occupies the space posterior to the teeth and anterior to the fauces or opening to the throat. It is bounded on the sides by cheeks and on the roof by an anterior bony structure called the hard palate and a posterior muscular area, the soft palate. The uvula, a cone-shaped extension of the soft palate, can be seen hanging down in front of the fauces. The floor of the oral cavity is formed by the tongue and associated muscles. Taste buds are found on the surface of the tongue. The bottom of the tongue is anchored posteriorly to the hyoid bone. Anteriorly, the membranous frenulum lingua anchors the tongue to the floor of the mouth. The tongue’s movement is controlled by extrinsic muscles that form the floor of the mouth and by intrinsic muscles that are part of the tongue itself. The movements of the tongue assist in speaking, swallowing, and forming food into a bolus.

Teeth, found in gum sockets, are the principal means of mechanical digestion in the mouth. Human teeth appear in two sets. The deciduous or milk teeth are the first to appear. There are usually ten in each jaw, and they are replaced by the second, permanent set during childhood. The permanent set consists of sixteen teeth in each jaw. The four incisors and two canines have sharp chiseled edges, which permit biting and tearing of food. The four premolars and six molars have flat surfaces that are used in grinding the food. Frequently, the third pair of molars or wisdom teeth do not erupt until later in adolescence. The crown of a tooth appears above the gum line while the roots are embedded in the gum socket. The small area between the crown and the root is called the neck. The crown is covered with enamel and the root with cementum. Dentin is beneath the covering in both areas and forms the bulk of the tooth. The central cavity of the tooth is filled with a soft membrane called pulp. Blood vessels and nerves are
embedded in the pulp.

At the rear of the mouth, the fauces or opening leads to the pharynx. The pharynx is a common passageway for the movement of air from nasal cavity to trachea and food from mouth to esophagus. The esophagus is a tube approximately twenty-five centimeters long. Most of the esophagus is located within the thoracic cavity, although the lower end of the tube pierces the diaphragm and connects with the stomach in the abdominal cavity. Both ends of the esophagus are controlled by a circular muscle called a sphincter. The movement of food through the esophagus is assisted by gravity and the contractions of the muscularis layer. No digestion is accomplished in either the pharynx or the esophagus.

The stomach, a J-shaped organ, is divided into four areas: the cardia, fundus, body, and pyloris. The cardia lies just below the sphincter at the juncture of esophagus and stomach, while the fundus is a pouch that pushes upward and to the left of the cardia. The large central area is the body, and the lower end of the stomach is the pyloris. Here another sphincter, the pyloric valve, controls the opening between stomach and intestine. The mucosa of the stomach is arranged in folds called rugae. The rugae permit distension of the organ as it fills. Gastric and mucus glands are present in the mucosa. The gastric glands produce and secrete enzymes that are specific for protein digestion, as well as hydrochloric acid, which creates the proper acid environment for enzyme action. The muscularis of the stomach wall has three layers of muscle with a circular, longitudinal, and oblique arrangement. The muscle arrangement facilitates the churning action that reduces the food to a semiliquid called
chyme. The pyloric valve relaxes under neuronal and hormonal influence, and the chyme is moved into the small intestine.

The site for the completion of digestion and the absorption of digested material is the small intestine. This tube, with a 2.5-centimeter diameter and a length of 6.4 meters, is coiled into the mid and lower abdomen. The first twenty-five centimeters of the small intestine constitute the duodenum. This is followed by the jejunum, which is 2.5 meters long. The ileum, at 3.6 meters, terminates at the ileocecal valve, which connects the small to the large intestine. The interior of the small intestine is characterized by the presence of fingerlike projections of the mucosa called villi that contain blood and lymph capillaries and circular folds of submucosa (the plicae circularis), both of which provide absorption surface for the digested food. Mucosal glands produce enzymes that contribute to the digestion of carbohydrates, lipids, and proteins. Enzymes from the pancreas and bile from the liver enter the small intestine at the duodenum and aid the chemical digestion.

The final compartment in the gastrointestinal system is the large intestine, sometimes called the bowel or colon. This tube, with a diameter of 6.5 centimeters and a length of 1.5 meters, is divided into the cecum; the ascending, transverse, and descending colon; the rectum; and the anal canal. The cecum is a blind pouch located just below the ileocecal valve. The fingerlike appendix is attached to the cecum. The ascending colon extends from the cecum up the right side of the abdomen to the underside of the liver, where it turns and runs across the body. The colon descends along the left side of the abdomen. The last few centimeters of colon form an S-shaped curve that gives the section its name, sigmoid colon. Three bands of longitudinal muscle called taeniae coli run the length of the colon. Contraction of these bands causes pouches or haustra to form in the colon, giving the tube a puckered appearance. The sigmoid colon leads into the rectum, a twenty-centimeter segment that terminates in a short anal canal. The anus is the opening from the anal canal to the exterior of the body.

Disorders and Diseases

Because the primary function performed in the gastrointestinal system is the physical and chemical preparation of food for cellular absorption and use, any malfunction of the process has implications for the overall metabolism of the body. Structural changes or abnormalities in the anatomy of the system interfere with the proper mechanical and chemical preparation of the food.

Teeth are the principal agents of mechanical digestion or mastication in the mouth. Dental caries or tooth decay involves a demineralization of the enamel through bacterial action. Disrupted enamel provides an entrance for bacteria to underlying tissues, resulting in infection and inflammation of the tissues. The resulting pain and discomfort interfere with the biting, chewing, and grinding of food. Three pairs of salivary glands secrete the water-based, enzyme-containing fluid called saliva. These glands can be the target of the virus that causes mumps. (Although the pain and swelling that are typical of this disease can prevent swallowing, the more important effect of the virus in males is the possible inflammation of the testes and subsequent sterility.)

The gastroesophageal sphincter at the lower end of the esophagus controls the movement of materials from the stomach into the esophagus. Relaxation of this sphincter allows a backflow of food (gastroesophageal reflux

) to occur. The acidity of the stomach contents damages the esophageal lining, and a burning sensation is experienced. Substances such as citric fruits, chocolate, tomatoes, alcohol, and nicotine as well as body positions that increase abdominal pressure, such as bending or lying on the side, induce heartburn or indigestion. A hiatal hernia
occurs when a defect of the diaphragm allows the lower portion of the esophagus and the upper portion of the stomach to enter the chest cavity; it causes
heartburn and difficulty in swallowing.

Pathologies and abnormalities of the stomach and intestines are studied in the medical science called gastroenterology. The stomach is the site of both mechanical and chemical digestion. Although small amounts of digested food begin to pass into the small intestine within minutes following a meal, the chyme usually remains in the stomach for three to five hours. Relaxation of the gastroesophageal sphincter will result in reflux; and stimulation by nerves from the medulla of the brain can cause the forceful emptying of stomach contents through the mouth. This is called vomiting and may be brought about by irritation, overdistension, certain foods, or drugs. Excessive vomiting results in dehydration, which in turn upsets electrolyte and fluid balance.

Chemical digestion in the stomach requires an acidic environment. This is provided by gastric glands, which secrete hydrochloric acid. The tissue lining the stomach protects it from this acidity and prevents self-digestion. Oversecretion of the gastric juices or a breakdown of the stomach lining can cause lesions or peptic ulcers to form in the mucosal lining. Gastritis, the inflammation of the stomach mucosa brought on by the ingestion of irritants such as alcohol and aspirin or an overactive nervous stimulation of the gastric glands, may be the underlying cause of ulcer formation. Ulcers can form in the lower esophagus, stomach, and duodenum because these are the organs that come in contact with gastric juice. The terms “gastric ulcer” and “duodenal ulcer” refer to peptic ulcers located in the stomach and the first portion of the small intestine, respectively.

Gastroenteritis could involve the stomach, the small intestine, or the large intestine. It is a disorder marked by nausea, vomiting, abdominal discomfort, and diarrhea. The condition has various causes and is known by several names. Bacteria are a common cause of the condition known as food poisoning
. Amoebas, parasites, and viruses can bring about the symptoms associated with intestinal influenza or travelers’ diarrhea. Allergic reactions to food or drugs may cause gastroenteritis.

Although diverticulitis may be found anywhere along the gastrointestinal tract, it is most commonly found in the sigmoid colon. This disorder results from the formation of pouches or diverticula in the wall of the tract. Undigested food and bacteria collect in the diverticula and react to form a hard mass. The mass interferes with the blood supply to the area and ultimately irritates and inflames surrounding tissue. Abscess, obstruction, and hemorrhage may develop. A diet lacking in fiber appears to be the major contributor to this disorder.

Colitis, or inflammation of the bowel, is accompanied by abdominal cramps, diarrhea, and constipation. It may be brought about by psychological stress, as in irritable bowel syndrome, or may be a manifestation of such disorders as chronic ulcerative colitis and Crohn’s disease.

A change in the rate of motility through the colon or large intestine results in one of two disorders: diarrhea or constipation. As food passes through the colon, water is reabsorbed by the body. If the food moves too quickly through the colon, then much of the water will remain in the feces and diarrhea results. Severe diarrhea affects electrolyte balance. Viral, bacterial, and parasitic organisms may initiate the rapid motility of substances through the colon. Another condition, called constipation, develops from sluggish motility. When the food remains for too long a time in the bowel, too much water is reabsorbed by the body. The feces then become dry and hard, and defecation is difficult. Lack of fiber in the diet and lack of exercise are the leading causes of constipation.

Hemorrhoids are varicose veins that develop in the rectum or anal canal. Varicose veins are the result of weakened venous valves. Factors such as pressure, lack of muscle tone as a result of aging, straining at defecation, pregnancy, and obesity are among the common contributors. Hemorrhoids become irritated and bleed when hard stools are passed.

Malignancies can occur at any point along the gastrointestinal tract. Cancers of the mouth are frequently associated with tobacco use. Esophageal cancer may be associated with heavy alcohol use, tobacco, or chronic reflux. Gastric cancer may be the result of chronic ulcer disease or heavy exposure to foods high in nitrites. Colon cancer is one of the leading causes of death in the United States, and it may be associated with certain genetic disorders or might arise spontaneously.

Disorders in the accessory organs contribute to the malfunctioning of the gastrointestinal system. Gallstones, cirrhosis of the liver, pancreatitis, and pancreatic cancer are among the major diseases affecting the digestive process. These disorders generally involve the obstruction of tubes or the destruction of glands, so that enzymes do not reach the intended site of digestion.

Perspective and Prospects

The proper functioning of the gastrointestinal system is dependent on the anatomical structure and health of the organs. The organs provide the site for the mechanical and chemical digestion of food, the absorption of food and water, and the elimination of waste material. Two factors play a primary role in causing anatomical abnormalities in digestion: aging and eating disorders.

The aging process gradually changes anatomical structure. For food to be chewed properly, teeth must be in good health. Dental caries, periodontal disease, and missing teeth prevent the proper mastication of food. Because of these problems, older people tend to avoid foods that require chewing. This may lead to an unbalanced diet. Another age-related change in the mouth is the atrophy of the salivary glands and other secretory glands, which interferes with chemical digestion and swallowing. A loss of muscle tone in the organ walls impedes mechanical digestion and slows down the movement of food through the system. Often, the elimination of waste material becomes difficult and constipation results.

Eating disorders such as anorexia nervosa and bulimia contribute to digestive malfunctioning. These disorders are most often associated with but are not limited to young women. Anorexia is self-imposed starvation, while bulimia is characterized by a binge-purge cycle that incorporates vomiting and/or abuse of laxatives. Both conditions induce nutrient deficiencies and upset water and electrolyte balances. The vomiting of the acid contents of the stomach damages esophageal, pharyngeal, and mouth tissue. It also destroys tooth enamel. In addition to the harm done to the gastrointestinal system, eating disorders affect several other systems, such as the reproductive system.

The field of medical science that studies and diagnoses digestive system disorders is gastroenterology. Gastroenterologists use several investigative techniques. Blood tests and stool examination are used to detect internal bleeding and deficiency disorders. For a time, X rays were the only nonsurgical means of obtaining information on the structure of internal organs. The advent of nuclear medicine
in the 1950s led to the use of radioisotopes in body scanning procedures. Instruments capable of a more detailed and direct visualization were developed, such as fiber optics and the fluoroscope. Fiber optics
involves the use of long, threadlike fibers of glass or plastic that transmit light into the organ and reflect the image back to the viewer; this method allows the physician to detect ulcers, lesions, neoplasms, and structural abnormalities. The fluoroscope uses X rays to permit continuous observation of motion within the organs.

The 1970s saw the development of more sophisticated scanning and imaging techniques. computed tomography (CT) scanning uses X-ray techniques to scan very thin slices of tissue and presents a defined, unobstructed view. Magnetic resonance imaging (MRI) can provide detailed information even to the molecular level; energies from powerful magnetic fields are translated into a visual representation of the structure being studied. Another technique, ultrasonography, passes sound waves through a body area, intercepts the echoes that are produced, and translates them into electrical impulses, which are recorded and interpreted by the physician.

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