The Gastrointestinal System - function, diseases and information

The gastrointestinal system converts ingested foods to nutrients the body can absorb and use. Physician specialists who treat gastrointestinal conditions are gastroenterologists. This section, “The Gastrointestinal System,” presents a discussion of gastrointestinal structures and their functions, an overview of gastrointestinal health and disorders, and entries about the health conditions that can affect the gastrointestinal system.

Functions of the Gastrointestinal System

TRADITIONAL CHINESE MEDICINE (TCM), a centuriesold philosophy of health centered on balance among the body’s systems and functions, views the torso as the “triple burner” or “triple heater.” Here the upper, middle, and lower segments of the body converge, becoming the core that distributes energy throughout the body much like a burner or heater. Western medicine shares a similar understanding, translated into the tangibility of physical structures and their functions.

The gastrointestinal system, also called the digestive or alimentary system, functions as the body’s furnace. Fuel—food—enters the gastrointestinal tract in raw form at the MOUTH. Some 20 hours or so later, the compressed residue—feces, also called stool—exits at the other end of the gastrointestinal tract, from the ANUS. Along its passage, the food undergoes numerous transformations as the gastrointestinal organs and structures break it down, mechanically and chemically, into particles and eventually into molecules of energy (NUTRIENTS) that the bloodstream can transport to cells throughout the body. That the typical adult eats three to five times (or more) in 24 hours, yet the body often passes a single BOWEL MOVEMENT in the same period is testament to the gastrointestinal system’s efficiency in extracting every molecule, literally, of useful matter from all that food.

The LIVER and PANCREAS produce numerous chemical substances to aid in breaking down the core nutrients of food—carbohydrate, protein, and fat—into molecules that can pass through the membrane of the small intestine to enter the bloodstream. The liver synthesizes BILE, a complex fluid containing water, electrolytes, cholesterol, biliary acids, and BILIRUBIN (400 to 800 milliliters per day). A network of channels, the BILE DUCTS, collect bile from the liver and transport it to the GALLBLADDER. The gallbladder extracts water from the bile to form a concentrated solution, which it stores until DIGESTIVE HORMONES signal the need to release bile for digestion. The bile flows through another duct, the common bile duct, mixes with pancreatic juices, and drains into the DUODENUM, the first part of the SMALL INTESTINE. INSULIN is the best known of the pancreatic products, though the pancreas synthesizes a number of other important hormones, DIGESTIVE ENZYMES, and juices essential for digestion. Digestive hormones orchestrate and synchronize the multitude of gastrointestinal functions.

Mechanical preparation: the mouth

The digestive journey begins with the mouth. Each mouthful of food passes between the crushing force of the TEETH, which can exert up to 3,500 pounds per square inch, at least two dozen times. The taste, texture, and smell of the food induce the SALIVARY GLANDS to release saliva, a watery liquid that contains the digestive enzyme amylase. Amylase begins to break down carbohydrates in the food into the sugar molecules that form them, getting a head start on extracting from food the body’s most significant fuel source. The lips and cheeks hold the food in the mouth while the tongue pushes the food under the teeth and against the palate (roof of the mouth). These actions grind the food and mix the particles with saliva to form a pastelike wad called a bolus. Finally the tongue pushes the bolus to the back of the THROAT for swallowing. A small projection of cartilaginous tissue that hangs at the back of the throat, the epiglottis, closes across the TRACHEA (windpipe) to direct the bolus of food down the ESOPHAGUS.

Wavelike contractions—PERISTALSIS—propel the bolus down the esophagus, a muscular tube about 12 inches long. The MUSCLE structure of the esophagus changes along its length, transitioning from striated muscle tissue that responds to voluntary control to smooth muscle tissue, completely under direction of the autonomic NERVOUS SYSTEM. Esophageal peristalsis thrusts the swallowed food bolus toward the STOMACH with such vigor that the food continues to its destination even if the person is upside down. A ring of muscle, the lower esophageal sphincter, opens to pass the bolus into the stomach then closes to keep it there.

Liquefication: the stomach

The pouchlike stomach can expand to five or six times its empty size to accommodate the meals that come its way. This is where the process of digestion gets under way; the stomach digests more than half the carbohydrates and about 20 percent of the protein that a meal contains. The stomach resides below the DIAPHRAGM, its upper portion resting under the apex of the HEART and near the SPLEEN, just under the left ribs, and its lower portion beneath the liver.

The inside of the stomach is the gastric mucosa, a thick layer of mucous membrane. A network of deep folds, called rugae, gives the gastric mucosa a furrowed appearance. At the bottom of the folds of the rugae are the gastric glands, which produce hydrochloric acid and digestive enzymes (gastric juices). Near the top of the folds are the cells that produce the mucus that protects the stomach’s lining from the gastric juices. When the stomach expands with food the rugae flatten, spreading the mucus and expanding the surface area of the stomach for thorough mixing of food with gastric juices. The gastric mucosa also secretes the digestive hormone gastrin.

Three layers of muscle give the stomach STRENGTH and FLEXIBILITY. The innermost layer wraps obliquely, or diagonally, around the gastric mucosa. The middle layer encircles the oblique muscle. The outer layer, the longitudinal muscle, envelopes the stomach lengthwise. Among them, these muscles give the stomach the ability to contract and convolute with considerable force as well as the ability to expand and contract for the volume of food it contains. These layers of muscle also give the stomach the ability to squeeze its contents downward into the small intestine for the rest of the digestive journey.

Food, in a semiliquid state after initial preparation in the mouth, enters at the top of the stomach, called the fundus, and flows downward across the rugae. Gastric juices immediately begin working to dissolve food particles, breaking them down into more basic compounds that the small intestine can digest. At the same time the stomach muscles contract, producing powerful contortions that further mix and liquefy the stomach’s contents. The combined chemical and mechanical actions produce chyme, a somewhat soupy solution the stomach then sends to the small intestine. The lower portion of the stomach is the pylorus, from the Greek for “gatekeeper.” Chyme exits the stomach through the pyloric sphincter, which relaxes to permit passage into the duodenum. Chyme trickles from the stomach over four to six hours.

Extracting nutrients: the small intestine

The small intestine is where nutrients move from the gut to the blood. The nearly 18 feet of small intestine loop back and forth within the abdominal cavity, framed inside the COLON. Two layers of smooth muscle, the outer longitudinal and the inner circular, form the walls of the small intestine. These muscles rhythmically contract to move digestive matter through the gastrointestinal tract. The intestinal mucosa, a thin mucous membrane, forms the inner lining. It produces a number of digestive enzymes and digestive hormones.

In the first segment of the small intestine, the duodenum, the intestinal mucosa is fairly smooth. Only 10 inches long, the duodenum handles the majority of digestive activity. In addition to receiving chyme from the stomach, the duodenum receives bile and pancreatic juices via the common bile duct, which enters the duodenum at a small port called the ampulla of Vater. These solutions complete the breakdown of foods into end-product nutrients. Bile transforms fats into fatty acids. Pancreatic and intestinal enzymes convert proteins to amino acids and polysaccharides (compound sugars) to monosaccharides (simple sugars). Further chemical interactions separate out vitamins and minerals. Monosaccharides and some electrolytes (salts and minerals) enter the bloodstream through the duodenum, which is also the major site for absorption of iron and calcium.

The watery mixture containing the remaining nutrients moves on to the middle and end segments of the small intestine for absorption. In these segments, the JEJUNUM and the ILEUM, millions of tiny projections called villi extend from the intestinal mucosa to vastly expand the mucosal surface area. A network of capillaries (tiny blood vessels) weaves through the villi. Nutrients pass through the mucosal membrane and into the capillaries, which transport them into the bloodstream and throughout the body. The jejunum, about seven feet long, absorbs the remaining monosaccharides and many amino acids, additional electrolytes, water-soluble vitamins (the B vitamins and vitamin C), folic acid, and minerals such as iron. The final segment of the small intestine, the ileum, is about 10 feet long. It absorbs fatty acids and the remaining amino acids, as well as vitamin B12 and the fat-soluble vitamins (vitamins A, D, E, and K). The ileum also reabsorbs bile salts. The journey through the small intestine takes 8 to 10 hours.

Compacting waste: the colon

The colon, also called the large intestine, collects and compacts the remnants of digestion for their elimination from the body, a process it accomplishes primarily by absorbing water. Like the small intestine, the colon’s wall contains two layers of muscle, the outer running lengthwise and the inner circling around, that rhythmically contract. The inner lining is flat mucous membrane. The CECUM, the first segment of the colon, is a pouchlike structure that receives digestive material from the ileum, which enters near its floor. The ileocecal valve maintains the passage for one-way movement. The cecum absorbs about a third of the water from the digestive material it receives. Peristalsis then carries the intestinal content from the cecum through the rest of the colon.

The main colon loops around the outer edge of the abdominal cavity. It contains five segments. The ascending colon rises from the cecum and travels up the body’s right side. At the gallbladder the colon takes a turn; the next segment is the transverse colon. The transverse colon extends across the top of the abdomen, with the liver and stomach above and the small intestine beneath. It takes a downward turn at the base of the stomach’s fundus, becoming the descending colon. The descending colon drops along the left perimeter of the abdominal cavity. The descending colon makes a staggered turn inward toward the midline of the body, becoming the sigmoid colon. These four segments of the colon are functionally contiguous, progressively dehydrating and compacting the digestive residue that moves through them. The final segment of the colon is the RECTUM, by which point digestive waste has reached the solid form known as feces or stool. The rectum stores stool until its expulsion from the body via the anus (bowel movement). The journey through the colon generally takes four to six hours, though can take longer.

Health and Disorders of the Gastrointestinal System

The gastrointestinal system represents an intricate balance of mechanical and chemical functions. Lifestyle and dietary habits can affect this balance. To help maintain this balance and preserve gastrointestinal health, gastroenterologists recommend

• Eating foods high in fiber such as fruits, vegetables, and whole grains and whole grain products. High-fiber foods move quickly through the gastrointestinal tract and result in residual bulk that helps keep stools soft, reducing the risk for conditions such as CONSTIPATION and HEMORRHOIDS.
• Reducing consumption of foods high in fats. Fats require more steps to digest, slowing movement through the gastrointestinal tract. Saturated (animal) fats are associated with an increased risk for COLORECTAL CANCER.
• Limiting consumption of products that contain CAFFEINE or ALCOHOL, which are frequent causes of DIARRHEA.
• Drinking plenty of noncaffeinated and nonalcoholic fluids throughout the day.
• Chewing food thoroughly before swallowing and remaining upright after eating.

Gastrointestinal symptoms are the second-leading reason for visits to the doctor, with diarrhea being the most common of these symptoms. Many gastrointestinal ailments are minor, such as INFECTION (GASTRITIS, GASTROENTERITIS, and COLITIS) and irritations such as DYSPEPSIA. The close proximity of the lower esophagus and the bottom of the heart gives rise to the term “heartburn,” an apt descriptor for the burning sensation that occurs when gastric contents bubble back up into the esophagus. GASTROESOPHAGEAL REFLUX DISORDER (GERD) develops when such backwash becomes chronic.

Chronic gastrointestinal conditions such as CELIAC DISEASE, DIVERTICULAR DISEASE, IRRITABLE BOWEL SYNDROME (IBS), and INFLAMMATORY BOWEL DISEASE (IBD) can significantly interfere with QUALITY OF LIFE. People who have conditions such as these must work closely with their doctors to develop effective treatment approaches and manage their lifestyles in ways that minimize symptoms and support gastrointestinal health.

Cancers are among the most serious gastrointestinal conditions and can involve any organ or structure of the gastrointestinal system. Though colorectal cancer remains the second-leading cause of deaths due to cancer in the United States, it also offers great opportunity for prevention as well as early detection and treatment. Doctors now know that detecting and removing intestinal polyps, fleshy growths that develop in the colon’s mucosa, via screening COLONOSCOPY eliminates the foundation for more than 90 percent of colorectal cancers.

Traditions in Medical History

Ancient doctors learned much about the inner structures and workings of the body from wounds that occurred on the battlefield. The writings of the Roman physician Celsus (14–37 C.E.) documented his recommendations to his students that they take advantage of such natural opportunities. One student who took the advice to heart was GALEN (130–200 C.E.), whose own teachings and writings would shape the understanding and practice of Western medicine for centuries. Galen learned much of the practice of medicine while treating soldiers and gladiators. Of the digestive process, Galen believed stomach liquefied food that then passed to the intestines. From the intestines the mixture traveled to the liver, where it mysteriously became blood that the veins carried around to the various tissues of the body. Though wrong in some fundamental ways, the extrapolation was not so far off from the reality.

In 1822, U.S. Army surgeon William Beaumont became the first physician to witness and explore the functions of the stomach in “real time.” In June of that year French-Canadian trader Alexis St. Martin suffered a musket wound to his left side that opened a fist-size hole in his stomach. St. Martin’s comrades brought him to Beaumont for treatment. Miraculously in an era of no antibiotics and limited surgical expertise, St. Martin survived. With great scientific curiosity, over several decades Beaumont observed the activities of St. Martin’s stomach through this window. He conducted experiments with various items of food tied to string that he periodically withdrew to assess the extent of their demolition in the stomach. He measured the volume and temperature of stomach juices. And he watched the digestive process as much as his schedule and St. Martin’s patience permitted.

Finally, in 1833 Beaumont published his findings in a book, Experiments and Observations on the Gastric Juice and the Physiology of Digestion. St. Martin lived 58 years with the hole in his stomach, outliving Beaumont by 27 years and dying at age 86. Beaumont’s detailed observations and experiments gave modern medicine the most extensive understanding of digestion possible until the 1940s when ANESTHESIA and ANTIBIOTIC MEDICATIONS made surgery practical, and surgeons could more carefully explore the stomach and other gastrointestinal structures.

Breakthrough Research and Treatment Advances

The 21st century arrived on the heels of amazing advances in medical and surgical treatments for gastrointestinal conditions. Among the most significant advances have been those in ORGAN TRANSPLANTATION, which result from a blend of improved surgical techniques, organ harvesting procedures, and immunosuppressive methods. In 1984 LIVER TRANSPLANTATION became the standard treatment for end-stage LIVER FAILURE, a milestone in its progression from experiment to therapeutic solution. Within 15 years surgeons in the United States were performing more than 5,000 liver transplantations a year. Surgeons are now exploring applications for transplantation technology in other conditions, such as to replace the severely diseased small intestine, stomach, and even pancreas. Though these transplant operations remain largely investigational, they hold great promise for people who have disorders such as CYSTIC FIBROSIS, SHORT BOWEL SYNDROME, DIABETES, and severe diverticulosis.

Other advances in diagnostic and operative procedures take advantage of fiberoptic technology. Endoscopic surgery has transformed oncegrueling operations, procedures such as open CHOLECYSTECTOMY, which often required up to 12 weeks of recuperation, to a few minor incisions and a third of the recovery time. Surgeons now can perform APPENDECTOMY, herniorrhaphy and hernioplasty, colon resection, and even operations on the stomach with minimally invasive techniques. COLOSTOMY (a surgically created opening through the abdominal wall for the passage of solid digestive waste), once nearly certain after most operations on the colon, now is often temporary or can be avoided altogether. Surgeons have developed methods for anastamosing, or connecting, the remaining segments of the bowel to restore near-normal function. Medications help soothe the bowel and control BACTERIA during HEALING.

The HUMAN GENOME PROJECT, the complete mapping of the human genetic structure, has led to discoveries that have altered the understanding, course of treatment, outlook, and prevention measures for a number of gastrointestinal disorders, including PEPTIC ULCER DISEASE, IBD, and familial cancers of the gastrointestinal tract. GENETIC SCREENING and GENETIC COUNSELING make it possible for people to learn whether they are at risk for many familial or hereditary disorders and take appropriate measures to minimize their likelihood for acquiring the condition. Numerous clinical trials are exploring investigational GENE THERAPY methods to treat or thwart hereditary disorders such as celiac disease and cystic fibrosis.

COLONOSCOPY, visualization of the entire colon using a flexible, lighted endoscope inserted through the anus, has the potential to eliminate 70 percent or more of colorectal cancer through early detection and removal of the adenomatous polyps that are most often the source of cancerous growths in the colon. Research continues the quest for a less intrusive approach, with the current focus on virtual colonoscopy and other procedures that allow for the visualization of the gastrointestinal tract without entering it (though virtual colonoscopy does not offer the opportunity to remove polyps; conventional colonoscopy remains the therapeutic option of choice for most polypectomies). Researchers are also looking for ways to use ENDOSCOPY to screen for other cancers that often go undetected until they are too advanced for treatment, hoping technology may offer similar preventive benefits for a broader range of gastrointestinal malignancies.