What are intestines?

Structure and Functions

After the initial process of digestion takes place in the stomach, food passes into the intestines, where the chemical action of several gastric juices separates out the nutritive content of fats, carbohydrates, and proteins. This nutritive material is then absorbed into the bloodstream through the walls of the intestines, while waste material is collected for excretion.

In adult humans, the
small intestine and large intestine together represent a total length of about nine meters (thirty feet). Both small and large intestines are hoselike muscular organs; the former is much longer, but substantially narrower, than the latter. The two intestines are joined at the cecum, which is located in the right-lower abdominal cavity. The appendix, which is sometimes called a “blind pouch” because it is unessential to the main processes performed in the gastrointestinal tract, projects from the cecum.

The physical disposition and function of the large intestine, or colon, are distinct from those of the small intestine. As the large intestinal tube leaves the cecum, it assumes a specific shape, following horizontal and vertical lines within the abdominal cavity. This is not the case with the small intestine, whose extensive length (about seven meters, or twenty-three feet) takes an intertwined “nesting” shape in the limited abdominal space available. By contrast, the colon, which is about 1.5 meters (5 feet) in length, has three easily identifiable sections: the ascending colon on the right side of the abdominal cavity, the transverse colon, and the descending colon on the left side of the abdomen. A final leftward bend in the colon at the sigmoid provides for its attachment to the rectum.

The two parts of the intestines carry out distinct functions in the overall digestive process. As food material passes from the lower end, or pylorus, of the stomach, only a part of the digestive process has occurred. Once partially digested food enters the small intestine, it is propelled through the intestine by means of a process of muscular contraction in the intestinal wall, which is called peristalsis.

As the food moves forward, different substances, some secreted from the lining of the intestine itself, and others—principally, bile and pancreatic fluid, which enter the upper intestine (duodenum) from the liver and pancreas—contribute to a further breaking down of food material.

Very small projections called villi are found along the interior surface of the small intestine. The villi absorb those portions of the food material that have been altered by the digestive process. From the villi, nutritive material is passed into the blood and lymphatic system for distribution to cells throughout the body. This process continues in the middle and end portions (jejunum and ileum, respectively) of the small intestine until, passing through the cecum, the remaining residue enters the large intestine.

The mixture of material contained in the large intestine, or colon, consists of indigestible food, bacteria, and substantial amounts of water. Most of the water is absorbed into the body through the walls of the colon, while the remaining waste material, or feces, is excreted through the rectum.

To understand how food materials are actually absorbed by the villi inside the small intestine and passed into the bloodstream, one must consider several chemical processes, according to the nature of the material in question.

For example, the triglyceride components of fat, chains of fatty acids attached to glycerol, are chemical compounds that do not dissolve in water (a major part of the main bloodstream). The pancreatic enzymes called lipases split the triglycerides into separate units of fatty acid. Once separated, these fatty acids become coated with bile salts secreted by the liver, a process that allows them to pass into the mucous cells lining the intestine. As this passage occurs, the coated fatty acids (micelles) resume their chainlike form as triglycerides. At this point, however, the triglycerides have assumed an altered chemical state. In this altered form, fats can be absorbed into the blood and carried throughout the body to be used as body “fuel” or, if unused, stored in fatty tissues.

Carbohydrates must be broken down into simple
sugars (glucose, galactose, and fructose) before they can be absorbed by the cell linings of the small intestine. This process occurs when the more complex carbohydrates (both starches and sugars) are split by the chemical effects of the enzyme amylase, which enters the small intestine from the salivary glands and the pancreas.

Finally, proteins, which contain the amino acids essential for the process of tissue formation in the body, must be split in several stages, the first of which occurs in the stomach itself. Here, proteins are partially broken down by the action of the gastric juices, mainly pepsin. Once protein material passes into the upper part of the small intestine, or duodenum, the process is accelerated by the influence of two main pancreatic enzymes: trypsin and chymotrypsin. These secretions cause the proteins to release amino acids in three forms: simple, dual, or triplicate bodies. It is not until these three forms are actually inside the cell walls of the small intestine that other enzymes split the dual and triplicate amino acids into their simplest single form, which can be absorbed into the veins that carry nourishment to the various organs of the body.

In the overall chemical process leading to the absorption of various body nutrients by the small and large intestines, there is a certain “absorptive specialization” in different zones of the gastrointestinal tract. Iron and calcium, for example, are absorbed in the duodenum, while proteins, fats, sugars, and all vitamins except vitamin B12 are absorbed in the jejunum. Finally, in the ileum, salt, vitamin B12, and bile salts are processed.

Disorders and Diseases

Doctors have always known that the digestive processes of the intestines can be affected in either positive or negative ways by the nature of the food that is consumed. In the simplest terms, negative reactions are manifested by the obvious effects of
indigestion and diarrhea. The control of such symptoms of improper or incomplete digestion may appear to the layperson to be a simple matter of using “over-the-counter” tablets such as laxatives or “antigas” pills. Treatment of the symptoms of indigestion, however, may provide only a superficial solution to a problem that is much more serious.

An area of important medical concern that goes beyond the general discomfort caused by imbalanced digestive functioning of the intestines involves peptic ulcers. A peptic ulcer is an open sore on the mucous membrane lining the gastrointestinal tract. The general label “peptic ulcer” was applied to this condition since the discovery, in the mid-1830s, of pepsin, the first clearly identified enzyme known to contribute to the chemical breakdown of ingested foods. Although later stages of research into the digestive process yielded much more extensive knowledge of the component elements of gastric juices, the specific name has remained attached to the general phenomenon of intestinal ulceration. The term “gastric ulcer” refers specifically to ulceration in the stomach lining.

Generally speaking, peptic ulcers occur when there is an imbalance between the task of digestion to be accomplished by the intestines and the amounts and levels of concentration of the gastric juices secreted into the gastrointestinal tract. When the amounts or concentrated strengths of gastric juices in the intestines exceed the level required for digestion (or flow into the intestine when no food has been ingested), these agents actually begin to digest the membranes of the intestine itself.

Various forms of treatment for intestinal ulcers have been developed, including both therapeutic drugs that have the capacity to counteract the corrosive effects of excessive gastric juices and, in the preventive vein, diets that contain natural combatants against intestinal disorders, especially high-fiber, unprocessed, or lightly processed foods.

In recent years, research into the causes of ulcers has extended into the field of gastrointestinal hormonal secretions originating not in the pancreas itself but in the intestine or stomach. These secretions reach the pancreas later through the bloodstream and stimulate its production of digestive juices. Such secretive processes may, if they fail to communicate properly balanced “codes” concerning the task of digestion that needs to take place in the intestines, cause an excessive supply (in volume or strength) of gastric juices, which can cause ulcerations to develop.

The most serious pathological condition that can affect the intestines is cancer of the colon
. Thought to develop from a degenerative process originating in benign polyps (stem-based tumors that may develop in areas of the organism lined with mucous membrane, such as the nose, the colon, and, in females, the uterus), cancer of the colon has registered a survival rate that is statistically higher than those of cancers in other vital organs of the abdominal cavity (the liver and stomach, in particular). This is partly because—if the cancer is discovered in time—substantial areas of the colon that have been attacked by cancer can be removed surgically without endangering the continued essential functioning of the intestines.

Almost all questions relating to the pathology of the intestinal tract are somehow connected with the type of food that is eaten. Thus, medical science has turned increasingly to publicizing preventive dietary practices that can have a bearing on all functions of the intestines, from the simplest level of discomfort to the most serious level of chronic diseases.

As stated above, a relatively recent and valuable contribution to the knowledge of natural ways to aid in the absorptive work of both the small intestine and the colon—and to reduce the dangers of ulceration and/or intestinal cancer—involves the role of the fiber content of foods. Fiber is generally described as consisting of polysaccharides and lignin, two plant substances that, more than any other nutritive material, retain their natural forms as plant cell walls and are not broken down by human digestive enzymes. The plant food that is richest in these materials is wheat bran, which contains about 40 percent fiber. As fiber-rich foodstuffs such as bran pass through the gastrointestinal tract, the fiber material they contain is subject to fermentation by anaerobic
bacteria in the colon. Two chemical results of this complex process seem to be the removal of deoxycholic acid from the bile and the reduction of the cholesterol saturation level of the bile. Both effects are deemed beneficial, since the reduction of deoxycholic acid and cholesterol in intestinal bile tends, at the very least, to reduce the likelihood of developing gallstones. Fiber-rich diets in combination with the reduction of excess weight became standards of preventive health care by the 1990s.

By the mid-twentieth century, typical personal diets in the Western world contained commercially refined foodstuffs that were rich in sugars and syrups, which are mainly fiber depleted. In addition to the specific disease-related factors mentioned above, medical science has noted that high consumption of fiber-depleted foods results in higher levels of energy intake (absorption of calories) during the digestive process that occurs in the intestines. In simple terms, when calorie intake exceeds the level required by the normally exercised body, the result is weight gain that may continue to the point of obesity.

Perspective and Prospects

Medical science began to become aware of the various digestive functions of hormonal secretions only in the first decades of the twentieth century. Although the early nineteenth century American army surgeon
William Beaumont
was the first doctor to discover the presence of gastric juices in the intestines, his analysis of digestive fluids remained quite elementary. Beaumont could easily identify hydrochloric acid in stomach secretions. He also took samples of bile from the intestinal tract and performed laboratory experiments that proved the role of bile in breaking down fatty materials. What remained unsolved were the identity and origins of other components of gastric juice and an explanation for their controlled secretion from surrounding organs in the abdominal cavity into the intestines. Beaumont’s view that mental concentration (including “negative mental concentration,” or anxiety) induced the flow of gastric juices proved eventually to be only partially correct.

It was only in 1902 that the British doctors Ernest Henry Starling and Sir William Maddock Bayliss were able to show that, in addition to nerve “signals,” certain chemical factors induced the flow of gastric juices, specifically from the pancreas into the intestinal tract. These doctors found that, in fact, the small intestine released into the bloodstream a “chemical transmitter” that, as it circulated to the other vital organs, stimulated the production and flow of the pancreatic juices necessary for digestion. They called this “chemical transmitter” secretin. To this initial agent would be added a whole category of secretions that are called “hormones,” a term taken from the Greek word for “urging on.”

A discovery was made in 1928 that helped to clarify the complex relationship of hormones, gastric juice secretion, and the carrying out of the digestive process by the small and large intestines. This was the discovery of pancreozymin, the second main “chemical transmitter” affecting the pancreas, by the American researcher Andrew Ivy. Pancreozymin was found to cause the release by the pancreas of an enzyme-rich fluid made up of three agents: trypsin, lipase, and amylase. Each agent proved to be an activator in the process of breaking down different nutrients (protein, fats, and carbohydrates, respectively).

Although the intestines are the ultimate destination of and seat of activity for the pancreatic juices released by command of this hormone (as well as of the last major digestion-linked hormone, gastrin, which was discovered in 1955), secretin alone has its origin in the intestines themselves. Both pancreozymin and gastrin are secreted from the stomach.

In time, researchers found that most gastrointestinal hormones are secreted by specialized cells that line the interior of the stomach. Such cells react at various levels according to the composition of the food that has been ingested, sending chemical signals, via the hormones they secrete, that determine the relative amounts and strengths of the several gastric juices that enter the intestines from the pancreas. A similar question of varied amounts and strengths of gastric juices was linked to the so-called vagus nerve function, which also activates pancreatic flow to the intestinal tract.

The functional relationship between these two activator agents—the one nervous and the other chemical—has become one of the primary interests of researchers who deal with the most common ailment attacking the intestinal organs: peptic ulceration.

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