What is the abdomen?

Structure and Functions

The abdomen is the portion of the body’s trunk that begins immediately below the diaphragm, which is the main respiratory muscle in the chest cavity, and extends to the lower pelvic region. The abdominal area is defined by a muscular wall made up of fatty tissue and skin, which determines the general shape of the body from the chest to the lower pelvis. The entire abdominal cavity is lined by a membrane called the peritoneum. This membrane encloses the essential organs of the abdomen: the stomach, small and large intestines, liver, gallbladder, bladder, pancreas, and kidneys. In females, the abdominal casing also contains the uterus, ovaries, and Fallopian tubes. At the front of the abdomen is the navel, essentially a scar which forms following the cutting of the umbilical cord after birth.

Any overview of the abdomen requires a composite view of the functions performed by each of the organs contained in it. With the exception of the female reproductive organs, all the organs contained in the abdominal cavity serve in one way or another in the process of food digestion, the transfer of diverse essential food by-products to the rest of the body, and the disposal of waste products via the urinary tract and the anal passage.

The esophagus is the tube through which all solid and liquid foods enter the stomach, which is the topmost organ in the abdominal cavity. Because it is essentially a bag, the stomach can assume different shapes and adjust in size to accommodate different volumes of food that reach it through the esophagus. In adult humans, the average capacity of the stomach is about one quart. The essential digestive function of the stomach is to convert foods from their original states to a general semiliquid state referred to as chyme.

This first stage of digestion is carried out by the chemical action of some thirty-five thousand gastric glands that make up the inner folds of the inner layer of the stomach, the gastric mucosa. As the gastric glands actively secrete gastric juice, the second layer of the stomach wall, which is muscle tissue, contracts and expands, providing the physical movement that is necessary for the gastric juice and food material to come into full contact.

Gastric juice actually begins to flow from the inner lining of the stomach even before food is present. This may occur when one smells food or even when one imagines the flavor of food. Among the component parts of gastric juice are the enzymes
pepsin and rennin, hydrochloric acid, and mucus, the latter of which protects the lining of the stomach from the effects of high acidity. Pepsin and rennin begin to break down different types of proteins when an optimum acid environment (a pH between 1 and 3) exists.

Once the initial stage of digestion has occurred, food passes from the stomach into the upper portion of the small intestine, or duodenum, via the pyloric sphincter. This passageway will not allow food to enter the small intestine until it is suitably modified by the action of the stomach.

In the small and large intestines, partially broken-down food is reduced further by the action of gastric juices that are either secreted into the intestines from other abdominal organs (the pancreas and liver, most notably) or secreted by the mucous membranes of the intestines themselves. It is in the small intestine that most of the breaking-down digestive work of gastric juices takes place. Food particles reach a certain level of decomposition so that they may be absorbed into the bloodstream through the mucous membranes of the intestine. The bulk of what is left is allowed to pass from the small intestine and through a gate-like passageway called the cecum and then to the large intestine or colon.

The function of the colon and the component juices that it contains is to separate out the three essential components that remain following the absorptive work of the small intestine: water, undigested foodstuff, and bacteria. Most of the water passes back into the body through the walls of the colon, while undigested food and bacteria are propelled farther down the gastrointestinal tract for eventual elimination as feces.

The importance of other organs in the abdomen—the liver, kidneys, pancreas, gallbladder, and bladder—is as complex as that of the intestines and in several cases goes beyond the basic function of digestion. Closest to the stomach and the digestive process itself, perhaps, is the action of the pancreas. The pancreas is the glandular organ located directly beneath the stomach. It is connected to the duodenum (the first and shortest segment of the small intestine), to which it provides pancreatic juice containing three digestive enzymes: trypsin, amylase, and lipase. These agents join the secretions of the small intestine, as well as bile flowing from the liver, to complete the digestive process that breaks down proteins, carbohydrates, and fats. They can then be absorbed through the walls of the intestine for the general nourishment of the body. In addition to its role in the digestive process, the pancreas possesses endocrine cells, called the islets of Langerhans, that secrete two hormones, insulin and glucagon, directly into the bloodstream. These two hormones work together to influence the level of sugar in the blood. When the insulin-secreting cells of the pancreas fail to function effectively, then diabetes mellitus may result.

Like the pancreas, the liver, which is the largest glandular organ of the body, shares in the digestive process by producing bile, a fluid essential for the emulsification of fats passing through the small intestine. Bile salts, as they are called, are stored in the gallbladder until they are released into the small intestine. This contribution to the digestive process, however, represents only a minimal part of the liver’s functions, many of which have vital effects on body functions far beyond the abdominal cavity. Because blood filled with oxygen flows into the liver from the aorta through the hepatic artery, on one hand, and blood containing digested food enters the liver from the small intestine via the portal vein, on the other, the relationship between “harmonizing” liver functions and the content of the blood is absolutely critical.

The metabolic cells that make up liver tissue, known as hepatic cells, are highly specialized. According to their specialized function, the hepatic cells in the four unequal-sized lobes of the liver may affect several factors: the amount of glycogen (converted and stored glucose) that should be reconverted to glucose and passed (for added energy) into the bloodstream; the conversion of excess carbohydrates and protein into fat; the counteraction of the harmful ammonia by-product of protein breakdown by the production of urea; the production of several essential components of blood, including plasma proteins and blood-clotting agents; the storing of key vitamins and minerals such as vitamins A, D, K, and B-12; and the removal of bacteria and other debris that collect in the blood itself—a function of the phagocytic, or Kupffer, cells in particular.

It is the next pair of vital abdominal organs, the kidneys, that separates many of the waste products associated with the liver’s metabolic functions, including urea and mineral salts, out of the blood and removes them from the body in the form of urine. This separation is performed by millions of tiny filtering agents called nephrons. Blood penetrates the interior of the kidney by way of an incoming arteriole that branches off from the main renal artery. After the filtering process has been completed, cleansed blood flows back into the main bloodstream via an outgoing arteriole and a system of blood vessels leading to the main renal vein. Waste materials remain, after filtering, in a tube-like extension of each nephron until they can be concentrated in the form of urine in a chamber in the middle of the kidney called the kidney pelvis. From this chamber, urine is propelled by muscular compression through the ureter tubes leading to the bladder, the last organ (in males) contained within the lower abdominal cavity. In addition to removing waste products from the blood, the kidneys can adjust the level in the blood of other substances—such as sodium, potassium, and calcium—that are needed by the body but that may exist in excess at certain times. Because the two kidneys perform exactly the same functions, it is possible to survive with only one healthy kidney.

Although obviously essential for temporary storage of urine and final elimination of liquid waste through the process of urination, the bladder is the least complicated organ in the abdominal cavity. The bladder is essentially a sac with a liquid capacity of about one pint. Its functions are governed by varied tension in and loosening of muscles in the walls of the sac and the external sphincter. When the pressure of collected urine reaches a certain point, nervous impulses cause the external sphincter to relax. Urine flow out of the bladder into the urethra tube can be controlled, up to a certain point, in humans and most mammals by conscious thought.

Disorders and Diseases

Given the concentration in the abdomen of vital regulatory organs, much medical research has focused on the pathology of this area of the body. Although there are a number of specific diseases that attack individual abdominal organs, the entire region is vulnerable to cancerous tumors. Medical science has tended to associate cancers in certain abdominal organs with dietary habits that are either of recent origin (consumption of highly processed foodstuffs in industrialized Western societies, for example) or geographically or ethnically distinctive—the East Asian, specifically Japanese, vulnerability to certain types of stomach cancer, for example. The latter vulnerability may, however, also be tied to dietary or other environmental considerations that vary in different populated areas of the globe.

Although cancers may strike any of the vital abdominal organs, chances of successful surgical intervention to remove tumors vary greatly according to the location of the cancer. Liver cancer, for example, is essentially untreatable through surgery, while the treatment of cancer of the colon has a significant success rate. This variation is partially attributable to the fact that the vital processes performed by the intestines may not be seriously threatened when a portion of the organ is removed in cancer surgery.

The most important specific diseases associated with the abdomen include peritonitis, hepatitis, and diabetes. Among these diseases, diabetes has received the most attention, both for its widespread impact on all sectors of the population and for the amount of research that has gone into the task of finding a cure. Diabetes occurs when the pancreas fails to produce enough insulin to metabolize the sugar substance glucose. A breakdown in this function impairs proper cell nourishment and results in excessive sugar in the blood and urine. This state, referred to as hyperglycemia, can affect a number of body functions outside the abdominal cavity, leading, for example, to atherosclerosis and vascular degeneration in general. Because many diabetes patients must inject insulin into their bodies to counteract a malfunctioning pancreas, an opposite, equally dangerous side effect, hyperinsulinism, may also occur. The most serious degenerative effect that menaces patients suffering from diabetes, however, occurs when the chemical and hormonal imbalance originating in the pancreas brings negative reactions to the kidneys, causing the latter to fail. Medical science has perfected various technical means for addressing this problem, most of which are connected with the mechanical process called dialysis.

Hepatitis is an inflammation that attacks the liver. The two common forms are hepatitis A (formerly called infectious hepatitis) and hepatitis B (formerly called serum hepatitis). Both are transmitted as a result of unsanitary conditions, the first in food and water supplies and the second when unsterile hypodermic needles or infected blood come into contact with the victim’s own bloodstream. Unlike most other diseases associated with the abdominal organs, hepatitis is extremely contagious. Hepatitis B can present dangers in using plasma supplied by donors, as there can be an incubation period from six weeks to six months before external signs of the disease occur.

Perhaps the most common abdominal disease, curable through the use of antibiotics if treated in time, is peritonitis. This is an acute inflammation of the peritoneum, the membrane that lines the entire abdominal cavity. It can occur as a result of direct bacterial invasion from outside the body or as a side effect of ruptures occurring in one of the organs contained in the abdomen. Peritonitis typically develops as a result of complications from appendicitis, bleeding ulcers, or a ruptured gallbladder.

Perspective and Prospects

The history of medical analysis of disorders of the abdominal area goes back as far as written history itself, ranging from simple indigestion and painful (and possibly fatal) gallstones to very serious and only recently understood diseases such as diabetes.

Perhaps the most noteworthy advancement in medical knowledge affecting the organs of the abdominal region has been the development of more sophisticated means to counteract the effects of kidney disorders. While there were some striking advances (but not full levels of success) in organ transplant surgery beginning in the 1970s, a technique called dialysis made remarkable strides. First used shortly after World War II as an effective but costly and physically limiting treatment, dialysis involves the use of a machine that receives blood pumped directly from the patient’s heart and processes this blood in place of the kidney. This involves filtering out excretory products, adding essential components that “refresh” blood needs (such as heparin to combat clotting as well as proper amounts of saline fluid), and then returning the blood to resume its vital function within the circulatory system.

Although the essential principles of dialysis did not change drastically in the last quarter of the twentieth century, levels of efficiency in a process that had to be repeated over a ten-hour period several times a week definitely did. Development of much smaller, portable dialysis devices made it possible for patients to follow their doctors’ instructions in carrying out their own treatment between hospital or office visits, thus lessening the chances of very dangerous crises at the outset of kidney failure.

The most notable hope for patients afflicted with kidney disorders is successful transplant from a healthy or recently deceased donor. By the early twenty-first century, transplants had also become foreseeable for those suffering from diseases that strike other organs in the abdominal cavity, especially the liver. Thus, healthy organ transplant technology can be said to represent one of the most important domains of future research, involving specialists of all the subsections of medicine relating to the abdominal cavity.

Bibliography

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De Wardener, H. E. The Kidney: An Outline of Normal and Abnormal Function. 5th ed. New York: Churchill Livingstone, 1985.

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