What is digestion?

Structure and Functions

In the most general terms, digestion is a multiple-stage process that begins by breaking down foodstuffs taken in by an organism. Some specialists consider that the actual process of digestion occurs after this breaking-down stage, when essential nutritional elements are absorbed into the body. Even after division of the digestive process into two main functions, there remains a third, by-product stage: disposal by the body of waste material in the form of urine and feces.

Several different vital organs, all contained in the abdominal cavity, contribute either directly or indirectly to the digestive process at each successive stage. Certain imbalances in the functioning of any one of these organs, or a combination, can lead to what is commonly called indigestion. Chronic imbalances in the functioning of any of the key digestive organs—the stomach, small intestine, large intestine (or colon), liver, gallbladder, and pancreas—may indicate symptoms of diseases that are far more serious than mere indigestion.

In a very broad sense, the process of digestion begins even before food that has been chewed and swallowed passes into the stomach. In fact, while chewing is underway, a first stage of glandular activity—the release of saliva by the salivary glands into the food being chewed (a process referred to as intraluminal digestion)—provides a natural lubricant to help propel masticated material down the esophagus. Although the esophagus does not perform a digestive function, its muscular contractions, which are necessary for swallowing, are like a preliminary stage to the muscular operation that begins in the stomach.

The human stomach has
two main sections: the baglike upper portion, or fundus, and the lower part, which is twice as large as the fundus, called the antrum. The function of the fundus is essentially to receive and hold foods that reach the stomach via the esophagus, allowing intermittent delivery into the antrum. Here two dynamic elements of the breaking-down process occur, one physical, the other chemical. The muscular tissue surrounding the antrum acts to churn the partially liquefied food in the lower stomach, while a series of what are commonly called gastric juices flow into the mixture held by the stomach.

The most active element that is secreted from special parietal cells in the mucous membranes lining the stomach is hydrochloric acid. The possibility of damage to the stomach lining is minimized (but not removed entirely) first by the chemical reaction between the acid and the mildly alkaline chewed food and second by the presence of other gastric juices in the antrum. Primary among these is the enzyme pepsin, which is secreted by a different set of specialized cells in the gastric lining. Secretions of both hydrochloric acid and pepsin become mixed and interact chemically with food materials, while the antrum itself moves in rhythmic pulses caused by muscular contractions (peristalsis). One of the key functions of pepsin during this stage is to break down protein molecules into shorter molecular strings of less complicated amino acids, which eventually serve as building material for many body tissues.

At a certain point, food materials are sufficiently reduced to pass beyond the antrum into the duodenum, the first section of the small intestine, where a different stage in the digestive process takes place. At this juncture, the partially broken-down food material is referred to as chyme. The transfer of food from one digestive organ to another is actually monitored by a special autonomic nerve, called the vagus nerve, which originates in the medulla at the head of the spinal cord. Although the vagus nerve innervates a number of vital zones in the abdominal cavity, its function here is quite specific: It adjusts the intensity of muscular movement in the stomach wall and thus limits the amount of food passing into the small intestine.

The exact amount of food that is allowed to enter the intestinal tract represents only part of the essential question of balance between agents contributing to the digestive process. The presence of a now slightly acidic food-gastric juice mixture in the duodenum sparks what is called an enterogastric reflex. Two hormones, secretin and cholecystokinin, begin to flow from the mucous membranes of the duodenum. These hormones serve to limit the acidic strength of stomach secretions and trigger reactions in the liver, gallbladder, and pancreas—other key organs that contribute to digestion as the chyme passes through the intestines.

While in the compact, coiled mass of the small intestine (compared to the thicker, but much shorter, colon, or large intestine), food materials, especially proteins, are broken down into one of twenty possible amino acid components by the chemical action of two pancreatic enzymes, trypsinogen and chymotrypsinogen, and two enzymes produced in the intestinal walls themselves, aminopeptidase and dipeptidase. It is interesting to note that the body, which is itself in large part constructed of protein material, has its own mechanism to prevent protein-splitting enzymes from devouring the very organs that produce them. Thus, when they leave the pancreas, both trypsinogen and chymotrypsinogen are inactive compounds. They become active “protein-breakers” only when joined by another enzyme—enterokinase—which is secreted from cells in the wall of the small intestine itself.

Other nutritional components contained in chyme interact chemically with other specialized enzymes that are secreted into the small intestine. Carbohydrate molecules, especially starch, begin to break down when exposed to the enzyme amylase in saliva. This process is intensified greatly when pancreatic amylase flows into the small intestine and mixes with the chyme. The products created when carbohydrates break down are simple sugars
, including disaccharides and monosaccharides, especially maltose. As these sugars are all broken down into monosaccharides, a final process that occurs in the wall of the small intestine itself (which contains more specialized enzymes such as maltase, sucrase, and lactase), they become the most rapidly assimilated body nutrients.

The process needed to break down fats is more complicated, since fats are water insoluble and enter the intestine in the form of enzyme-resistant globules. Before the fat-splitting enzyme lipase can be chemically active, bile, a fluid produced by the liver and stored in the gallbladder, must be present. Bile serves to dissolve fat globules into tiny droplets that can be broken down for absorption, like all other nutritive elements, into the body via the epithelial lining of the intestinal wall. Such absorption is locally specialized. Iron and calcium pass through the epithelial lining of the duodenum. Protein, fat, sugars, and vitamins pass through the lining of the jejunum, or middle small intestine. Finally, salt, vitamin B12, and bile salts pass through the lining of the lower small intestine, or ileum.

It is this stage that many scientists consider to be the true process of digestion. Absorption occurs through enterocytes, which are specialized cells located on the surface of the epithelium. The surface of the epithelium is increased substantially by the existence of fingerlike projections called villi. These tiny protrusions are surrounded by the fluid elements of chemically altered food. Specialized enterocyte cells selectively absorb these elements into the capillaries that are inside each of the hundreds of thousands of villi. From the capillaries, the nutrients enter the blood and are carried by the portal vein to the liver. This organ carries out the essential chemical processes that prepare fats, carbohydrates, and proteins for their eventual delivery, through the main bloodstream, to various parts of the body.

Elements that are left after the enzymes in the small intestine have done their work are essentially waste material, or feces. These pass from the small intestine to the large intestine, or colon, through a dividing passageway called the cecum. The disposal of waste materials may or may not be considered to be technically part of the main digestive process.

After essential amounts of water and certain salts are absorbed into the body through the walls of the colon, the remaining waste material is expulsed from the bowels through the rectum and anus. If any prior stage in the digestive process is incomplete or if chemical imbalances have occurred, the first symptoms of indigestion may manifest themselves as bowel movement irregularities.

Disorders and Diseases

Malfunctions in any of the delicate processes that make up digestion can produce symptoms that range from what is commonly called simple indigestion to potentially serious diseases of the gastrointestinal tract. Functional indigestion, or dyspepsia, is one of the most common sources of physical discomfort experienced not only by human beings but by most animals as well. Generally speaking, dyspepsias are not the result of organic disease, but rather of a temporary imbalance in one of the functions described above. There are many possible causes of such an imbalance, including nervous stress and changes in the nature and content of foods eaten.

The most common causes of dyspepsia and their symptoms, although serious enough in chronic cases to require expert medical attention, are far less dangerous than diseases afflicting one of the digestive organs. Such diseases include gallstones, pancreatitis, peptic ulcers (in which excessive acid causes lesions in the stomach wall), and, most serious of all, cancers afflicting any of the abdominal organs.

Dyspepsia may stem from either physical or chemical causes. On the physical side, it is clear that an important part of the digestive process depends on muscular or nerve-related impulses that move partially digested food through the gastrointestinal tract. When, for reasons that are not yet fully understood, the organism fails to coordinate such physical reactions, spasms may occur at several points from the esophagus through to the colon. If extensive, such muscular contractions can create abdominal pains that are symptomatic of at least one category of functional indigestion.

Problems of motility, or physical movement of food materials through the digestive tract, may also cause one common discomfort associated with indigestion: heartburn. This condition occurs when the system fails to move adequate quantities of the mixture of food and gastric juices, including hydrochloric acid, from the stomach into the duodenum. The resultant backup of food forces part of the acidic liquid mass into the esophagus, causing instant discomfort.

Insufficient motility may also cause delays in the movement of feces through the colon, resulting in constipation. Just as the vagus nerve monitors the muscular movements that are necessary to move food from the stomach to the small intestine, an essential gastrocolic reflex, tied to the organism’s nervous system, is needed to ensure a constant rhythm in the movement of feces into the rectum for elimination. If this function is delayed (as a result of nervous stress in some individuals, or because of the dilated physical state of the colon in aged persons), food residues become too tightly compressed in the bowels. As the colon continues to carry out its normal last-stage digestive function of reabsorbing essential water from waste material before it is eliminated, the feces become drier and even more compacted, making defecation difficult and sometimes painful.

Most other imbalances in digestive functions are chemical in nature. Highly spiced or unfamiliar foods frequently upset the balance in the body’s chemical digestion. Symptoms may appear either in the abdomen itself (in particular, a bloated stomach accompanied by what is commonly called gas, a symptom of chemical disharmony in the digestive process) or in the stool. If the chemical breakdown of chyme is incomplete because of an imbalance in the proportion (either excessive or inadequate) of enzymes secreted into the stomach or intestines, the normal process of absorption will not take place, creating one of a number of symptoms of indigestion.

The most common symptom of indigestion is diarrhea, which can result from a variety of causes. Because movement in the bowels is affected by different nerve signals, some diarrhea attacks may be linked to nonchemical reactions, such as extreme nervousness. Relaxation of the sphincter, however, as well as the rise in the contractile pressure of the lower colon that precedes defecation (the gastroileal reflex), is also affected by the presence of gastrointestinal hormones, particularly gastrin itself. An imbalance in the amount of concentration of such components in the gastrointestinal tract (attributable to incomplete digestive chemistry) tends to relax the bowels to such a degree that elimination cannot be prevented except through determined mental resistance. It is important to note that if diarrhea continues for an extended time, its effect on the body is not simply the loss of essential body nutrients that pass through the bowels without being fully digested; the inability of the colon to reabsorb into the body an adequate proportion of the water content from the feces can lead to
dehydration of the organism, especially in infants.

In most areas of the world, there is widespread consensus that treatment of indigestion is a matter of taking over-the-counter drugs whose function is to right the imbalance in some of the chemical processes described above. In theory as well as in practice, such treatments do work, since the basic chemical imbalance, if it is has not extended beyond the point of indigestion (in the case of peptic ulcers, for example), is fairly easily diagnosed, even by pharmacists. Increasingly, however, the public is becoming aware that digestion can be aided, and indigestion avoided, by paying closer attention to dietary habits, particularly the importance of increasing fiber intake to facilitate the digestive process. Critical advances are also being made in knowledge of the potentially harmful effects on digestion of chemical additives to processed foods.

Perspective and Prospects

Historical traces of the medical observation of indigestion, as well as the prescription of remedies, can be found as far back as ancient Egypt. A famous medical text from about 1600 BCE known as the Ebers Papyrus contains suggested remedies (mainly herbal drugs) for digestive ailments, as well as instructions for the use of suppositories to loosen the lower bowel. For centuries, however, such practical advice for treating indigestion was never accompanied by an adequate theoretical conception of the digestion function itself.

In the medieval Western world, many erroneous guidelines for understanding the digestive process were handed down from the works of Galen of Pergamum (129–ca. 199 CE). Galen taught that food material passed from the intestines to the liver, where it was transformed into blood. At this point, a vital life-giving spirit, or “pneuma,” gave the blood power to drive the body. Similar misconceptions would continue until, following the work of William Harvey (1578–1657), medical science gained more accurate knowledge of the circulatory function of the bloodstream. By the eighteenth century, rapid advances had been made in studies of the function of the stomach and intestines, notably by the French naturalist René de Réaumur (1683–1757), who demonstrated that food is broken down by gastric juices in the stomach, and by the Italian physiologist Lazzaro Spallanzani (1729–1799), who discovered that the stomach itself is the source of gastric juices.

It was an American army surgeon, William Beaumont (1785–1853), who wrote what became, until well into the twentieth century, the most complete medical guide to digestive functions. Beaumont carried out direct clinical observations of the actions of gastric juices in humans. He also observed the way in which the anticipation of eating can spark not only the secretion of such fluids but also the muscular stimuli that promote motility in the digestive process. Soon after Beaumont’s findings were published, the German physiologist Theodor Schwann (1810–1882) first isolated pepsin. Others would show that a variety of enzymes in the gastrointestinal tract are secreted by different organs in the abdomen, notably the pancreas.

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