DIGESTION

CONCEPT

Digestion is the process whereby the foods we eat pass through our bodies and are directed toward the purposes of either providing the body with energy or building new cellular material, such as fat or muscle. The parts of food that the body cannot use, along with other wastes from the body, are eliminated in the form of excrement. Aspects of digestion, particularly the production of waste and intestinal gas, are not exactly topics for polite conversation, yet without these and other digestive processes, life for humans and other organisms would be impossible. The functioning of digestion itself is like that of a well-organized, cohesive sports team or even of a symphony orchestra: there are many parts and players, each with an indispensable role.

HOW IT WORKS

Nutrients

For digestion to occur, of course, it is necessary first to have something to digest—namely, nutrients. What follows is a cursory overview of nutrients and nutrition, subjects covered in much more depth within the essay of that name. Nutrients include proteins, carbohydrates, fats, minerals, and vitamins. In addition to these nutrients, animal life requires other materials, not usually considered nutrients, which include water, oxygen, and something that greatly aids the process of food digestion and elimination of wastes: fiber.

PROTEINS AND CARBOHYDRATES.

Proteins are large molecules built from long chains of amino acids, which are organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur-bonded in characteristic formations. Proteins serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body's immune system, and making enzymes. (An enzyme is a protein material that speeds up chemical reactions in the bodies of plants and animals.) Good examples of dietary proteins include eggs, milk, cheese, and other dairy products. Incomplete proteins, or ones lacking essential amino acids—those amino acids that are not produced by the human body—include peas, beans, lentils, nuts, and cereal grains.

Carbohydrates are compounds that consist of carbon, hydrogen, and oxygen. Their primary function in the body is to supply energy. When a person ingests more carbohydrates than his or her body needs at the moment, the body converts the excess into a compound known as glycogen. It then stores the glycogen in the liver and muscle tissues, where it remains, a potential source of energy for the body to use in the future, though if it is not used soon, it may be stored as fat. The carbohydrate group comprises sugars, starches, cellulose (a type of fiber), and various other chemically related substances.

LIPIDS, VITAMINS, AND MINERALS.

Lipids include all fats and oils and are distinguished by the fact that they are soluble (i.e., capable of being dissolved) in oily or fatty substances but not in water. In the body, lipids supply energy much as carbohydrates do, only much more slowly. Lipids also protect the organs from shock and damage and provide the body with insulation from cold, toxins, and other threats. Processed, saturated fats (fats that have been enhanced artificially to make them more firm) are extremely unhealthy, and consumption of some types of animal fat (e.g., pork fat) is also inadvisable. On the other hand, vegetable fats, such as those in avocados and olive oil, as well as the animal fats in such fish as tuna, mackerel, and salmon can be highly beneficial.

Vitamins are organic substances that, in extremely small quantities, are essential to the nutrition of most animals and some plants. In particular, they work with enzymes in regulating metabolic processes—that is, the chemical processes by which nutrients are broken down and converted into energy or used in the construction of new tissue or other material in the body. Vitamins do not in themselves provide energy, however, and thus they do not qualify as a form of nutrition. Much the same is true of minerals, except that these are inorganic substances, meaning that they do not contain chemical compounds made of carbon and hydrogen.

The Digestive System

To supply the body with the materials it needs for energy and the building of new tissue, nutrients have to pass through the digestive system. The latter is composed of organs (an organ being a group of tissues and cells, organized into a particular structure, that performs a specific function within an organism) and other structures through which nutrients move. The nutrients pass first through the mouth and then through the esophagus, stomach, small intestine, and large intestine, or colon. Collectively, these structures are known as the alimentary canal.

Nutrients advance through the alimentary canal to the stomach and small intestine, and waste materials continue from the small intestine to the colon (large intestine) and anus. Along the way, several glands play a role. A gland is a cell or group of cells that filters material from the blood, processes that material, and secretes it either for use again in the body or to be eliminated as waste. Among the glands that play a part in the digestive process are the salivary glands, liver, gallbladder, and pancreas. (The last three are examples of glands that are also organs.) The glands with a role in digestion secrete digestive juices containing enzymes that break down nutrients chemically into smaller molecules that are absorbed more easily by the body. There are also hormones involved in digestion-there are, for example, glandular cells in the lining of the stomach that make the hormone gastrin.

FROM THE MOUTH TO THE STOMACH.

The first stage of digestion is ingestion, in which food is taken into the mouth and then broken down into smaller pieces by the chewing action of the teeth. To facilitate movement of the food through the mouth and along the tongue, it is necessary for saliva to be present. Usually, the sensations of sight, taste, and smell associated with food set in motion a series of neural responses that induce the formation of saliva by the salivary glands in the mouth. Amylase, an enzyme in the saliva, begins the process of breaking complex carbohydrates into simple sugars. (The terms simple and complex in this context refer to chemical structures.)

By the time it is ready to be swallowed, food is in the form of a soft mass known as a bolus. The action of swallowing pulls the food down through the pharynx, or throat, and into the esophagus, a tube that extends from the bottom of the throat to the top of the stomach. (Note that for the most part, we are using human anatomy as a guide, but many aspects of the digestive process described here also apply to other higher animals, particularly mammals.) The esophagus does not take part in digestion but rather performs the function of moving the bolus into the stomach.

A wavelike muscular motion termed peristalsis, which consists of alternating contractions and relaxations of the smooth muscles lining the esophagus, moves the bolus through this passage. At the place where the esophagus meets the stomach, a powerful muscle called the esophageal sphincter acts as a valve to keep food and stomach acids from flowing back into the esophagus and mouth. (Although the most well-known sphincter muscle in the body is the one surrounding the anus, sometimes known simply as " the sphincter," in fact, sphincter is a general term for a muscle that surrounds, and is able to control the size of, a bodily opening.)

FROM THE STOMACH TO THE SMALL INTESTINE.

Chemical digestion begins in the stomach, a large, hollow, pouchlike muscular organ. While food is still in the mouth, the stomach begins its production of gastric juice, which contains hydrochloric acid and pepsin, an enzyme that digests protein. Gastric juice is the material that breaks down the food. Once nerves in the cheeks and tongue are stimulated by the food, they send messages to the brain, which, in turn, alerts nerves in the stomach wall, stimulating the secretion of gastric juice before the bolus itself arrives in the stomach. Once the bolus touches the stomach lining, it triggers a second release of gastric juice, along with mucus that helps protect the stomach lining from the action of the hydrochloric acid. Three layers of powerful stomach muscles churn food into a thick liquid called chyme, which is pumped gradually through the pyloric sphincter, which connects the stomach small intestine.

THE SMALL INTESTINE.

The names of the small and large intestines can be confusing, rather like those of Upper and Lower Egypt in ancient history. In both cases, the adjectives seem to refer to one thing but actually refer to something else entirely. Thus, it so happens that Upper Egypt was south of Lower Egypt (because it was "upper" in elevation, not latitude), while the small intestine is, in fact, much longer than the large intestine. The reason is that small refers to its diameter rather than its length: though it is about 23 ft. (7 m) long, the small intestine is only 1 in. (2.5 cm) in diameter, while the large intestine, only 5 ft. (1.5 m) in length, is 3 in. (7.6 cm) across.

The small intestine, which connects the stomach and large intestine, is in three sections: the duodenum, jejunum, and ileum. About 1 ft. (0.3 m) long, the duodenum breaks down chyme from the stomach with the aid of the pancreas and gallbladder. The pancreas, a large gland located below the stomach, secretes pancreatic juice, which contains three enzymes that break down carbohydrates, fats, and proteins, into the duodenum through the pancreatic duct. The gallbladder empties bile, a yellowish or greenish fluid from the liver, into the duodenum when chyme enters that portion of the intestine. Although bile does not contain enzymes, it does have bile salts that help dissolve fats.

Digested carbohydrates, fats, proteins, and most of the vitamins, minerals, and iron in food are absorbed in the jejunum, which is about 4 ft. (1.2 m) long. Aiding this absorption are up to five million tiny finger-like projections called villi, which greatly increase the surface area of the small intestine, thus accelerating the rate at which nutrients are absorbed into the bloodstream. The remainder of the small intestine is taken up by the ileum, which is smaller in diameter and has thinner walls than the jejunum. It is the final site for absorption of some vitamins and other nutrients, which enter the circulatory system in plasma, a watery liquid in which red blood cells also are suspended.

As it moves through the circulatory system, plasma takes with it amino acids, enzymes, glycerol (a form of alcohol found in fats), and fatty acids, which it directs to the body's tissues for energy and growth. Plasma also contains waste products from the breakdown of proteins, including creatinine, uric acid, and ammonium salts. These constituents are moved to the kidneys, where they are filtered from the blood and excreted in the urine. But, of course, urine is not the only waste product excreted by the body; there is also the solid waste, processed through the large intestine, or colon.

THE LARGE INTESTINE AND BEYOND.

Like the small intestine, the large intestine is in segments. It rises up on the right side of the body (the ascending colon), crosses over to the other side underneath the stomach (the transverse colon), descends on the left side, (the descending colon), and forms an S shape (the sigmoid colon) before reaching the rectum and anus. In addition to its function of pumping solid waste, the large intestine removes water from the waste products—water that, when purified, will be returned to the bloodstream. In addition, millions of bacteria in the large intestine help produce certain B vitamins and vitamin K, which are absorbed into the bloodstream along with the water.

After leaving the sigmoid colon, waste passes through the muscular rectum and then the anus, the last point along the alimentary canal. In all, the movement of food through the entire length of the alimentary tract takes from 15 to 30 hours, with the majority of that time being taken up by activity in the colon. Food generally spends about three to five hours in the stomach, another four to five hours in the small intestine, and between five and 25 hours in the large intestine.

The transit time, or the amount of time it takes for food to move through the system, is a function of diet: for a vegetarian who eats a great deal of fiber, it will be on the short end, while for a meat eater who has just consumed a dinner of prime rib, it will take close to the maximum time. People who eat diets heavy in red meat or junk foods are also likely to experience a buildup, over time, of partially digested material on the linings of their intestines. Obviously, this is not a healthy situation, and to turn it around, a person may have to change his or her diet and perhaps even undergo some sort of colon-cleansing program. There is an easy way to test transit time in one's system: simply eat a large serving of corn or red beets, and measure how long it takes for these to fully work their way through the digestive system.

REAL-LIFE APPLICATIONS

Digestive Disorders

It is hard to watch more than a few minutes of commercial television without seeing advertisements for fast foods and other varieties of junk food or stomach-relief medicine or both. There is a connection, of course: a society glutted on greasy drive-through burgers and thick-crust pizzas needs something to cure the upset stomachs that result.

Indigestion is a general condition that, as its name suggests, involves an inability to digest food properly. Heartburn, sometimes called acid indigestion, is a specific type of indigestion that occurs when the stomach produces too much hydrochloric acid. The latter is essential to digestion, but if a person eats a giant Polish sausage, a spicy-hot bowl of jambalaya, or some other hard-to-digest food (as opposed to a healthy meal of baked fish with brown rice and spinach, for instance), the stomach may produce too much of the acid.

Heartburn is so named because it causes a sharp pain behind the breastbone, which might feel like a heart attack. It also may produce acid reflux, in which the stomach acid backs up into the esophagus. If you have ever experienced what might be called a leap of vomit, in which a burp is associated with the rise of burning, foul-tasting bile through the esophagus, then you have firsthand knowledge of acid reflux and heartburn.

Digestive tract diseases, such as dyspepsia, sometimes can cause chronic indigestion, but more often than not, people experience indigestion as a result of eating too quickly or too much, consuming high-fat foods, or eating in a stressful situation. (This is why you might feel sick to your stomach when eating lunch at school on the day of a difficult test, a fight, or a romantic trauma, such as a breakup or asking for a first date.) Smoking, excessive drinking, fatigue, and the consumption of medications that irritate the stomach lining also can contribute to indigestion. In addition, it is a good idea not to eat too soon before going to bed, since this can produce heartburn.

COMMERCIAL ANTACIDS.

Most nonprescription stomach-relief medicine is in the form of an antacid, which, as the term suggests, is a substance that works against acids in the stomach. Chemically, the opposite of an acid is a base, or an alkaline substance, the classic example being sodium bicarbonate or sodium hydrogen carbonate (NaHCO₃)—that is, baking soda. Baking soda alone can perform the function of an antacid, but the taste is rather unpleasant, and for this reason most antacid products combine it with other chemicals to enhance the flavor.

One famous commercial stomach remedy actually uses acid. This is Alka-Seltzer, but the presence of citric acid has more to do with marketing than with the chemistry of the stomach. The citric acid, often used as a sweetener, imparts a more pleasant flavor than the bitter taste of alkaline antacids, and, moreover, when Alka-Seltzer tablets are placed in water, the acid reacts chemically with the sodium bicarbonate to create the product's trademark fizz.

Ultimately, all antacids (Alka-Seltzer included) work because the bases in the product react with the acids in the stomach. This is a chemical process called neutralization, in which the acid and base cancel out each other, producing water and a salt in the process. (Table salt, or sodium chloride, is just one of many salts, all of which are formed by the chemical bonding of a metal with a nonmetal—in the case of table salt, sodium and chlorine, respectively.) Thanks to this process, acid in the stomach of a heartburn sufferer is neutralized.

ULCERS.

There are some digestive disorders that cannot be cured by Alka-Seltzer or its many competitors, such as Rolaids, Maalox, Mylanta, Tums, Milk of Magnesia, or Pepto-Bismol. Instead of just occasional indigestion or heartburn, a person may be afflicted with a sore in one part of the digestive tract, which may be either a stomach ulcer or a duodenal ulcer. Stomach ulcers, which form in the lining of the stomach, are called peptic ulcers because they form with the help of stomach acid and pepsin. Duodenal ulcers, which are more common, tend to be smaller than stomach ulcers and heal more quickly. Any ulcer, whether a small sore or a deep cavity, leaves a scar in the alimentary canal.

Until the early 1990s, physicians generally maintained that personal behavior and conditions, such as stress and poor diet, were the principal factors behind ulcer. Medical researchers eventually came to believe, however, that the culprit was a certain bacterium, which can live undetected in the mucous lining of the stomach. This bacterium irritates and weakens the lining, making it more susceptible to damage by stomach acids. As many as 80% of all stomach ulcers may be caused by such a bacterial infection. With this newfound knowledge, ulcer patients today are more likely to be treated with antibiotics and antacids rather than special diets or expensive medicines.

Ways to Improve Digestion

There are numerous ways to improve digestion, by changing either the way one eats or the things one eats. In the first category, it is important to eat only when you are really hungry and to eat slowly and chew food thoroughly. Drinking liquids with a meal is probably not a good practice; it is better to wait until you are finished, so as not to interfere with the action of digestive fluids. Certainly, smoking and excessive drinking have a negative impact on digestion, whereas regular exercise has a positive influence.

In the category of diet, it is a good idea to minimize one's intake of red meat, such as steak. Although most people find red meat tasty, and it can be a good supplier of dietary iron in limited proportions, the digestion of red meat requires the production of much more stomach acid, and thus it places a great burden on the digestive system. It is also wise to eliminate as many processed foods as possible, including sweets and junk foods, and to eat as many natural foods as one can manage.

In general, one can hardly go wrong with raw vegetables, which are just about the best thing a person can eat—not only because of their digestive properties but also because many of them are packed so full of vitamins and minerals. (Note that vegetables are best when raw and fresh, since cooking removes many of the nutrients. Canned vegetables usually are both nutrient-poor and full of sodium or even synthetic chemicals. Frozen vegetables are much better than canned ones, but they still do not compare nutritionally with fresh vegetables.)

A good diet includes a great deal of fiber, indigestible material that simply passes through the system, assisting in the peristaltic action of the alimentary canal and in the process of eliminating waste. Cellulose, found in most raw fruits and vegetables, is an example of fiber, also called bulk or roughage. Yogurt may be a beneficial food, because it includes "good" bacteria (a topic we discuss near the conclusion of this essay) that assist the digestive process. Some foods, such as raw bean sprouts, papaya, figs, and pineapple, contain enzymes that appear to assist the body in digesting them.

In addition, one of the greatest "foods" for aiding digestion is not a food at all, but water, of which most people drink far too little. Some experts claim that a person should drink eight 8 oz. (0.24 l) glasses a day, but others maintain that a person should drink half as many ounces of water as his or her weight in pounds. In other words, a person who weighed 100 lb. (45.36 kg) would drink 50 oz. (1.48 l) of water a day, whereas a person who weighed 150 lb. (68.04 kg) would drink 75 oz. (2.22 l). A good rule of thumb for metric users, instead of the 2:1 pounds-to-ounces ratio, would be 30:1 kilograms to liters. Note also that tap water may contain chemicals or other impurities, and therefore consuming it in large quantities is not advisable. A much better alternative is bottled or filtered water.

Human Waste

Many years ago, both a serious book and a comedic movie had the title Everything You Always Wanted to Know about Sex (But Were Afraid to Ask). Just as the title of David R. Reuben's 1969 book inspired Woody Allen's 1972 movie, which was very loosely based on it, the "Everything you always wanted to know … " motif inspired a whole array of imitators. Often such titles play on the very fact that hardly anyone wants to know, and certainly no one is afraid to ask, about the topic in question. A good example is an on-line article by central Asia authority Mark Dickens entitled "Everything You Always Wanted to Know about Tocharian But Were Afraid To Ask" (http://www.oxuscom.com/eyawtkat.htm). The joke, of course, is that most people have never heard of Tocharian, a central Asian language. Nonetheless, one subject ranks with sex as something everyone wonders about but most are afraid to ask.

Even the name of that topic creates problems, since people have so many euphemisms for it: "number two," for instance, or BM (short for bowel movement). There are baby-and child-oriented terms for this process and product, the bodily control of which can be a major problem for a very young human being, and, of course, there is at least one grown-up term for it that will not be mentioned here. People even have nicknames for animal dung, such as pies, patties, or chips. For the sake of convenience, let us call the process defecation, and the product human waste (or excrement or feces) and admit that everyone has wondered how something as pleasant as food can, after passing through the alimentary canal, turn into something as unpleasant as the final product.

The average person excretes some 7 lb. (3.2 kg) of feces per day, an amount equal to a little more than 1 ton (0.91 tonnes) per year. This waste is made up primarily of indigestible materials as well as water, salts, mucus, cellular debris from the intestines, bacteria, and cellulose and other types of fiber. Like the human body itself, these waste products are mostly water: about 75%, compared with 25% solid matter. Much of what goes into producing excrement has nothing to do with what enters the digestive system, so even if a person were starving he or she would continue to excrete feces.

COLORATION.

What about the color and the smell? The color of feces comes from bilirubin, a reddish-yellow pigment found in blood and bile, which passes through the liver and enters the small intestine via the gallbladder. Later, as it passes through the large intestine, it is degraded by the action of bacteria, a process that turns it brown and gives feces its characteristic color.

Not surprisingly, disorders involving the red blood cells, liver, or gallbladder can change the color of human waste. A person with gallstones or hepatitis (a disease characterized by inflammation of the liver) is likely to excrete grayish-brown feces, while anemia (a condition that involves a lack of red blood cells) may be associated with a yellowish stool. A person experiencing bleeding in the gastrointestinal tract (the stomach and intestines) may produce waste the color of black tar. In addition, foods with distinctive colors and textures also can affect theappearance of stools.

FRIENDLY BACTERIA.

Beforeaddressing the smell of feces, which is the result of action by bacteria in the colon, it is worth saying a few words about those single-cell organisms themselves. This is especially important inlight of the fact that bacteria have a bad reputation that is not entirely deserved. Without question, there are harmful microbes in the world, but a world completely free of these organisms would be one in which humans and other animals would be unable to live. In fact, we have amutually beneficial relationship, a type of symbiosis (see Symbiosis) with the microorganisms in our alimentary canals, particularly in the colon.

Bacteria live in the guts—a term that refers to all or part of the alimentary canal—of most animals, where they assist in such difficult digestiveactivities as the processing of chewed grasses. The latter is heavy in cellulose, and to digest it, cows, sheep, deer, and other grass eaters (known as ruminants) have stomachs with several compartments. The first of these compartments is called therumen, and it serves as home to millions of bacteria, which assist in breaking down the heavy fibers.

Humans' bacterial symbiotic partners (actually, this is a type of symbiosis known as mutualism, in which both creatures benefit) include bacteria of the species Escherichia coli, or E. coli. The name is no doubt familiar to most readers from its appearance in the news in connection with horror stories involving E. coli poisoning in food or local water supplies. Certainly, E. coli can be extremely harmful when it is outside the human gut, but inside the gut it is humans' friend.

E. coli is a coprophile (literally, "excrement lover"), meaning that it depends on feces for survival. Fecal matter itself can contain all manner of harmful substances associated with the decomposition of foods or with the body's efforts to rid itself of toxins (including pathogens, or disease-carrying parasites—see Parasites and Parasitology), so anything associated with feces is dirty and potentially dangerous. It is for this reason that E. coli can cause serious illness or death if it gets into other parts of the body.

As long as it stays where it belongs, however, E. coli not only aids in the digestive process but also provides the body with vitamin K, essential for proper blood clotting, as well as vitamin B₁₂, thiamine, and riboflavin. Every person carries millions and millions of these helpful fellow travelers; even though a single bacterium weighs almost nothing in human terms, the combined weight of all the helpful, "good" bacteria in our guts is a staggering 7 lb. (3.2 kg).

INTESTINAL GAS.

As those bacteria do their work, they generate vast quantities of gases, which are by-products of the chemical processes that play a part in breaking down the foods passing through the gut. Among these gaseous products are hydrogen sulfide, a foul-smelling substance that can be toxic in large quantities. Unlike carbon monoxide, which has no odor, few people are in danger of dying from inhalation of hydrogen sulfide—even though it is abundant in nature—because the smell is enough to dissuade anyone from inhaling it for long periods of time. (Incidentally, gas companies include traces of hydrogen sulfide with natural gas. By itself, natural gas is odorless, but when a leak occurs, a homeowner will smell hydrogen sulfide and alert the gas company.)

Hydrogen sulfide is just one of many unpleasant-smelling chemical products that result from bacterial action on solids in the gut. Others include indole, skatole, ammonia, and mercaptans, though the most distinctive-smelling of all are indole and skatole, which come primarily from the digestion of an amino acid known as tryptophan. In addition, the particular foods a person eats, as well as the specific bacterial residents (some harmful) of his or her gut, can affect the odor of intestinal gas.

When gases pass outside the rectum, the result is flatulence (of course, there are other, less polite words for it), which is the subject of much schoolboy humor. Even inside the body, intestinal gas can make noise and cause embarrassment, in the form of borborygmus—intestinal rumbling caused by moving gas. As for the flammability of intestinal gas, it probably results from the high proportion of hydrogen, an extremely flammable gas.

WHERE TO LEARN MORE

Avraham, Regina. The Digestive System. New York: Chelsea House Publishers, 1989.

Ballard, Carol. The Stomach and Digestive System. Austin, TX: Raintree Steck-Vaughn, 1997.

Digestive System Diseases. Karolinska Institutet (Web site). <http://www.mic.ki.se/Diseases/c6.html>.

The Human Body's Digestive System Theme Page. Community Learning Network (Web site). <http://www.cln.org/themes/digestive.html>.

Medline Plus: Digestive System Topics. National Library of Medicine, National Institutes of Health (Web site). <http://www.nlm.nih.gov/medlineplus/digestivesystem.html>.

Morrison, Ben. The Digestive System. New York: Rosen Publishing Group, 2001.

National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health (Web site). <http://www.niddk.nih.gov/index.htm>

Parker, Steve, and Ian Thompson. Digestion. Brookfield, CT: Copper Beech Books, 1997.

Pathophysiology of the Digestive System. Colorado State University (Web site). <http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/>.

Richardson, Joy. What Happens When You Eat? Illus. Colin Maclean and Moira Maclean. Milwaukee, WI: Gareth Stevens Publishing, 1986.

KEY TERMS

ALIMENTARY CANAL:

The entire length of tube that extends from the mouth to the anus, including the esophagus, stomach, and small and large intestines. Nutrientspass through the alimentary canal to the stomach and small intestine, and waste materials from these nutrients (and from other sites in the body) pass from the small intestine to the colon (large intestine) and anus.

AMINO ACIDS:

Organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations. Strings of amino acids make up proteins.

BILE:

A yellowish or greenish digestive fluid excreted by the liver.

BOLUS:

A term for a chewed mass of food making its way through the initial portions of the alimentary canal.

CARBOHYDRATES:

Naturally occurring compounds, consisting of carbon, hydrogen, and oxygen, whose primary function in the body is to supply energy. Included in the carbohydrate group aresugars, starches, cellulose, and various other substances.

CELLULOSE:

A polysaccharide that is the principal material in the cell walls ofplants. Cellulose also is found in such natural fibers as cotton and is used as a raw material in manufacturing such products as paper.

COLON:

The large intestine, through which waste materials pass on their way to excretion through the anus.

COMPOUND:

A substance in which atoms of more than one element are bonded chemically to one another.

ENZYME:

A protein material that speeds up chemical reactions in the bodies of plants and animals.

FIBER:

Indigestible material in food that simply passes through the digestivesystem, assisting in the peristaltic action of the alimentary canal and in the processing of waste. Examples of fiber, also called bulk or roughage, include cellulose.

GASTROINTESTINAL TRACT:

The stomach and intestines.

GLAND:

A cell or group of cells that filters material from the blood, processes that material, and secretes it either for use again in the body or to be eliminated as waste.

GLUCOSE:

A type of sugar that occurs widely in nature. Glucose is the form in which animals usually receive carbohydrates.

GLYCOGEN:

A white polysaccharide that is the most common form in which carbohydrates are stored in animal tissues, particularly muscle and liver tissues.

GUT:

A term that refers to all or part of the alimentary canal. Although the word is considered a bit crude in everyday life, physicians and biological scientists concerned with this part of the anatomy use itregularly.

HEMOGLOBIN:

An iron-containing pigment in red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide from them.

LIPIDS:

Fats and oils, which dissolve in oily or fatty substances but not in water-based liquids. In the body, lipids supply energy in slow-release doses, protect organs from shock and damage, and provide insulation for the body, for instance, from toxins.

METABOLISM:

The chemical process by which nutrients are broken down and converted into energy or used in the construction of new tissue or other material in the body.

MINERALS:

Inorganic substances that, in a nutritional context, serve a function similar to that of vitamins. Minerals may include chemical elements, particularly metallic ones, such as calcium or iron, as well as some compounds.

ORGAN:

A group of tissues and cells, organized into a particular structure, that performs a specific function within anorganism.

ORGANIC:

At one time, chemists used the term organic only in reference to living things. Now the word is applied to compounds containing carbon and hydrogen.

PERISTALSIS:

A series of involuntary muscle contractions that force bolus, and later waste, through the alimentary canal.

POLYSACCHARIDE:

A complexsugar, in which the molecules are composed of many glucose subunits arranged in a chain. Polysaccharides can be broken down chemically to produce simple sugars, or monosaccharides.

PROTEINS:

Large molecules built from long chains of amino acids. Proteins serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body's immune system, and making enzymes.

SPHINCTER:

A general term for a muscle that surrounds and is able to control the size of a bodily opening.

SYMBIOSIS:

A biological relationship in which (usually) two species live in close proximity to each other and interact regularly in such a way as to benefit one or both of the organisms.

TISSUE:

A group of cells, along with the substances that join them, that forms part of the structural materials in plants oranimals.

VITAMINS:

Organic substances that, in extremely small quantities, are essential to the nutrition of most animals and some plants. In particular, vitamins work with enzymes in regulating metabolic processes; they do not in themselves provide energy, however, and thus vitamins alone do not qualify as a form of nutrition.

DIGESTION

DIGESTION. Digestion can occur at many levels in the body; generally, it refers to the breakdown of macro-molecules or a matrix of cells, or tissues, into smaller molecules and component parts. This particular section will focus on digestion of food in the gastrointestinal tract: the process that is required to obtain essential nutrients from the food we eat. The gastrointestinal tract (GIT) is a highly specialized organ system that allows humans to consume food in discrete meals as well as in a very diverse array of foodstuffs to meet nutrient needs. Figure 1 contains a schematic of the GIT and illustrates the organs of the body with which food comes into contact during its digestion. These organs include the mouth, esophagus, stomach, small intestine, and large intestine; in addition, the pancreas and liver secrete into the intestine. The system is connected to the vascular, lymphatic, and nervous systems; however, the function of these systems in gastrointestinal physiology is beyond the scope of this article, which focuses primarily on the process of breaking down macromolecules and the matrix of food.

Mechanical Aspects of Digestion

Food is masticated in the mouth. Chewing breaks food into smaller particles that can mix more readily with the GIT secretions. In the mouth, saliva lubricates the food bolus so that it passes readily through the esophagus to the stomach. The sensory aspects of food stimulate the flow of saliva, which not only lubricates the bolus of food but is protective and contains digestive enzymes. Swallowing is regulated by sphincter actions to move the bolus of food into the stomach. The motility of the stomach continues the process of mixing food with the digestive secretions, now including gastric juice, which contains acid and some digestive enzymes. The action of the stomach continues to break down food into smaller particles prior to passage to the intestine. The mixture of food and digestive juices is referred to as digesta, or chyme. The stomach, which after a meal may contain more than a liter of material, regulates the rate of digestion by metering chyme into the small intestine over several hours. Several factors can slow the rate of gastric emptying; for example, solids take longer to empty than liquids, mixtures relatively high in lipid take longer to empty, and viscous, or thick, mixtures take longer to empty than watery, liquid contents.

In the upper part of the small intestine, the duodenum, receptors appear to influence the rate of gastric emptying either through hormonal or neural signals. Peristaltic motor activity in the small intestine propels chyme along the length of the intestine, and segmentation allows mixing with digestive juices in the intestine, which include pancreatic enzymes, bile acids, and sloughed intestinal cells. Digestion of macronutrients, which began in the mouth, continues in the small intestine, where the intestinal surface provides an immense absorptive surface to allow absorption of digested molecules into circulation. While the intestine from the outside appears to be a tube, the lining of the inner surface contains tissue folds and villi that are lined with intestinal cells, each with microvilli, or a brush border, which greatly amplify the absorptive surface. The intestinal cells can absorb compounds by several cell membrane–mediated transport mechanisms and then transform them into compounds, or complexes, that can enter circulation through the blood, or lymphatic, system.

What is not digested and absorbed passes into the large intestine. In this organ, water and electrolytes are reabsorbed, and the movements of the large intestine allow mixing of the contents with the microflora of bacteria and other microbes that are naturally present in the large intestine. These microbes continue the process of digesting the chyme. Eventually the residue enters the rectum and the anal canal, and stool is formed, which is defecated. Transit time of a non-digestible marker from mouth to elimination in the stool varies considerably: normal transit time is typically twenty-four to thirty-six hours, but can be as long as seventy-two hours in otherwise healthy individuals.

Breakdown of Macromolecules in Foods

Foods are derived from the tissues of plants and animals as well as from various microorganisms. For absorption of nutrients from the gut to occur, the cellular and molecular structure of these tissues must be broken down. The mechanical actions of the GIT help disrupt the matrix of foods, and the macromolecules, including proteins, carbohydrates and lipids, are digested through the action of digestive enzymes. This digestion produces smaller, lower molecular weight molecules that can be transported into the intestinal cells to be processed for transport in blood, or lymph.

Proteins are polymers of amino acids that in their native structure are three-dimensional. Many cooking or processing methods denature proteins, disrupting their tertiary structure. Denaturation, which makes the peptide linkages more available to digestive enzymes, is continued in the stomach with exposure to gastric acid. In addition, digestion of the peptide chain begins in the stomach with the enzyme pepsin. Once food enters the small intestine, enzymes secreted by the pancreas continue the process of hydrolyzing the peptide chain either by cleaving amino acids from the C-terminal end, or by hydrolyzing certain peptide bonds along the protein molecule. The active forms of the pancreatic enzymes include trypsin, chymotrypsin, elastase, and carboxypeptidase A and B. This process of protein digestion produces small peptide fragments and free amino acids. The brush border surface of the small intestine contains peptidases, which continue the digestion of peptides, either to smaller peptide fragments or free amino acids, and these products are absorbed by the intestinal cells.

Carbohydrates are categorized as digestible or non-digestible. Digestible carbohydrates are the various sugar-containing molecules that can be digested by amylase or the saccharidases of the small intestine to sugars that can be absorbed from the intestine. The predominant digestible carbohydrates in foods are starch, sucrose, lactose (milk sugar), and maltose. Glycogen is a glucose polymer found in some animal tissue; its structure is similar to some forms of starch. Foods may also contain simple sugars such as glucose or fructose that do not need to be digested before absorption by the gut. Alpha amylase, which hydrolyzes the alpha one to four linkages in starch, is secreted in the mouth from salivary glands and from the pancreas into the small intestine. The action of amylase produces smaller carbohydrate segments that can be further hydrolyzed to sugars by enzymes at the brush border of the intestinal cells. This hydrolysis step is closely linked with absorption of sugars into the intestinal cells.

Non-digestible carbohydrates cannot be digested by the enzymes in the small intestine and are the primary component of dietary fiber. The most abundant polysaccharide in plant tissue is cellulose, which is a glucose polymer with beta one to four links between the sugars. Amylase, the starch-digesting enzyme of the small intestine, can only hydrolyze alpha links. The non-digestible carbohydrates also include hemicelluloses, pectins, gums, oligofructose, and inulin. While non-digestible, they do affect the digestive process because they provide bulk in the intestinal contents, hold water, can become viscous, or thick, in the intestinal contents, and delay gastric emptying. In addition, non-starch polysaccharides are the primary substrate for growth of the microorganisms in the large intestine and contribute to stool formation and laxation. Products of microbial action include ammonia, gas, and short-chain fatty acids (SCFA). SCFA are used by cells in the large intestine for energy and some appear in the circulation and can be used by other cells in the body for energy as well. Thus, while dietary fiber is classified as non-digestible carbohydrate, the eventual digestion of these polysaccharides by microbes does provide energy to the body. Current research is focused on the potential effect of SCFA on the health of the intestine and their possible role in prevention of gastrointestinal diseases.

For dietary lipids to be digested and absorbed, they must be emulsified in the aqueous environment of the intestinal contents; thus bile salts are as important as lipolytic enzymes for fat digestion and absorption. Dietary lipids include fatty acids esterified to a glycerol backbone (mono-, di-or triglycerides); phospholipids; sterols, which may be esterified; waxes; and the fat-soluble vitamins, A, D, E, and K. Digestion of triglycerides (TG), phospholipids (PL), and sterols illustrate the key factors in digestion of lipids. Lipases hydrolyze ester bonds and release fatty acids. In TG and PL, the fatty acids are esterified to a glycerol backbone, and in sterols, to a sterol nucleus such as cholesterol. Lipases that digest lipids are found in food, and are secreted in the mouth and stomach and from the pancreas into the small intestine. Lipases in food are not essential for normal fat digestion; however, lipase associated with breast milk is especially important for newborn infants. In adults the pancreatic lipase system is the most important for lipid digestion. This system involves an interaction between lipase, colipase, and bile salts that leads to rapid hydrolysis of fatty acids from TG. An important step in the process is formation of micelles, which allows the lipid aggregates to be miscible in the aqueous environment of the intestine. In mixed micelles, bile salts and PL function as emulsifying agents and are located on the surface of these spherical particles. Lipophilic compounds such as MG, DG, free sterols, and fatty acids, as well as fat-soluble vitamins, are in the core of the particle. Micelles can move lipids to the intestinal cell surface, where the lipids can be transported through the cell membrane and eventually packaged by the intestinal cells for transport in blood or lymph. Most absorbed lipid is carried in chylomicrons, large lipoproteins that appear in the blood after a meal and which are cleared rapidly in healthy individuals. Bile salts are absorbed from the lower part of the small intestine, returned to the liver, and resecreted into the intestine, a process referred to as enterohepatic circulation. It is important to note that bile salts are made from cholesterol, and drugs such as cholestyramine or diet components such as fiber that decrease the amount of bile salt reabsorbed from the intestine help to lower plasma cholesterol concentrations.

Regulation of Gastrointestinal Function

Regulation of the gastrointestinal response to a meal involves a complex set of hormone and neural interactions. The complexity of this system derives from the fact that part of the response is directed at preparing the GIT to digest and absorb the meal that has been consumed in an efficient manner and also at signaling short-term satiety so that feeding is terminated at an appropriate point. Traditionally, physiologists have viewed the regulation in three phases: cephalic, gastric, and intestinal. In the cephalic phase, the sight, smell, and taste of foods stimulates the secretion of digestive juices into the mouth, stomach, and intestine, essentially preparing these organs to digest the foods to be consumed. Experiments in which animals are sham fed so that food consumed does not actually enter the stomach or intestine demonstrate that the cephalic phase accounts for a significant portion of the secretion into the gut. The gastric and intestinal phases occur when food and its components are in direct contact with the stomach or intestine, respectively. During these phases, the distension of the organs with food as well as the specific composition of the food can stimulate a GIT response.

The GIT, the richest endocrine organ in the body, contains a vast array of peptides; however, the exact physiological function of each of these compounds has not been established. Five peptides, gastrin, cholecystokinin (CCK), secretin, gastric inhibitory peptide (GIP), and motilin are established as regulatory hormones in the GIT. Multiple aspects have been investigated to understand their release and action. For example, CCK is located in the upper small intestine; protein and fat stimulate its release from the intestine, while acid inhibits its secretion. Once released, it can inhibit gastric emptying and stimulate secretion of acid and pancreatic juice and contraction of the gall bladder. In addition, it stimulates motility and growth in the GIT and regulates food intake and insulin release. Among the other established gastrointestinal peptides, secretin stimulates secretion of fluid and bicarbonate from the pancreas, gastrin stimulates secretion in the stomach, GIP inhibits gastric acid secretion, and motilin stimulates the motility of the upper GIT. In addition to investigating the various factors causing release of these hormones and the response to them, physiologists are also interested in the interactions among hormones as well as those with the nervous system, since the response to a meal involves release of many factors.

Obtaining food and digesting it efficiently are paramount to survival. The human GIT system most likely evolved during the period when the species acquired its food primarily through hunting and gathering. The over-lapping regulatory systems, combined with an elevated capacity to digest food and absorb nutrients, insured that humans used food efficiently during periods in which scarcity might occur.

See also Eating; Intestinal Flora; Microbiology .

BIBLIOGRAPHY

Cordian, L. "Cereal Grains: Humanity's Double-edged Sword." World Review of Nutrition and Dietetics 84 (1999): 19–73.

Johnson, Leonard R., ed. Gastrointestinal Physiology. St. Louis, Mo.: Mosby, 1997.

Barbara O. Schneeman

Digestion

Digestion is the process of breaking down food into molecules that cells can absorb. Carbohydrates, proteins, nucleic acids, and fats are broken down into their smallest units (monomers) by digestive enzymes. These hydrolytic enzymes break chemical bonds through a reaction that requires water. Each hydrolytic enzyme is named after the substances it hydrolyzes. For example, carbohydrases break carbohydrates into single sugars (monosaccharides), proteases break proteins into amino acids, nucleases break nucleic acids into nucleotides, and lipases hydrolyze fats to fatty acids.

The Digestive System

The digestive system of humans consists of a one-way digestive tract with several specialized chambers along the way—mouth, stomach, small intestine, and large intestine. Each chamber has a specific function. In the human digestive system, digestion starts in the mouth. There food is physically digested by the action of our jaws and teeth, which break it down into smaller pieces, increasing the surface area for the digestive enzymes to work on. While being chewed, the food is mixed with saliva. Saliva makes the food slippery for swallowing, and also contains the enzyme amylase, a carbohydrase that breaks down starch into smaller polysaccharides. Once food is swallowed it passes along the digestive system through the activity of peristalsis (wavelike smooth muscle contractions). The passage of food from one chamber to the next is regulated by sphincters, or ringlike muscles.

Once the food arrives in the stomach, it is mixed with gastric juice, which contains hydrochloric acid (HCL) and pepsin. HCL kills most swallowed bacteria, and breaks down most food into individual cells, further increasing the surface area for enzyme attack. Pepsin (a protease) begins the hydrolysis of proteins into smaller polypeptides. The main site of human digestion and absorption of nutrients is the small intestine. There accessory glands of the digestive system, such as the pancreas and the liver, secrete their products into the digestive tract. The pancreas secretes bicarbonate ions that neutralize the acid from the stomach, protecting the digestive enzymes of the small intestine. The pancreas also releases a carbohydrase (pancreatic amylase), which continues the carbohydrate digestion started by salivary amylase in the mouth. The resulting disaccharides (e.g., maltose) are further hydrolyzed into monosaccharides (e.g., glucose) by enzymes (e.g., maltase), which are built into the membranes of cells lining the small intestine. This is also where sugar is absorbed from the lumen of the digestive tract into the cells lining the digestive tract.

Other enzymes that are produced and released by the pancreas are pro-teases (e.g., trypsin), nucleases, and lipases. Proteases attached to the membrane of cells lining the walls of the digestive tract are responsible for hydrolysis of small peptides to amino acids (which are then absorbed), and attached nucleases process nucleotides whose components are also taken up.

For the pancreatic lipases to be efficient, the lipid clumps that form in the watery environment of the digestive tract have to be broken into tiny droplets, which will increase the surface area for the lipases to work on. This is the function of bile salts, which are produced by the liver and stored in the gall bladder before being released into the small intestine. The fatty acids resulting from the action of the lipases are then absorbed.

While digestion and absorption of nutrients are taking place in the small intestine, peristalsis slowly pushes the content of the small intestine into the large intestine, where water and ion absorption are taking place. In the large intestine, populations of bacteria live on material that is not digestible by humans. As a byproduct of their metabolism, they produce gas as well as vitamins such as vitamin K that can be absorbed.

Control and Digestion

How is it possible to digest food molecules and not the cells of the digestive system itself, which is made of the same molecules? Most enzymes are produced in an inactive form called zymogens, that do not affect the cells that produce them. These zymogens are then released into the digestive system where they are activated. The cells lining the digestive tract are protected from the active enzymes by a thick layer of mucus. However, the mucus lining is constantly eroded, and if the lining is eroded faster (e.g., by acid-resistant bacteria) than it is regenerated, stomach ulcers may occur.

To make efficient use of all the nutrients contained in food, control and coordination of the digestion process is crucial. This is accomplished by several negative feedback systems. For example, when we see, smell or taste food, our brain signals the stomach to secrete gastric juice. When food proteins are actually present in the stomach, they trigger the release of the hormone gastrin into the blood stream. This hormone causes the cells in the stomach wall to release even more gastric juice, ensuring that proteins in the stomach are properly predigested. If the stomach becomes too acidic, the release of gastrin—and thus the production of gastric juice—ceases. When the acidic content of the stomach enters the small intestine, another hormone, secretin, is released into the blood stream from the cells lining the small intestine. Secretin causes the pancreas to dump bicarbonate from the pancreas into the small intestine, buffering the acid. When amino acids or fatty acids are detected in the small intestine, another hormone, choleocystokinin (CCK), is released from the intestinal cells into the blood. CCK triggers the gall bladder to release bile and the pancreas to release its digestive enzymes. At the same time, CCK inhibits the peristalsis of the stomach thus slowing down food transport. This allows enough time for the digestion of the food already present in the small intestine before more food from the stomach arrives.

see also Digestive System; Homeostasis.

Kathrin F. Stanger-Hall

Bibliography

Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology, 5th ed. Menlo Park, CA: Addison Wesley Longman, Inc., 1999.

Eckert, Roger, David Randall, and George Augustine. Animal Physiology, 3rd ed. New York: W. H. Freeman and Company, 1988.

Kapit, Wynne, Robert I. Macey, and Esmail Meisami. The Physiology Coloring Book. Cambridge, MA: Harper Collins Publishers, 1987.

Purves, William K., Gordon Orian, and Craig H. Heller. Life: The Science of Biology,4th ed. Sunderland, MA: Sinauer Associates, 1994.

Digestion

Digestion breaks down foods into nutrient molecules that are small enough to be absorbed into an animal's circulatory system. Following digestion, nutrients are delivered to cells, where energy is extracted from their chemical bonds. Digestion often begins with a mechanical tearing apart of food into smaller pieces, which are then chemically dismantled in a stepwise fashion.

Because digestive chemicals are harsh, food processing in an animal's body takes place in compartments. Some single-celled organisms, such as protista, sequester food particles in food vacuoles, where enzymes break them down. Simple multicellular organisms, such as hydra and flatworms, have one-opening digestive systems. They must digest the nutrients and expel the waste before eating anew. Digestive systems of more complex animals have two openings, allowing simultaneous ingestion, digestion, and excretion. Roundworms have the simplest two-opening digestive system, which is little more than a tube. Enzymes in the tube break down the food, and nutrients are absorbed from there into the body fluids. More complex animals, such as vertebrates, have a gastrointestinal tract that is specialized into compartments where digestive enzymes and other substances process the food. Waves of muscular contraction called peristalsis help to move the food along.

Digestive system specializations reflect the lifestyles of their owners. Birds eat and digest nearly all the time. They can store food in an enlarged sac called a crop, and have a muscular organ called a gizzard that uses small pebbles to grind food. In some migratory species, the intestines actually enlarge before a long flight, enabling the animal to obtain energy throughout the journey. Ruminants such as cows have several stomachs, which contain cellulose -digesting bacteria, enabling them to digest grasses.

In humans, digestion begins at the mouth, where teeth tear food into small pieces, and the enzyme salivary amylase begins the breakdown of starch. During swallowing, food travels quickly through the esophagus , landing in the stomach. Here, the food is churned and further mechanically broken down as it mixes with gastric juice into a slurry called chyme. Each day, 40 million cells that line the stomach's interior release up to three quarts of gastric juice, which consists of water, mucus, salts, hydrochloric acid, and the enzyme pepsin, which breaks down protein into peptides. Hydrochloric acid unwinds proteins and kills many microorganisms.

After a length of time that reflects the components of the meal, a drawstringlike muscular structure called the pyloric sphincter at the stomach's exit opens, and chyme squirts into the duodenum, the first ten inches of the small intestine. The next two segments are the jejunum and the ileum. In addition to peristalsis, the small intestine undergoes localized muscle contractions that slosh the chyme back and forth, exposing it to several types of digestive enzymes. Trypsin and chymotrypsin continue the breakdown of peptides, and then peptidases break these down further into amino acids . Carbohydrases and pancreatic amylase continue the carbohydrate digestion that began in the mouth, and nucleases break down deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Bile, which is produced in the liver and stored in the gallbladder, emulsifies fats, which are then chemically digested by lipases into fatty acids and monoglycerides. The pancreas secretes trypsin, chymotrypsin, amylase, lipase, and nucleases.

Absorption of most of the products of digestion occurs in the small intestine. Water, electrolytes , and minerals are absorbed in the large intestine. The remaining material, which consists mostly of bacteria, bile, cellulose and shed intestinal lining cells, is compacted into feces in the rectum, and exits the body through the anus.

see also Digestive System

Ricki Lewis

Bibliography

Alexander, R. McNeill. "News of Chews: The Optimization of Mastication." Nature 391 (1998): 329–331.

Johnson, Leonard R., and Thomas A. Gerwin. Gastrointestinal Physiology. New York: Mosby, 2001.

digestion The breakdown of a complex compound into its constituent parts, achieved either chemically or enzymically. Most frequently refers to the digestion of food, which means breakdown by digestive enzymes of proteins to amino acids, starch to glucose, fats to glycerol and fatty acids. These breakdown products are then absorbed into the bloodstream. See also gastro‐intestinal tract.