digestion

1. Physiology.
   1. The process by which food is converted into substances that can be absorbed and assimilated by the body. It is accomplished in the alimentary canal by the mechanical and enzymatic breakdown of foods into simpler chemical compounds.
   2. The result of this process.
   3. The ability to digest food.
2. The process of decomposing organic matter in sewage by bacteria.
3. Assimilation of ideas or information; understanding.

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.

The process whereby the constituents of food are broken down into simpler organic chemicals that can be absorbed through the lining of the gut. Mastication in the mouth and churning in the stomach assist mixing with the secreted juices which contain the necessary enzymes to catalyse the chemical processes. The linings of the mouth, stomach, and small intestine, together with the pancreas, contribute three main types of enzyme: amylases for carbohydrates, lipases for fats, and proteases for proteins. The end products are simple sugars, lipids, peptides and amino acids.

— Sheila Jennett

See alimentary system; eating; enzymes.

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.

The process by which complex food is broken down into simple compounds by chemical processes. Fats are broken down into glycerol and fatty acids, proteins into amino acids, and starch into glucose. Chemical digestion depends on enzymes secreted by cells lining the alimentary canal or by cells in the pancreas. Chemical digestion is aided by chewing which physically breaks down large chunks of food into smaller pieces; this is sometimes called mechanical digestion. Fat digestion is accelerated by bile secreted by the liver. Bile is stored in the gall bladder until needed in the small intestine where it emulsifies fat, increasing the surface area on which enzymes can act.

noun

The process of absorbing and incorporating, especially mentally: absorption, assimilation. See accept/reject.

The conversion of food into absorbable substances in the GI tract.

(click to enlarge)
Major organs of the human digestive system. Food taken in by the mouth is guided by the tongue as … (credit: © Merriam-Webster Inc.)

Process of dissolving and chemically converting food for absorption by cells. In the mouth, food is chewed, mixed with saliva, which begins to break down starches, and kneaded by the tongue into a ball for swallowing. Peristalsis propels it through the esophagus and the rest of the alimentary canal. In the stomach, food mixes with acid and enzymes, which further break it down. The mixture, called chyme, enters the duodenum, the first part of the small intestine. Bile from the liver breaks up fat globules. Enzymes from the pancreas and intestinal glands act on specific molecules, breaking carbohydrates down into simple sugars, proteins into amino acids, and fats into glycerol and fatty acids. These products are absorbed by the bloodstream. Indigestible substances, such as fibre, pass into the large intestine, where water and ions are reabsorbed and feces held for excretion.

For more information on digestion, visit Britannica.com.

The process that breaks down large food molecules in the alimentary canal into smaller substances, which can be absorbed from the gut into the blood stream. Digestion involves mechanical processes, such as chewing, and chemical processes involving enzymes.

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.

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

The breaking down of food, which is made up of complex organic molecules, into smaller molecules that the body can absorb and use for maintenance and growth.

1. the act or process of converting food into chemical substances that can be absorbed into the blood and utilized by the body tissues.
2. the subjection of a substance to prolonged heat and moisture, so as to disintegrate and soften it.
Digestion is accomplished by physically breaking down, churning, diluting and dissolving the food substances, and also by splitting them chemically into simpler compounds. Carbohydrates are eventually broken down to monosaccharides (simple sugars); proteins are broken down into amino acids; and fats are absorbed as fatty acids, monoglycerides and glycerol (glycerin).
The digestive process takes place in the alimentary canal or digestive system. The salivary glands, liver, gallbladder and pancreas are located outside the alimentary canal, but they are considered accessory organs of digestion because their secretions provide essential enzymes and other substances.

• avian d. — differs markedly from mammals in the mouth; there are no teeth, dental functions being performed by the beak and the muscular gizzard; the esophagus, in other than owls and insectivorous species, has one or two crops, dilations where ingesta are held temporarily.
• enzymatic d. — most digestive processes in monogastric animals are enzymatic brought about by enzymes secreted into the lumen of the gastrointestinal tract and enzymes located at the brush borders of the intestinal epithelium.
• d. error — any disruption of the normal digestive process; caused by abnormal ingesta, either chemically or physically, or by an error in the physiological and biochemical processes which constitute digestion.
• gastric d. — digestion by the action of gastric juice.
• impaired d. — see maldigestion.
• intestinal d. — digestion by the action of intestinal juices, bile and pancreatic juice.
• luminal phase d. — the stage of the digestion of fats that goes on in the lumen of the intestine; as distinct from the mucosal phase that occurs in the epithelial cells.
• pancreatic d. — digestion by the action of pancreatic juice.
• peptic d. — gastric digestion by pepsin.
• primary d. — digestion occurring in the gastrointestinal tract.
• ruminant d. — characterized by the fermentative functions that are carried on in the forestomachs. Cellulose is readily digested with the output of short-chain fatty acids being the chief energy source for the animal. Nonprotein nitrogen is utilized by the ruminal bacteria for the manufacture of protein which is later available for the satisfaction of the animal's protein needs.
• salivary d. — the change of starch into maltose by the saliva; most marked in humans.
• d. tests — see starch digestion test, lactose digestion test, gelatin digestion test.

n.

The conversion of victuals into virtues. When the process is imperfect, vices are evolved instead -- a circumstance from which that wicked writer, Dr. Jeremiah Blenn, infers that the ladies are the greater sufferers from dyspepsia.

For the industrial process, see anaerobic digestion.

For the treatment of precipitates in analytical chemistry, see precipitation (chemistry)#Digestion.

"Gut" and "Guts" redirect here. For other uses, see Gut (disambiguation).

"Entrails" redirects here. For the practice of reading entrails, see Extispicy.

Digestion is the mechanical and chemical breaking down of food into smaller components, to a form that can be absorbed, for instance, into a blood stream. Digestion is a form of catabolism; a break-down of macro food molecules to smaller ones.

In mammals, food enters the mouth, being chewed by teeth, with chemical processing beginning with chemicals in the saliva from the salivary glands. Then it travels down the esophagus into the stomach, where acid both kills most contaminating microorganisms and begins mechanical break down of some food (eg denaturation of protein), and chemical alteration of some. After some time (typically an hour or two in humans, 4-6 hours in dogs, somewhat shorter duration in house cats, ...), the results go through the small intestine, through the large intestine, and are excreted during defecation.^[1]

Other organisms use different mechanisms to digest food.

Contents 1 Digestive systems 1.1 Invasive digestion 1.2 Secretion systems 1.2.1 Channel transport system 1.2.2 Molecular syringe 1.2.3 Conjugation machinery 1.2.4 Release of outer membrane vesicles 1.3 Phagosome 1.4 Gastrovascular cavity 1.5 Specialized organs and behaviors 1.5.1 Beaks 1.5.2 Tongue 1.5.3 Teeth 1.5.4 Crop 1.5.5 Abomasum 1.5.6 Specialized behaviors 1.6 In earthworms 2 Overview of vertebrate digestion 3 Human digestion process 3.1 Phases of gastric secretion 3.2 Oral cavity 3.3 Pharynx 3.4 Esophagus 3.5 Stomach 3.6 Small intestine 3.7 Large intestine 4 Fat digestion 5 Digestive hormones 6 Significance of pH in digestion 7 Uses of animal gut by humans 8 See also 9 Footnotes 10 References 11 External links

Digestive systems

Digestive systems take many forms. Some organisms, eg nearly all spiders, simply secrete biotoxins and digestive chemicals (eg, enzymes)s into the extracellular environment prior to ingestion of the consequent "soup". In others, once potential nutrients or food is inside the organism, digestion can be conducted to a vesicle or a sac-like structure, through a tube, or through several specialized organs aimed at making the absorption of nutrients more efficient.

Invasive digestion

Viruses "digest" through the invasion of cells, thereby reaching "food" held within cells and their vacuoles. Specific viral capsid proteins match receptors on host cell surfaces and are used to fuse the membranes of the virus and the target cell. The cell membrane is then (1) punctured and an opening established, (2) the host cell is induced to endocytose the virus, and the resulting vacuole is either punctured or digested, or (3) some portion of the host plasma membrane, cell wall, or capsule is digested.^[2] The viral capsid or genome is injected into the host cell's cytoplasm.

Secretion systems

Main article: Secretion

Bacteria use several systems to obtain nutrients from other organisms in the environment.

Channel transport system

In a channel transport system several proteins form a contiguous channel traversing the inner and outer membranes of the bacteria. It is a simple system, which consists of only three protein subunits: the ABC protein, membrane fusion protein (MFP), and outer membrane protein (OMP). This secretion system transports various molecules, from ions, drugs, to proteins of various sizes (20 - 900 kDa). The molecules secreted vary in size from the small Escherichia coli peptide colicin V, (10 kDa) to the Pseudomonas fluorescens cell adhesion protein LapA of 900 kDa.^[3]

Molecular syringe

A molecular syringe is used through which a bacterium (e.g. certain types of Salmonella, Shigella, Yersinia) can inject proteins into eukaryotic cells. One such mechanism was first discovered in Y. pestis and showed that toxins could be injected directly from the bacterial cytoplasm into the cytoplasm of its host's cells rather than simply be secreted into the extracellular medium.^[4]

Conjugation machinery

Schematic drawing of bacterial conjugation. Conjugation diagram 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, brings the two cells together. 3- The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell. 4- Both cells recircularize their plasmids, synthesize second strands, and reproduce pili; both cells are now viable donors.

The conjugation machinery of some bacteria (and archaeal flagella) is capable of transporting both DNA and proteins. It was discovered in Agrobacterium tumefaciens, which uses this system to introduce the Ti plasmid and proteins into the host which develops the crown gall (tumor). ^[5]. The VirB complex of Agrobacterium tumefaciens is the prototypic system.^[6]

The nitrogen fixing Rhizobia are an interesting case, wherein conjugative elements naturally engage in inter-kingdom conjugation. Such elements as the Agrobacterium Ti or Ri plasmids contain elements that can transfer to plant cells. Transferred genes enter the plant cell nucleus and effectively transform the plant cells into factories for the production of opines, which the bacteria use as carbon and energy sources. Infected plant cells form crown gall or root tumors. The Ti and Ri plasmids are thus endosymbionts of the bacteria, which are in turn endosymbionts (or parasites) of the infected plant.

The Ti and Ri plasmids are themselves conjugative. Ti and Ri transfer between bacteria uses an independent system (the tra, or transfer, operon) from that for inter-kingdom transfer (the vir, or virulence, operon). Such transfer creates virulent strains from previously avirulent Agrobacteria.

Release of outer membrane vesicles

In addition to the use of the multiprotein complexes listed above, Gram-negative bacteria possess another method for release of material: the formation of outer membrane vesicles.^[7] Portions of the outer membrane pinch off, forming spherical structures made of a lipid bilayer enclosing periplasmic materials. Vesicles from a number of bacterial species have been found to contain virulence factors, some have immunomodulatory effects, and some can directly adhere to and intoxicate host cells. While release of vesicles has been demonstrated as a general response to stress conditions, the process of loading cargo proteins seems to be selective.^[8]

Phagosome

A phagosome is a vacuole formed around a particle absorbed by phagocytosis. The vacuole is formed by the fusion of the cell membrane around the particle. A phagosome is a cellular compartment in which pathogenic microorganisms can be killed and digested. Phagosomes fuse with lysosomes in their maturation process, forming phagolysosomes. In humans, Entamoeba histolytica can phagocytose red blood cells.^[9]

Trophozoites of Entamoeba histolytica with ingested erythrocytes

Gastrovascular cavity

The gastrovascular cavity functions as a stomach in both digestion and the distribution of nutrients to all parts of the body. Extracellular digestion takes place within this central cavity which is lined with the gastrodermis, the internal layer of epithelium. This cavity has only one opening to the outside that functions as both a mouth and an anus: waste and undigested matter is excreted through the mouth/anus, which can be described as an incomplete gut.

Aboral end

Oral end

Mouth

Oral end

Aboral end

    Exoderm

    Gastroderm

    Mesoglea

    Digestive cavity

Medusa (left) and polyp (right)^[10]

In a plant such as the Venus Flytrap that can make its own food through photosynthesis, it does not eat and digest its prey for the traditional objectives of harvesting energy and carbon, but mines prey primarily for essential nutrients (nitrogen and phosphorus in particular) that are in short supply in its boggy, acidic habitat.^[11]

Venus Flytrap (Dionaea muscipula) leaf

Specialized organs and behaviors

Catalina Macaw exhibits its seed shearing beak.

Squid beak and ruler for size comparison.

Teeth of a Carcharodon megalodon.

Rough illustration of a ruminant digeestive system.

To aid in the digestion of their food animals were created with organs such as beaks, tongues, teeth, a crop, gizzard, and others.

Beaks

Macaws primarily eat seeds, nuts, and fruit, using their impressive beaks to open even the toughest seed. First they scratch a thin line with the sharp point of the beak, then they shear the seed open with the sides of the beak.

The mouth of the squid is equipped with a sharp horny beak mainly made of chitin^[12] and cross-linked proteins. It is used to kill and tear prey into manageable pieces. The beak is very robust, but does not contain any minerals, unlike the teeth and jaws of many other organisms, including marine species.^[13] The beak is the only indigestible part of the squid.

Tongue

Main article: Tongue

The tongue is skeletal muscle on the floor of the mouth that manipulates food for chewing (mastication) and swallowing (deglutition). It is sensitive and kept moist by saliva. The underside of the tongue is covered with a smooth mucous membrane. The tongue is utilised to roll food molecules into a bolus before being transported down the esophagus through the use of peristalsis. The sublingual region underneath the front of the tongue is a location where the oral mucosa is very thin, and underlain by a plexus of veins. This is an ideal location for introducing certain medications to the body. The sublingual route takes advantage of the highly vascular quality of the oral cavity, and allows for the speedy application of medication into the cardiovascular system, by passing the gastrointestinal tract.

Teeth

Main article: Teeth

Teeth (singular, tooth) are small whitish structures found in the jaws (or mouths) of many vertebrates that are used to tear, scrape, milk and chew food. Teeth are not made of bone, but rather of tissues of varying density and hardness. The shape of an animal's teeth is related to its diet. For example, plant matter is hard to digest, so herbivores have many molars for chewing.

The teeth of carnivores are shaped to kill and tear meat, using specially-shaped canine teeth. Herbivores' teeth are made for grinding food materials, in this case, plant parts.

Crop

A crop, or croup, is a thin-walled expanded portion of the alimentary tract used for the storage of food prior to digestion. In some birds it is an expanded, muscular pouch near the gullet or throat. In adult doves and pigeons, the crop can produce crop milk to feed newly hatched birds.^[14]

Certain insects may have a crop or enlarged oesophagus.

Abomasum

Main article: Abomasum

Herbivores have evolved cecums (or an abomasum in the case of ruminants). Ruminants have a fore-stomach with four chambers. These are the rumen, reticulum, omasum, and abomasum. In the first two chambers, the rumen and the reticulum, the food is mixed with saliva and separates into layers of solid and liquid material. Solids clump together to form the cud (or bolus). The cud is then regurgitated, chewed slowly to completely mix it with saliva and to break down the particle size.

Fiber, especially cellulose and hemi-cellulose, is primarily broken down into the volatile fatty acids, acetic acid, propionic acid and butyric acid in these chambers (the reticulo-rumen) by microbes: (bacteria, protozoa, and fungi). In the omasum water and many of the inorganic mineral elements are absorbed into the blood stream.

The abomasum is the fourth and final stomach compartment in ruminants. It is a close equivalent of a monogastric stomach (eg, those in humans or pigs), and digesta is processed here in much the same way. It serves primarily as a site for acid hydrolysis of microbial and dietary protein, preparing these protein sources for further digestion and absorption in the small intestine. Digesta is finally moved into the small intestine, where the digestion and absorption of nutrients occurs. Microbes produced in the reticulo-rumen are also digested in the small intestine.

Specialized behaviors

A flesh fly "blowing a bubble". One explanation of this behaviour is that the fly regurgitates its food into a bubble in order to increase the concentration of its food by evaporating excessive water content

Regurgitation has been mentioned above under abomasum and crop, referring to crop milk, a secretion from the lining of the crop of pigeons and doves with which the parents feed their young by regurgitation.^[15].

Many sharks have the ability to turn their stomachs inside out and evert it out of their mouths in order to get rid of unwanted contents (perhaps developed as a way to reduce exposure to toxins).

Other animals, such as rabbits and rodents, practice coprophagia behaviors - eating specialized feces in order to re-digest food, especially in the case of roughage. Capybara, rabbits, hamsters and other related species do not have a complex digestive system as do, for example, ruminants. Instead they extract more nutrition from grass by giving their food a second pass through the gut. Soft fecal pellets of partially digested food are excreted and generally consumed immediately. They also produce normal droppings, which are not eaten.

Young elephants, pandas, koalas, and hippos eat the feces of their mother, probably to obtain the bacteria required to properly digest vegetation. When they are born, their intestines do not contain these bacteria (they are completely sterile). Without them, they would be unable to get any nutritional value from many plant components.

In earthworms

An earthworm's digestive system consists of a mouth, pharynx, esophagus, crop, gizzard, and intestine. The mouth is surrounded by strong lips which act like a hand to grab pieces of dead grass, leaves, and weeds, with bits of soil to help chew. The lips break the food down into smaller pieces. In the pharynx the food is lubricated by mucus secretions for easier passage. The esophagus adds calcium carbonate to neutralize the acids formed by food matter decay. Temporary storage occurs in the crop where food and calcium carbonate are mixed. The powerful muscles of the gizzard churn and mix the mass of food and dirt. When the churning is complete, the glands in the walls of the gizzard add enzymes to the thick paste which aid in the chemical breakdown of the organic matter. By peristalsis the mixture is sent to the intestine where friendly bacteria continue chemical breakdown. This releases carbohydrates, protein, fat, and various vitamins and minerals for absorption into the body.

Overview of vertebrate digestion

In most vertebrates, digestion is a multi-stage process in the digestive system, starting from ingestion of raw materials, most often other organisms. Ingestion usually involves some type of mechanical and chemical processing. Digestion is separated into four steps:

1. Ingestion: placing food into the mouth (entry of food in the digestive system),
2. Mechanical and chemical breakdown: mastication and the mixing of the resulting bolus with water, acids, bile and enzymes in the stomach and intestine to break down complex molecules into simple structures,
3. Absorption: of nutrients from the digestive system to the circulatory and lymphatic capillaries through osmosis, active transport, and diffusion, and
4. Egestion: Removal of undigested materials from the digestive tract through defecation.

Underlying the process is muscle movement throughout the system through swallowing and peristalsis. Each step in digestion requires energy, and thus imposes an "overhead charge" on the energy made available from absorbed substances. Differences in that overhead cost are important influences on lifestyle, behavior, and even physical structures. Examples may be seen in humans, who differ considerably from other hominids (lack of hair, smaller jaws and musculature, different dentition, length of intestines, cooking, etc).

The major part of digestion takes place in the small intestine. The large intestine primarily serves as a site for fermentation of indigestible matter by gut bacteria and for resorption of water from digesta before excretion.

In mammals, preparation for digestion begins with the cephalic phase in which saliva is produced in the mouth and digestive enzymes are produced in the stomach. Mechanical and chemical digestion begin in the mouth where food is chewed, and mixed with saliva to begin enzymatic processing of starches. The stomach continues to break food down mechanically and chemically through churning and mixing with both acids and enzymes. Absorption occurs in the stomach and gastrointestinal tract, and the process finishes with defecation.^[1]

Human digestion process

The digestive system

The whole digestive system is around 9 meters long. In a healthy human adult this process can take between 24 and 72 hours.

Phases of gastric secretion

• Cephalic phase - This phase occurs before food enters the stomach and involves preparation of the body for eating and digestion. Sight and thought stimulate the cerebral cortex. Taste and smell stimulus is sent to the hypothalamus and medulla oblongata. After this it is routed through the vagus nerve and release of acetylcholine. Gastric secretion at this phase rises to 40% of maximum rate. Acidity in the stomach is not buffered by food at this point and thus acts to inhibit parietal (secretes acid) and G cell (secretes gastrin) activity via D cell secretion of somatostatin.
• Gastric phase - This phase takes 3 to 4 hours. It is stimulated by distention of the stomach, presence of food in stomach and decrease in pH. Distention activates long and myentric reflexes. This activates the release of acetylcholine which stimulates the release of more gastric juices. As protein enters the stomach, it binds to hydrogen ions, which lowers the pH of the stomach to around pH 1-3. Inhibition of gastrin and HCl secretion is lifted. This triggers G cells to release gastrin, which in turn stimulates parietal cells to secrete HCl. HCl release is also triggered by acetylcholine and histamine.
• Intestinal phase - This phase has 2 parts, the excitatory and the inhibitory. Partially-digested food fills the duodenum. This triggers intestinal gastrin to be released. Enterogastric reflex inhibits vagal nuclei, activating sympathetic fibers causing the pyloric sphincter to tighten to prevent more food from entering, and inhibits local reflexes.

Oral cavity

Main article: Mouth (human)

In humans, digestion begins in the oral cavity where food is chewed. Saliva is secreted in large amounts (1-1.5 litres/day) by three pairs of exocrine salivary glands (parotid, submandibular, and sublingual) in the oral cavity, and is mixed with the chewed food by the tongue. There are two types of saliva. One is a thin, watery secretion, and its purpose is to wet the food. The other is a thick, mucous secretion, and it acts as a lubricant and causes food particles to stick together and form a bolus. The saliva serves to clean the oral cavity and moisten the food, and contains digestive enzymes such as salivary amylase, which aids in the chemical breakdown of polysaccharides such as starch into disaccharides such as maltose. It also contains mucin, a glycoprotein which helps soften the food into a bolus.

Swallowing transports the chewed food into the oesophagus, passing through the oropharynx and hypopharynx. The mechanism for swallowing is coordinated by the swallowing center in the medulla oblongata and pons. The reflex is initiated by touch receptors in the pharynx as the bolus of food is pushed to the back of the mouth.

Pharynx

Main article: Human pharynx

The pharynx is the part of the neck and throat situated immediately posterior to (behind) the mouth and nasal cavity, and cranial, or superior, to the esophagus. It is part of the digestive system and respiratory system. Because both food and air pass through the pharynx, a flap of connective tissue, the epiglottis closes over the trachea when food is swallowed to prevent choking or aspiration.

The oropharynx is that part of the pharynx which lies behind the oral cavity and is lined by stratified squamous epithelium. The nasopharynx lies behind the nasal cavity and like the nasal passages is lined with ciliated columnar pseudostratified epithelium.

Like the oropharynx above it the hypopharynx (laryngopharynx) serves as a passageway for food and air and is lined with a stratified squamous epithelium. It lies inferior to the upright epiglottis and extends to the larynx, where the respiratory and digestive pathways diverge. At that point, the laryngopharynx is continuous with the oesophagus. During swallowing, food has the "right of way", and air passage temporarily stops.

Esophagus

Main article: esophagus

The esophagus is a narrow muscular tube about 25 centimeters long which starts at pharynx at the back of the mouth, passes through the thoracic diaphragm, and ends at the cardiac orifice of the stomach. The wall of the esophagus is made up of two layers of smooth muscles, which form a continuous layer from the esophagus to the open and contract slowly, over long periods of time. The inner layer of muscles is arranged circularly in a series of descending rings, while the outer layer is arranged longitudinally. At the top of the esophagus, is a flap of tissue called the epiglottis that closes during swallowing to prevent food from entering the trachea (windpipe). The chewed food is pushed down the esophagus to the stomach through peristaltic contraction of these muscles. It takes only about seven seconds for food to pass through the esophagus and no digestion takes place.

Stomach

Main article: Stomach

The stomach is a small,'J'-shaped pouch with walls made of thick, elastic muscles, which stores and helps break down food. Food which has been reduced to very small particles is more likely to be fully digested in the small intestine, and stomach churning has the effect of assisting the physical disassembly begun in the mouth. In ruminants who are able to digest fibrous material (primarily cellulose), use fore-stomachs and repeated chewing to further the disassembly. Rabbits and some other animals pass material through their entire digestive systems twice. Most birds ingest small stones to assist in mechanical processing in gizzards.

Food enters the stomach through the cardiac orifice where it is further broken apart and thoroughly mixed with gastric acid, pepsin and other digestive enzymes to break down proteins. The enzymes in the stomach also have an optimum, meaning that they work at a specific pH and temperature better than any others. The acid itself does not break down food molecules, rather it provides an optimum pH for the reaction of the enzyme pepsin and kills many microorganisms that are ingested with the food. It can also denature proteins. This is the process of reducing polypeptide bonds and disrupting salt bridges which in turn causes a loss of secondary, tertiary or quaternary protein structure. The parietal cells of the stomach also secrete a glycoprotein called intrinsic factor which enables the absorption of vitamin B-12. Other small molecules such as alcohol are absorbed in the stomach, passing through the membrane of the stomach and entering the circulatory system directly. Food in the stomach is in semi-liquid form, which upon completion is known as chyme.

The transverse section of the alimentary canal reveals four (or five, see description under mucosa) distinct and well developed layers within the stomach:

• Serous membrane, a thin layer of mesothelial cells that is the outermost wall of the stomach.
• Muscular coat, a well-developed layer of muscles used to mix ingested food, composed of three sets running in three different alignments. The outermost layer runs parallel to the vertical axis of the stomach (from top to bottom), the middle is concentric to the axis (horizontally circling the stomach cavity) and the innermost oblique layer, which is responsible for mixing and breaking down ingested food, runs diagonal to the longitudinal axis. The inner layer is unique to the stomach, all other parts of the digestive tract have only the first two layers.
• Submucosa, composed of connective tissue that links the inner muscular layer to the mucosa and contains the nerves, blood and lymph vessels.
• Mucosa is the extensively folded innermost layer. It can be divided into the epithelium, lamina propria, and the muscularis mucosae, though some consider the outermost muscularis mucosae to be a distinct layer, as the it develops from the mesoderm rather than the endoderm (thus making a total of 5 layers). The epithelium and lamina are filled with connective tissue and covered in gastric glands that may be simple or branched tubular, and secrete mucus, hydrochloric acid, pepsinogen and renin. The mucus lubricates the food and also prevents hydrochloric acid from acting on the walls of the stomach.

Small intestine

Main article: Small intestine

After being processed in the stomach, food is passed to the small intestine via the pyloric sphincter. The majority of digestion and absorption occurs here after the milky chyme enters the duodenum. Here it is further mixed with three different liquids:

• Bile, which emulsifies fats to allow absorption, neutralizes the chyme and is used to excrete waste products such as bilin and bile acids.
• Pancreatic juice made by the pancreas.
• Intestinal enzymes of the alkaline mucosal membranes. The enzymes include maltase, lactase and sucrase (all three of which process only sugars), trypsin and chymotrypsin.

As the pH level changes in the small intestines and gradually becomes basic, more enzymes are activated further that chemically break down various nutrients into smaller molecules to allow absorption into the circulatory or lymphatic systems. Small, finger-like structures called villi, each of which is covered with even smaller hair-like structures called microvilli improve the absorption of nutrients by increasing the surface area of the intestine and enhancing speed at which nutrients are absorbed. Blood containing the absorbed nutrients is carried away from the small intestine via the hepatic portal vein and goes to the liver for filtering, removal of toxins, and nutrient processing.

The small intestine and remainder of the digestive tract undergoes peristalsis to transport food from the stomach to the rectum and allow food to be mixed with the digestive juices and absorbed. The circular muscles and longitudinal muscles are antagonistic muscles, with one contracting as the other relaxes. When the circular muscles contract, the lumen becomes narrower and longer and the food is squeezed and pushed forward. When the longitudinal muscles contract, the circular muscles relax and the gut dilates to become wider and shorter to allow food to enter.

Large intestine

Main article: Large intestine

After the food has been passed through the small intestine, the food enters the large intestine. Within it, digesta is retained long enough to allow fermentation due to the action of gut bacteria, which breaks down some of the substances which remain after processing in the small intestine; some of the breakdown products are absorbed. In humans, these include most complex saccharides (at most three disaccharides are digestible in humans). In addition, in many vertebrates, the large intestine resorbs fluid; in a few, with desert lifestyles, this resorption makes continued existence possible.

In humans, the large intestine is roughly 1.5 meters long, with three parts: the cecum at the junction with the small intestine, the colon, and the rectum. The colon itself has four parts: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. The large intestine absorbs water from the bolus and stores feces until it can be egested. Food products that cannot go through the villi, such as cellulose (dietary fiber), are mixed with other waste products from the body and become hard and concentrated feces. The feces is stored in the rectum for a certain period and then the stored feces is eliminated from the body due to the contraction and relaxation through the anus. The exit of this waste material is regulated by the anal sphincter.

Fat digestion

The presence of fat in the small intestine produces hormones which stimulate the release of lipase from the pancreas and bile from the gallbladder. The lipase (activated by acid) breaks down the fat into monoglycerides and fatty acids. The bile emulsifies the fatty acids so they may be easily absorbed.

Short- and some medium chain fatty acids are absorbed directly into the blood via intestine capillaries and travel through the portal vein just as other absorbed nutrients do. However, long chain fatty acids and some medium chain fatty acids are too large to be directly released into the tiny intestinal capillaries. Instead they are absorbed into the fatty walls of the intestine villi and reassembled into triglycerides. The triglycerides are coated with cholesterol and protein (protein coat) into chylomicron particles.

Within the villi, the chylomicron enters a lymphatic capillary called a lacteal, which merges into larger lymphatic vessels. It is transported via the lymphatic system and the thoracic duct up to a location near the heart (where the arteries and veins are larger). The thoracic duct empties the chylomicrons into the bloodstream via the left subclavian vein. At this point the chylomicrons can transport the triglycerides, largely to the liver for further processing, or to fat tissue for storage.

Digestive hormones

There are at least four hormones that aid and regulate the digestive system in mammals. There are variations across the vertebrates, as for instance in birds. Arrangements are complex and additional details are regularly discovered. For instance, more connections to metabolic control (largely the glucose-insulin system) have been uncovered in recent years.

• Gastrin - is in the stomach and stimulates the gastric glands to secrete pepsinogen(an inactive form of the enzyme pepsin) and hydrochloric acid. Secretion of gastrin is stimulated by food arriving in stomach. The secretion is inhibited by low pH .
• Secretin - is in the duodenum and signals the secretion of sodium bicarbonate in the pancreas and it stimulates the bile secretion in the liver. This hormone responds to the acidity of the chyme.
• Cholecystokinin (CCK) - is in the duodenum and stimulates the release of digestive enzymes in the pancreas and stimulates the emptying of bile in the gall bladder. This hormone is secreted in response to fat in chyme.
• Gastric inhibitory peptide (GIP) - is in the duodenum and decreases the stomach churning in turn slowing the emptying in the stomach. Another function is to induce insulin secretion.

Significance of pH in digestion

Digestion is a complex process which is controlled by several factors. pH plays a crucial role in a normally functioning digestive tract. In the mouth, pharynx, and esophagus, pH is typically about 6.8, very weakly acidic. Saliva controls pH in this region of the digestive tract. Salivary amylase is contained in saliva and starts the breakdown of carbohydrates into monosaccharides. Most digestive enzymes are sensitive to pH and will not function in a low-pH environment like the stomach. A pH below 7 indicates an acid, while a pH above 7 indicates a base; the concentration of the acid or base, however, does also play a role.

pH in the stomach is very acidic and inhibits the breakdown of carbohydrates while there. The strong acid content of the stomach provides two benefits, both serving to denature proteins for further digestion in the small intestines, as well as providing non-specific immunity, retarding or eliminating various pathogens.

In the small intestines, the duodenum provides critical pH balancing to activate digestive enzymes. The liver secretes bile into the duodenum to neutralise the acidic conditions from the stomach. Also the pancreatic duct empties into the duodenum, adding bicarbonate to neutralize the acidic chyme, thus creating a neutral environment. The mucosal tissue of the small intestines is alkaline, creating a pH of about 8.5, thus enabling absorption in a mild alkaline in the environment.^{[dubious – discuss]}

Uses of animal gut by humans

• The stomachs of calves have commonly been used as a source of rennet for making cheese.
• The use of animal gut strings by musicians can be traced back to the third dynasty of Egypt. In the recent past, strings were made out of lamb gut. With the advent of the modern era, musicians have tended to use strings made of silk, or synthetic materials such as nylon or steel. Some instrumentalists, however, still use gut strings in order to evoke the older tone quality. Although such strings were commonly referred to as "catgut" strings, cats were never used as a source for gut strings.
• Sheep gut was the original source for natural gut string used in racquets, such as for tennis. Today, synthetic strings are much more common, but the best strings are now made out of cow gut.
• Gut cord has also been used to produce strings for the snares which provide the snare drum's characteristic buzzing timbre. While the snare drum currently almost always uses metal wire rather than gut cord, the North African bendir frame drum still uses gut for this purpose.
• "Natural" sausage hulls (or casings) are made of animal gut, especially hog, beef, and lamb. Similarly, Haggis is traditionally boiled in, and served in, a sheep stomach.
• Chitterlings, a kind of food, consist of thoroughly washed pig's gut.
• Animal gut was used to make the cord lines in longcase clocks and for fusee movements in bracket clocks, but may be replaced by metal wire.
• The oldest known condoms, from 1640 AD, were made from animal intestine.^[16]

See also

• Nutrition
• Human gastrointestinal tract

Footnotes

1. ^ ^{a b} Maton, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 0-13-981176-1. OCLC 32308337. 
2. ^ Smith NL, Taylor EJ, Lindsay AM, Charnock SJ, Turkenburg JP, Dodson EJ, Davies GJ, Black GW (Dec 2006). "Structure of a group A streptococcal phage-encoded virulence factor reveals a catalytically active triple-stranded β-helix". Proc Natl Acad Sci U S A. 102 (49): 17652–7. doi:10.1073/pnas.0504782102. PMID 16314578. 
3. ^ Wooldridge K (editor) (2009). Bacterial Secreted Proteins: Secretory Mechanisms and Role in Pathogenesis. Caister Academic Press. ISBN 978-1-904455-42-4. 
4. ^ Salyers, A. A. & Whitt, D. D. (2002). Bacterial Pathogenesis: A Molecular Approach, 2nd ed., Washington, D.C.: ASM Press. ISBN 1-55581-171-X
5. ^ Cascales E & Christie P.J. (2003). "The versatile Type IV secretion systems". Nat Rev Microbiol 1 (2): 137–149. doi:10.1038/nrmicro753. 
6. ^ Christie PJ, Atmakuri K, Jabubowski S, Krishnamoorthy V & Cascales E. (2005). "Biogenesis, architecture, and function of bacterial Type IV secretion systems". Ann Rev Microbiol 59: 451–485. doi:10.1146/annurev.micro.58.030603.123630. 
7. ^ Chatterjee, SN and J Das. "Electron microscopic observations on the excretion of cell wall material by Vibrio cholerae." "J.Gen.Microbiol." "49" : 1-11 (1967) ; Kuehn, MJ and NC Kesty. "Bacterial outer membrane vesicles and the host-pathogen interaction." Genes Dev. 19(22):2645-55 (2005)
8. ^ McBroom, AJ and MJ Kuehn "Release of outer membrane vesicles by Gram-negative bacteria is a novel envelope stress response." Mol. Microbiol. 63(2):545-58 (2007)
9. ^ Boettner DR, Huston CD, Linford AS, et al. (January 2008). "Entamoeba histolytica phagocytosis of human erythrocytes involves PATMK, a member of the transmembrane kinase family". PLoS Pathog. 4 (1): e8. doi:10.1371/journal.ppat.0040008. PMID 18208324. PMC 2211552. http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.0040008. 
10. ^ Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004). Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 76–97. ISBN 0030259827. 
11. ^ Leege, Lissa. "How does the Venus flytrap digest flies?". Scientific American. http://www.sciam.com/article.cfm?id=how-does-the-venus-flytra. Retrieved 2008-08-20. 
12. ^ Clarke, M.R. (1986). A Handbook for the Identification of Cephalopod Beaks. Oxford: Clarendon Press. ISBN 0-19-857603-X. 
13. ^ Miserez, A; Li, Y; Waite, H; Zok, F (2007). "Jumbo squid beaks: Inspiration for design of robust organic composites". Acta Biomaterialia 3: 139–149. doi:10.1016/j.actbio.2006.09.004. 
14. ^ Gordon John Larkman Ramel (2008-09-29). "The Alimentary Canal in Birds". http://www.earthlife.net/birds/digestion.html. Retrieved 2008-12-16. 
15. ^ Levi, Wendell (1977). The Pigeon. Sumter, S.C.: Levi Publishing Co, Inc. ISBN 0853900132. 
16. ^ "World's oldest condom". Ananova. 2008. http://www.ananova.com/news/story/sm_1870958.html?menu=news.quirkies.sexlife. Retrieved 2008-04-11. 

References

• Kimball's Biology Pages, Digestion
• Chemistry lecture
• American Journal of Physiology, article

External links

• Human Physiology - Digestion
• Berkeley Anatomy lecture on the digestive system
• NIH guide to digestive system

v • d • e Human organ systems Cardiovascular system · Endocrine system · Gastrointestinal tract (Digestion) · Immune system · Integumentary system · Lymphatic system · Muscular system · Musculoskeletal system · Nervous system · Reproductive system · Respiratory system · Skeletal system · Urinary system

v • d • e Digestive system, physiology: gastrointestinal physiology GI tract Upper GI Exocrine Chief cells (Pepsinogen) · Parietal cells (Gastric acid, Intrinsic factor) · Goblet cells (Mucus) Processes Swallowing · Vomiting Fluids Saliva · Gastric juice Lower GI Enteric nervous system Meissner's plexus · Auerbach's plexus Endocrine/paracrine G cells (gastrin) · D cells (somatostatin) · ECL cells (Histamine) enterogastrone: I cells (CCK) · K cells (GIP) · S cells (secretin) Enteroendocrine cells · Enterochromaffin cell · APUD cell Border Brunner's glands · Paneth cells · Enterocytes Fluids Intestinal juice Processes Segmentation contractions · Migrating motor complex · Borborygmus · Defecation Either/both Processes Peristalsis (Interstitial cell of Cajal) · Gastrocolic reflex · Digestion Accessory Fluids Bile · Pancreatic juice Processes Enterohepatic circulation Abdominopelvic Peritoneal fluid digestive system navs: anat of tract,glands,perit,diaphragm/physio/dev, noncongen/congen/congen of d+w/neoplasia, symptoms+signs/eponymous, proc

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Dansk (Danish)
n. - fordøjelse, optagelse, nedbrydning

Nederlands (Dutch)
spijsvertering

Français (French)
n. - (Anat, Chim, fig) digestion

Deutsch (German)
n. - Verdauung, Digestion

Ελληνική (Greek)
n. - πέψη, χώνευση

Italiano (Italian)
digestione

Português (Portuguese)
n. - digestão (f)

Русский (Russian)
пищеварение, усвоение

Español (Spanish)
n. - digestión

Svenska (Swedish)
n. - (mat)smältning

中文（简体）(Chinese (Simplified))
消化力, 领悟

中文（繁體）(Chinese (Traditional))
n. - 消化力, 領悟

한국어 (Korean)
n. - 소화, 동화력

日本語 (Japanese)
n. - 消化, 消化力, 吸収

العربيه (Arabic)
‏(الاسم) عمليه الهضم الطعام, القدرة على هضم الطعام‏

עברית (Hebrew)
n. - ‮עיכול‬

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