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.
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 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 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.
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.
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.)
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.
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.
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.
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.
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.
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
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.
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.
What about the color and the smell? The color of feces comes from
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.
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).
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.
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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.
Organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations. Strings of amino acids make up proteins.
A yellowish or greenish digestive fluid excreted by the liver.
A term for a chewed mass of food making its way through the initial portions of the alimentary canal.
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.
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.
The large intestine, through which waste materials pass on their way to excretion through the anus.
A substance in which atoms of more than one element are bonded chemically to one another.
A protein material that speeds up chemical reactions in the bodies of plants and animals.
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.
The stomach and intestines.
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.
A type of sugar that occurs widely in nature. Glucose is the form in which animals usually receive carbohydrates.
A white polysaccharide that is the most common form in which carbohydrates are stored in animal tissues, particularly muscle and liver tissues.
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.
An iron-containing pigment in red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide from them.
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.
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.
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.
A group of tissues and cells, organized into a particular structure, that performs a specific function within anorganism.
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.
A series of involuntary muscle contractions that force bolus, and later waste, through the alimentary canal.
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.
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.
A general term for a muscle that surrounds and is able to control the size of a bodily opening.
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.
A group of cells, along with the substances that join them, that forms part of the structural materials in plants oranimals.
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.