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digestive system, in the animal kingdom, a group of organs functioning in digestion and assimilation of food and elimination of wastes. Virtually all animals have a digestive system. In the vertebrates (phylum Chordata, subphylum Vertebrata) the digestive system is very complex. It consists of the gastrointestinal tract (gut), an extensive tube extending from the mouth to the anus, through which the swallowing, digestion, and assimilation of food and the elimination of waste products are accomplished.
The Human Digestive System
In the digestive system, ingested food is converted into a form that can be absorbed into the circulatory system for distribution to and utilization by the various tissues of the body. This is accomplished both physically, by mastication in the mouth and churning of the stomach, and chemically, by secretions and enzymes of the gastrointestinal tract. Beginning at the mouth, all food passes through the alimentary canal (pharynx, esophagus, stomach, and intestines) before it reaches the anus, where undigested matter is eliminated as waste. The outer walls of the digestive tract are composed of layers of muscle and tissue that undergo waves of contraction (peristalsis), thereby pushing the food along its digestive path. The inner lining contains glands that secrete the acids and enzymes necessary to break down food into a form utilizable by the body.
Digestion begins in the mouth, where chewing reduces the food to fine texture, and saliva moistens it and begins the conversion of starch into simple sugars by means of an enzyme, salivary amylase. The food is then swallowed, passing through the pharynx and down the muscular esophagus, or gullet, to the expanded muscular pouchlike section of the gastrointestinal tract, the stomach. Specialized cells in the stomach secrete digestive enzymes and gastric juices, which act on the partially digested food. The stomach also physically churns and mixes the food. The stomach secretions include the enzyme pepsin, which acts on proteins; hydrochloric acid, essential for the action of pepsin; and an enzyme, gastric lipase, which begins the breakdown of fats. The gastric juices of young children contain, in addition to those just mentioned, the enzyme rennin, which acts on milk. Some foods, including simple sugars and alcohol, are absorbed directly through the stomach wall and do not remain in the stomach. Most food, however, is not absorbed in the stomach and passes into the duodenum (first section of the small intestine) in the form of a thick liquid called chyme.
Digestive enzymes from the pancreas and bile from the liver act on the chyme in the duodenum. These enzymes include pancreatic lipase, which breaks down fats into glycerol and fatty acids; pancreatic amylase, which continues the breakdown of starches and most other carbohydrates into disaccharides; and trypsin and erepsin, which break down whole and partially digested proteins (proteoses and peptones) into amino acids, the end products of protein digestion. Bile is essential for emulsifying large fat globules into smaller ones that are more easily digested by pancreatic lipase. In addition, intestinal juices are secreted by small glands in the intestinal wall called the crypts of Lieberkühn. Like the pancreatic juices, intestinal juices contain enzymes that continue the digestion of proteins and fats and also contain three enzymes that break down disaccharides into glucose, galactose, and fructose (simple sugars). The digested food is absorbed into the circulatory and lymphatic systems through small fingerlike projections of the intestinal wall, called villi. Undigested material passes into the large intestine, where most of the water is absorbed and the solid material, or feces, is excreted through the anus.
See J. E. Morton, Guts: The Form and Function of the Digestive System (1967); H. W. Davenport, Physiology of the Digestive Tract: An Introductory Text (3d ed. 1971).
The vertebrate digestive system consists of the digestive tract and ancillary organs that serve for the acquisition of food and assimilation of nutrients required for energy, growth, maintenance, and reproduction. Food is ingested, reduced to particles, mixed with digestive fluids and enzymes, and propelled through the digestive tract. Enzymes produced by the host animal and microbes indigenous to the digestive tract destroy harmful agents and convert food into a limited number of nutrients, which are selectively absorbed. The digestive systems of vertebrates show numerous structural and functional adaptations to their diet, habitat, and other characteristics. Carnivores, which feed exclusively on other animals, and species that feed on plant concentrates (seeds, fruit, nectar, pollen) tend to have the shortest and simplest digestive tract. The digestive tract tends to be more complex in omnivores, which feed on both plants and animals, and most complex in herbivores, which feed principally on the fibrous portions of plants.
Gut structure and function can also vary with the habitat and other physiological characteristics of a species. The digestive tract of fish has adaptations for a marine or fresh-water environment. The basal metabolic rate per gram of body weight increases with a decrease in the body mass. Therefore, small animals must process larger amounts of food per gram of body weight, thus limiting their maximum gut capacity and digesta retention time.
Because of wide species variations, the digestive system of vertebrates is best described in terms of the headgut, foregut, midgut, pancreas, biliary system, and hindgut. The headgut consists of the mouthparts and pharynx, which serve for the procurement and the initial preparation and swallowing of food. The foregut consists of an esophagus for the swallowing of food and, in most species, a stomach that serves for its storage and initial stages of digestion. The esophagus of most vertebrates is lined with a multilayer of cells that are impermeable to absorption. In most birds it contains the crop, an outpocketing of its wall that provides for the temporary storage of food. A stomach is present in all but the cyclostomes and some species of advanced fish and in the larval amphibians. In most vertebrates it consists of a dilated segment of the gut that is separated from the esophagus and midgut by muscular sphincters or valves. This is often referred to as a simple stomach. However, in birds these functions are carried out by the crop (storage), proventriculus (secretion), and gizzard (grinding or mastication). In most vertebrates, a major portion of the stomach is lined with a proper gastric mucosa (epithelium), which secretes mucus, hydrochloric acid (HCl), and pepsinogen. The distal (pyloric) part of the stomach secretes mucus and bicarbonate ions (HCO-3), and its muscular contractions help reduce the size of food particles and transfer partially digested food into the midgut. The stomach of reptiles and most mammals has an additional area of cardiac mucosa near its entrance, which also secretes mucus and bicarbonate ions. See Esophagus
The midgut or small intestine is the principal site for the digestion of food and the absorption of nutrients. It is lined with a single layer of cells that secrete mucus and fluids, contain enzymes that aid in the final stages of carbohydrate and protein digestion, and absorb nutrients from the lumen into the circulatory system. The surface area of the lumen can be increased by a variety of means, such as folds and pyloric ceca (blind sacs) in fish. In higher vertebrates the lumen surface is increased by the presence of villi, which are macroscopic projections of the epithelial and subepithelial tissue.
The lumen surface is also expanded by a brush border of microvilli on the lumen-facing (apical) surface of the midgut absorptive cells in all vertebrates. The brush border membranes contain enzymes that aid in the final digestion of food and mechanisms that provide for the selective absorption of nutrients. The lumenal surface area of the human small intestine is increased 10-fold by the presence of villi and an additional 20-fold by the microvilli, resulting in a total surface area of 310,000 in.2 (2,000,000 cm2).
Digestion in the midgut is aided by secretions of digestive enzymes and fluid by pancreatic tissue, and secretion of bile by the liver. Pancreatic tissue is distributed along the intestinal wall, and even into the liver, of some species of fish. However, the pancreas is a compact organ in sharks, skates, rays, many teleosts, and all other vertebrates. The liver is a compact organ in all vertebrates. One of its many functions is the secretion of bile. In most vertebrates, the bile is stored in the gallbladder and released into the intestine as needed, but a gallbladder is absent in some species of fish and mammals. Bile salts serve to emulsify lipids and increase their surface area available for digestion by the water-soluble lipase. See Gallbladder, Liver, Pancreas
The hindgut is the final site of digestion and absorption prior to defecation or evacuation of waste products. The hindgut of fish, amphibian larvae, and a few mammals is short and difficult to distinguish from the midgut. However, the hindgut of adult amphibians and reptiles, birds, and most mammals is a distinct segment, which is separated from the midgut by a muscular sphincter or valve. It also tends to be larger in diameter. Thus, the midgut and hindgut of these animals are often referred to as the small intestine and the large intestine. See Intestine
The hindgut of some reptiles and many mammals includes a blind sac or cecum near its junction with the midgut. A pair of ceca are present in the hindgut of many birds and a few mammalian species. The remainder of the hindgut consists of the colon and a short, straight, terminal segment, which is called the rectum in mammals. The digestive and urinary tracts exit separately from the body of most species of fish and mammals. However, in adult amphibians and the reptiles, birds, and some mammals, this segment terminates in a chamber called the cloaca, which also serves as an exit for the urinary and reproductive systems. The hindgut or, where present, the cloaca terminates in the anus. See Colon, Urinary system
The hindgut is similarly lined with a single layer of absorptive and mucus-secreting cells. However, it lacks villi, and (with the exception of the cecum of birds) its absorptive cells lack digestive enzymes and the ability to absorb most nutrients. One major function of the hindgut is to reabsorb the fluids secreted into the upper digestive tract and (in animals that have a cloaca) excreted in the urine. It also serves as the principal site for the microbial production of nutrients in the herbivorous reptiles and birds and in most herbivorous mammals. Thus, the hindgut tends to be longest in animals that need to conserve water in an arid environment, and has a larger capacity in most herbivores.
The digestion of food, absorption of nutrients, and excretion of waste products require the mixing of ingesta with digestive enzymes and the transit of ingesta and digesta through the digestive tract. In all vertebrates other than the cyclostome the contents are mixed and moved by an inner layer of circular muscle and an outer layer of muscle that runs longitudinally along the tract. The initial act of deglutition (swallowing) and the final act by which waste products are defecated from the digestive tract are effected by striated muscle. This type of muscle is characterized by rapid contraction and is controlled by extrinsic nerves. However, the esophagus of amphibians, reptiles, and birds, and the entire gastrointestinal tract of all vertebrates are enveloped by smooth muscle. This smooth muscle contracts more slowly, and its rate of contraction is partly independent of external stimulation. See Muscle
Nerve and endocrine tissue
The initial act of deglutition and final act of defecation are under the voluntary control of the central nervous system. However, the remainder of the digestive system is subject to the involuntary control of nerves which release a variety of neurotransmitting or neuromodulating agents that either stimulate or inhibit muscular contractions and the secretions of glands and cells. The motor and secretory activities of the digestive system are also under the control of a wide range of other substances produced by endocrine cells that are released either distant from (hormones) or adjacent to (paracrine agents) their site of action. Although there are some major variations in the complement and activities of the neurotransmitters, neuromodulators, hormones, and paracrine agents, their basic patterns of control are similar.
The anatomy of the human digestive system is similar to that of other mammalian omnivores (see illustration). The teeth and salivary glands are those of a mammalian omnivore, and the initial two-thirds of the esophagus is enveloped by striated muscle. A simple stomach is followed by an intestine, whose length consists of approximately two-thirds small bowel and one-third large bowel. The structures of the pancreas and biliary system show no major differences from those of other mammals. During early fetal development, a distinct, conical cecum is present and continues to grow until the sixth month of gestation. However, unlike other primates, the cecum recedes to become little more than a bulge in the proximal colon by the time of birth. The colon continues to lengthen after the birth and is sacculated throughout its length like that of the apes and a few monkeys but few other mammals.
The major physiological activities of the digestive system are motility, secretion, digestion, and absorption. Each activity can be affected by diet and, in the cold-blooded species, is reduced with a decrease in body temperature.
The mastication of food and the movement of ingesta and digesta through the digestive tract are controlled by the motor activity of muscular contractions. Pressure of food against the palate and back of the mouth stimulates a nerve reflex that passes through a deglutition center in the brain. This reflex closes the entrance into the respiratory system and stops respiration, to prevent the inspiration of food into the lungs, and initiates muscular contractions that pass food into the esophagus. The food (bolus) is then passed down the esophagus and into the stomach by a moving wave of muscular contractions (peristalsis) accompanied by inhibition of the esophageal sphincters. The multicompartmental forestomach of ruminants undergoes a continuous series of complex, repetitive contractions that are controlled by the central nervous system. However, the gastric motility of most species and the intestinal motility of all vertebrates are controlled partially by the intrinsic characteristics of their smooth muscle cells. The result is production of either stationary (mixing) contractions of the stomach and intestine or a series of peristaltic contractions that carry digesta on through the tract.
Digestion is accomplished by enzymes produced by the digestive system (endogenous enzymes) or by bacteria that are normal residents of the digestive tract. Plant and animal starches are converted to oligosaccharides (short-chain structures) and disaccharides by amylase, which is secreted by the salivary glands of some species and the pancreas of all vertebrates. The end products of starch digestion, plus the dietary disaccharides, are converted to monosaccharides by enzymes in the brush border of the absorptive epithelial cells lining the small intestine. Vertebrates do not produce enzymes capable of digesting the structural polysaccharides of plants.
Lipids are digested into alcohols, monoglycerides, and fatty acids by lipases and esterases, which are secreted predominantly by the pancreas. However, the lipases are water-soluble enzymes that can attack their substrate only at a lipid-water interface. Therefore, the lipids must be emulsified in order to provide the surface area required for efficient digestion. Emulsification is accomplished by the release of bile salts secreted by the liver and released into the midgut.
Dietary protein is first broken down into long chains of amino acids (polypeptides) by gastric pepsin and pancreatic trypsin. The polypeptides are then attacked by other pancreatic proteases (chymotrypsin, carboxypeptidase, elastase) to form tripeptides, dipeptides, and amino acids. All of these enzymes are secreted in an inactive form to prevent the self-digestion of the secretory cells prior to their release. Pepsin is activated by the acidity resulting from the secretion of hydrochloric acid (HCl) into the stomach, and trypsin is activated by an enzyme (enterokinase) that is secreted by intestinal epithelium. Tri- and dipeptides are digested into amino acids by enzymes in the brush border and contents of midgut absorptive cells. Nucleic acids are digested by pancreatic ribonucleases into pentose sugars, purines, and pyrimidines.
Substantial numbers of bacteria can be found in all segments of the gastrointestinal tract, but the highest numbers are present in those segments in which digesta are retained for prolonged periods of time at a relatively neutral pH. Indigenous bacteria help protect the animal from pathogenic microorganisms by stimulating immunity and competing for substrates. They also convert dietary and endogenous substances that are not digested by endogenous enzymes into absorbable nutrients. Many species of indigenous bacteria can ferment sugars, starches, and structural carbohydrates into short-chain fatty acids. The short-chain fatty acids, which are predominantly acetic, propionic, and butyric acids, are readily absorbed and serve as an additional source of energy. These bacteria also synthesize microbial protein and the B-complex vitamins that may be useful to their host. The contributions of indigenous bacteria to the production and conservation of nutrients are greatest in herbivores. Although it has been estimated that short-chain fatty acid absorption provides 4% of total maintenance energy requirement by dogs and 6–10% of the maintenance energy required by humans, they can account for 30% of the maintenance energy of rabbits and up to 70% of maintenance energy of horses and ruminants. See Bacterial physiology and metabolism
The epithelial cells that line the gastrointestinal tract are closely attached to one another at their lumen-facing border by tight junctions, which are relatively impermeable to most substances other than water. Therefore, the major restriction for the absorption of most substances from the lumen into the blood is the apical and basolateral membranes of these cells. Lipid-soluble substances can be transported across the apical cell membranes by passive diffusion down their concentration gradient. The short- and medium-chain fatty acids that result from lipid digestion in the small intestine pass directly into the blood. However, the monoglycerides and long-chain fatty acids are resynthesized into triglycerides by the epithelial cells in the midgut and incorporated into small spheres (chylomicrons), which are transported across the basolateral membrane into the lymphatic system. Fat-soluble vitamins, long-chain alcohols, and other lipids also appear to be incorporated into chylomicrons and to enter the lymphatic system.
The intestinal cell membranes are relatively impermeable to the passive diffusion of water-soluble monosaccharides, amino acids, vitamins, and minerals that constitute a major portion of the required nutrients. These nutrients are selectively transferred across the intestinal cell membranes by carrier-mediated transport. Membrane carriers combine with the nutrient at one membrane surface and pass it across the membrane for release at the opposing surface. Some simply facilitate the diffusion of a substance down its concentration gradient; others are capable of transporting a nutrient against its concentration gradient, which requires either a direct or indirect investment of cellular energy. See Cell membranes
The metabolic processes of the body require a number of different minerals. Some such as iron, calcium, sodium, and chloride are required in relatively large quantities. Others such as manganese and zinc are labeled trace minerals because they are required in only minute amounts.
The nutrient that is required in largest quantity for digestion, absorption, metabolism, and excretion of waste products is water. Because it readily diffuses across cell membranes down its concentration gradient, the net secretion or absorption of water is determined by the net secretion or absorption of all other substances. Sodium, chloride, and bicarbonate are the principal ions that are present in the extracellular fluids that bathe the body cells of all vertebrates and that are transported across cell membranes. Therefore, the transport of these electrolytes is the major driving force for the secretion or absorption of water.
the alimentary organs in animals and man. The digestive system supplies the body with energy and building materials that are needed to restore cells and tissues, which are constantly being destroyed in the normal course of vital activity. In most animals, the digestive system is a tube with an oral opening for food intake and an anus for elimination of unassimilated digestive products.
Lower animals lack digestive systems, while intracellular digestion is characteristic of protozoans. In the most primitive multicellular animals, digestion is carried out by specialized individual cells—choanocytes and pinacocytes in sponges and parenchymal cells in acoelomate turbellarians. The structure of the digestive system varies greatly among invertebrates. In coelen-terates the system consists of an enteron, which is lined with endoderm and communicates with the outside only through an oral opening. Hydroids possess a simple sacciform enteron, but in other coelenterates the digestive system is divided into a central “stomach” and peripheral chambers or canals and is called the gastrovascular system. In coral polyps and ctenophorans, the edges of the mouth are turned inward to form the stomodaeum, which is ectodermal. The digestive system of platyhelminths also lacks an anus. It consists of an endodermal midgut, which is branched to varying degrees among the large worms, and a muscular foregut, or pharynx.
Nemertines, nemathelminths, and other invertebrates have, in addition to a foregut and midgut, an ectodermal hindgut with an anal opening. The pharynx of mollusks includes a horny jaw, a radula, and salivary glands. Mollusks also have an esophagus, a stomach, a capacious digestive gland (which functions as a liver), small intestine, and a hindgut. Cephalopod mollusks have, in addition, a pancreas.
The digestive system of arthropods is also highly complex. For example, the midgut in arachnids and crustaceans is furnished with a large digestive gland. The excretory organs—the Malpighian tubules—open into the hindgut in arachnids, myriapods, and insects. Over the course of evolution, tapeworms and acanthocephalids lost their digestive systems as a result of a long endoparasitic mode of life; members of the phylum Pogonophora lost theirs in conjunction with possessing a protective tube.
In lower chordates, the anterior portion of the intestine opens to the outside through metameric branchial clefts.
In vertebrates and man, the digestive system consists of an oral cavity, pharynx, esophagus, stomach, intestine, liver, and pancreas. In vertebrates that breathe through gills, the pharynx is lined with branchial clefts and thus serves not only fo • passing food from the mouth to the esophagus but also for breathing. The stomach is simple in most fishes, amphibians, reptiles, and predatory and omnivorous mammals; it is complex in ruminants, cetaceans, and some birds and fishes. The digestive tract of vertebrates is connected with a glandular system.
Terrestrial vertebrates have variously differentiated salivary glands. Some vertebrates with an intestine have other additional glandular organs; for example, many fishes have pyloric glands, and elasmobranchs have a rectal gland. The intestine of most vertebrates and man is differentiated into several sections that are morphologically and functionally distinct.
REFERENCESShmal’gauzen, I. I. Osnovy sravnitel’noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1947.
Beklemishev, V. N. Osnovy sravnitel’noi anatomii bespozvonochnykh, 3rd ed, vol. 2. Moscow, 1964.
A. V. IVANOV