digestive system

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 ChordataChordata
, phylum of animals having a notochord, or dorsal stiffening rod, as the chief internal skeletal support at some stage of their development. Most chordates are vertebrates (animals with backbones), but the phylum also includes some small marine invertebrate animals.
..... Click the link for more information. , 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 systemcirculatory system,
group of organs that transport blood and the substances it carries to and from all parts of the body. The circulatory system can be considered as composed of two parts: the systemic circulation, which serves the body as a whole except for the lungs, and the
..... Click the link for more information. 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 mouthmouth,
entrance to the digestive and respiratory tracts. The mouth, or oral cavity, is ordinarily a simple opening in lower animals; in vertebrates it is a more complex structure.
..... Click the link for more information. , all food passes through the alimentary canal (pharynxpharynx
, area of the gastrointestinal and respiratory tracts which lies between the mouth and the esophagus. In humans, the pharynx is a cone-shaped tube about 4 1-2 in. (11.43 cm) long.
..... Click the link for more information. , esophagusesophagus
, portion of the digestive tube that conducts food from the mouth to the stomach. When food is swallowed it passes from the pharynx into the esophagus, initiating rhythmic contractions (peristalsis) of the esophageal wall, which propel the food along toward the stomach.
..... Click the link for more information. , stomachstomach,
saclike dilation in the gastrointestinal tract between the esophagus and the intestines, forming an organ of digestion. The stomach is present in virtually all vertebrate animals and in many invertebrates.
..... Click the link for more information. , and intestinesintestine,
muscular hoselike portion of the gastrointestinal tract extending from the lower end of the stomach (pylorus) to the anal opening. In humans this fairly narrow (about 1 in./2.
..... Click the link for more information. ) 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 amylaseamylase
, enzyme having physiological, commercial, and historical significance, also called diastase. It is found in both plants and animals. Amylase was purified (1835) from malt by Anselme Payen and Jean Persoz.
..... Click the link for more information. . 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 juicesgastric juice,
thin, strongly acidic (pH varying from 1 to 3), almost colorless liquid secreted by the glands in the lining of the stomach. Its essential constituents are the digestive enzymes pepsin and rennin (see rennet), hydrochloric acid, and mucus.
..... Click the link for more information. , which act on the partially digested food. The stomach also physically churns and mixes the food. The stomach secretions include the enzyme pepsinpepsin,
enzyme produced in the mucosal lining of the stomach that acts to degrade protein. Pepsin is one of three principal protein-degrading, or proteolytic, enzymes in the digestive system, the other two being chymotrypsin and trypsin.
..... Click the link for more information. , 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 pancreaspancreas
, glandular organ that secretes digestive enzymes and hormones. In humans, the pancreas is a yellowish organ about 7 in. (17.8 cm) long and 1.5 in. (3.8 cm) wide. It lies beneath the stomach and is connected to the small intestine at the duodenum (see digestive system).
..... Click the link for more information. and bilebile,
bitter alkaline fluid of a yellow, brown, or green color, secreted, in man, by the liver. Bile, or gall, is composed of water, bile acids and their salts, bile pigments, cholesterol, fatty acids, and inorganic salts.
..... Click the link for more information. from the liverliver,
largest glandular organ of the body, weighing about 3 lb (1.36 kg). It is reddish brown in color and is divided into four lobes of unequal size and shape. The liver lies on the right side of the abdominal cavity beneath the diaphragm.
..... Click the link for more information. 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 trypsintrypsin,
enzyme that acts to degrade protein; it is often referred to as a proteolytic enzyme, or proteinase. Trypsin is one of the three principal digestive proteinases, the other two being pepsin and chymotrypsin.
..... Click the link for more information. 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.

Bibliography

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).

Digestive system

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.

Anatomy

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.

Musculature

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

Human digestive system

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.

Physiology

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.

Motility

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

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

Absorption

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.

Digestive System

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.

REFERENCES

Shmal’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

digestive system

[də′jes·tiv ‚sis·təm] (anatomy) A system of structures in which food substances are digested.

digestive system

digestive

[dĭ-jes´tiv] pertaining to digestion.digestive system the organs that have as their particular function the ingestion, digestion, and absorption of food or nutritive elements. They include the mouth, teeth, tongue, pharynx, esophagus, stomach, and intestines. The accessory organs of digestion, which contribute secretions important to digestion, include the salivary glands, pancreas, liver, and gallbladder. (See also color plates.)Mouth. The mouth is the entrance to the alimentary canal; in it the teeth, tongue, and jaws begin the process of digestion by mastication. saliva is secreted into the mouth by three separate pairs of glands (the salivary glands) located under the tongue, inside the lower jaw, and in the cheek. Saliva softens and lubricates the food, and dissolves some of it; it also contains an enzyme called ptyalin that begins the conversion of starches into sugar. Saliva also moistens the inside of the mouth, the tongue, and the teeth, and rinses them after the food has departed on the next stage of its journey.
Four passageways meet at the back of the throat: the oral and nasal passages, the larynx, and the esophagus. In the act of swallowing, the entrances to the nasal passages and the larynx are each sealed off momentarily by the soft palate and the epiglottis, so that the food can pass into the esophagus without straying into the tract" >respiratory tract. Stomach. Propelled by rhythmic muscular contractions called peristalsis, the food moves rapidly through the esophagus, past the sphincter" >cardiac sphincter (a circular muscle at the base of the esophagus) and into the stomach. Here the peristaltic motions are stronger and more frequent, occurring at the rate of three per minute, churning, liquefying, and mixing the foods with the gastric juice. In the juice are the enzymes pepsin and lipase and, in infants, rennin; a secretion called mucin, which coats and protects the stomach lining; and hydrochloric acid. Together the pepsin and hydrochloric acid begin the splitting of the proteins in the food. The lipase in the stomach is a rather weak fat-splitting enzyme, able to act only on fats that are already emulsified, such as those in cream and the yolk of egg; the intestine has a stronger lipase, and it is there that most fats are digested.
The average adult stomach holds about 1.5 liters. The stomach reaches its peak of digestive activity nearly 2 hours after a meal and may empty in 3 to 4½ hours; a heavy meal may take as long as 6 hours to pass into the small intestine. Small Intestine. The food leaves the stomach in the form of chyme, a thick, liquid mixture. It passes through the pylorus, a sphincter muscle opening from the lower part of the stomach into the duodenum. This sphincter is closed most of the time, opening each time a peristaltic wave passes over it. The stomach is much wider than the rest of the canal and also has a J-shaped curve at its bottom, so that the passage of food through the pylorus is automatically slowed until the food is of the right consistency to flow through the narrow opening into the intestine.
The small intestine is about 6 meters (20 feet) long. The lining of the small intestine has deep folds and fingerlike projections called villi that give it a surface of about 9 square meters (100 square feet) through which absorption of food can take place.
The duodenum, a C-shaped curve with a length of about 25 cm (10 in), is the first and widest part of the small intestine. Into it flows the pancreatic juice, with enzymes that break down starch, protein, and fats. The common bile duct also empties into the duodenum. The bile emulsifies fats for the action of the fat-splitting enzymes.
Just below the duodenum is the jejunum, the longest portion of the small intestine, and beyond that is the ileum, the last and narrowest section of the small intestine. Along this whole length, carbohydrates, proteins, and fats are broken down into sugars, amino acids, fatty acids, and glycerin. The lining of the small intestine absorbs these nutrient compounds as rapidly as they are produced. The bulky and unusable parts of the diet pass into the large intestine.Large Intestine. At the junction of the small and large intestines is the valve" >ileocecal valve, so called because it is at the end of the ileum and the beginning of the cecum. A small blind tube called the appendix" >vermiform appendix is attached to the cecum. The longer part of the large intestine is called the colon and is divided into the ascending, transverse, and descending colon, and the sigmoid flexure, an S-shaped bend at the distal end of the colon. The sigmoid colon empties into the rectum.
Along the 1.7 meters (5.5 feet) or so of the large intestine, the liquid in the waste is gradually reabsorbed through the intestinal walls. Thus the waste is formed into fairly solid feces and pushed down into the rectum for eventual evacuation. This takes from 10 to 20 hours. The evacuation consists of bacteria, cells cast off from the intestines, some mucus, and such indigestible substances as cellulose. The normal dark brown color of the feces" >feces is caused by bile pigments.

system

[sis´tem] 1. a set or series of interconnected or interdependent parts or entities (objects, organs, or organisms) that act together in a common purpose or produce results impossible by action of one alone. 2. an organized set of principles or ideas. adj., adj systemat´ic, system´ic.
The parts of a system can be referred to as its elements or components; the environment of the system is defined as all of the factors that affect the system and are affected by it. A living system is capable of taking in matter, energy, and information from its environment (input), processing them in some way, and returning matter, energy, and information to its environment as output.
An open system is one in which there is an exchange of matter, energy, and information with the environment; in a closed system there is no such exchange. A living system cannot survive without this exchange, but in order to survive it must maintain pattern and organization in the midst of constant change. Control of self-regulation of an open system is achieved by dynamic interactions among its elements or components. The result of self-regulation is referred to as the steady state; that is, a state of equilibrium. homeostasis is an assemblage of organic regulations that act to maintain steady states of a living organism.
A system can be divided hierarchically into subsystems, which can be further subdivided into sub-subsystems and components. A system and its environment could be considered as a unified whole for purposes of study, or a subsystem could be studied as a system. For example, the collection of glands in the endocrine system can be thought of as a system, each endocrine gland could be viewed as a system, or even specific cells of a single gland could be studied as a system. It is also possible to think of the human body as a living system and the endocrine system as a subsystem. The division of a system into a subsystem and its environment is dependent on the perspective chosen by the person studying a particular phenomenon.

Systems, subsystems, and suprasystems. Within the environment there are suprasystems, such as human society, and systems within the suprasystem, such as the educational and industrial systems and the health care delivery system. Within the health care delivery system are subsystems, such as the patient, family members, the nurse, the physician, and allied health care professionals and paraprofessionals.alimentary system digestive system.apothecaries' system see apothecaries' system" >apothecaries' system.autonomic nervous system see autonomic nervous system.avoirdupois system see avoirdupois system" >avoirdupois system.behavioral system in the behavioral system model of nursing, the patterned, repetitive, and purposeful behaviors of an individual.cardiovascular system the heart and blood vessels, by which blood is pumped and circulated through the body; see also circulatory system.CD system (cluster designation) a system for classifying markers" >cell-surface markers expressed by lymphocytes based on a computer analysis of monoclonal antibodies against hla antigens, with antibodies having similar specificity characteristics being grouped together and assigned a number (CD1, CD2, CD3, etc.); these CD numbers are also applied to the specific antigens recognized by the various groups of monoclonal antibodies. See also antigen" >CD antigen.centimeter-gram-second system (CGS) (cgs) a system of measurements in which the units are based on the centimeter as the unit of length, the gram as the unit of mass, and the second as the unit of time.central nervous system see central nervous system.centrencephalic system the neurons in the central core of the brainstem from the thalamus to the medulla oblongata, connecting the hemispheres" >cerebral hemispheres.circulatory system see circulatory system.client system in the general systems framework and theory of goal attainment" >general systems framework and theory of goal attainment, the composite of physiological, psychological, sociocultural, and developmental variables that make up the total person.colloid system (colloidal system) colloid (def. 3).conduction system (conductive system (of heart)) the system of atypical cardiac muscle fibers, comprising the sinoatrial and atrioventricular nodes, internodal tracts, atrioventricular bundle, bundle branch, and terminal ramifications into the Purkinje network.digestive system see digestive system.Emergency Medical Services (EMS) system a comprehensive program designed to provide services to the patient in the prehospital setting. The system is activated when a call is made to the EMS operator, who then dispatches an ambulance to the patient. The patient receives critical interventions and is stabilized at the scene. A communication system allows the health care workers at the scene to contact a trauma center for information regarding further treatment and disposition of the patient, followed by transportation of the patient to the most appropriate facility for treatment.endocrine system the system of ductless glands and other structures that produce internal secretions (hormones) that are released directly into the circulatory system, influencing metabolism and other body processes; see endocrine glands.environmental control system unit" >environmental control unit.expert system a set of computer programs designed to serve as an aid in decision making.extrapyramidal system see extrapyramidal system.gateway system a software interface between an online searcher and one or more search systems, facilitating the use of the system by searchers who are unfamiliar with it, or with online retrieval in general.genitourinary system the organs concerned with production and excretion of urine, together with the reproductive organs. (See Plates.) Called also urogenital system.haversian system a canal" >haversian canal and its concentrically arranged lamellae, constituting the basic unit of structure in compact bone (osteon).

Haversian system: Structures of compact and spongy bone with the central haversian canal surrounded by the lamellae. From Applegate, 2000.health care system see health care system.heterogeneous system a system or structure made up of mechanically separable parts, as an emulsion or suspension.His-Purkinje system the intraventricular conduction system from the bundle of His to the distal Purkinje fibers, which carries the impulse to the ventricles.Home Health Care Classification system see home health care classification system.homogeneous system a system or structure made up of parts that cannot be mechanically separated, as a solution.hypophyseoportal system (hypophysioportal system) (hypothalamo-hypophysial portal system) the venules connecting the hypothalamus with the sinusoidal capillaries of the anterior lobe of the pituitary gland; they carry releasing substances to the pituitary.immune system see immune system.interpersonal system in the general systems framework and theory of goal attainment, two or more individuals interacting in a given situation.lay health system a system comprising an informal referral network and sources of treatment outside the formal biomedical sources of health care; it includes individual consultation and information-seeking through significant others and peers concerning health behaviors, symptoms, and evaluation of treatment before, during, and after consultation with health care professionals.legal system in the omaha system, anything connected with law or its administration; it includes legal aid, attorney, courts, or Child Protective Services (CPS), and many other agencies and officials.limbic system a system of brain structures common to the brains of all mammals, comprising the phylogenetically old cortex (archipallium and paleopallium) and its primarily related nuclei. It is associated with olfaction, autonomic functions, and certain aspects of emotion and behavior.lymphatic system see lymphatic system.lymphoid system the lymphoid tissue of the body, collectively; it consists of primary (or central) lymphoid tissues, the bone marrow, and thymus, and secondary (or peripheral) tissues, the lymph nodes, spleen, and gut-associated lymphoid tissue (tonsils, Peyer's patches).lymphoreticular system the lymphoid and reticuloendothelial systems considered together; see also lymphoreticular disorders.metric system see metric system.mononuclear phagocyte system the group of highly phagocytic cells that have a common origin from stem cells of the bone marrow and develop circulating monocytes and tissue macrophages, which develop from monocytes that have migrated to connective tissue of the liver (kupffer's cells), lung, spleen, and lymph nodes. The term has been proposed to replace reticuloendothelial system, which includes some cells of different origin and does not include all macrophages.nervous system see nervous system.nursing system in the self-care model of nursing, all the actions and interactions of nurses and patients in nursing practice situations; nursing systems fall into three categories: wholly compensatory, partly compensatory, and supportive-educative.Omaha system see omaha system.oxygen delivery system a device that delivers oxygen through the upper airways to the lungs at concentrations above that of ambient air. There are two general types: the fixed performance or high flow type, which can supply all of the needs of a patient for inspired gas at a given fractional inspired oxygen; and the variable performance or low flow type, which cannot supply all of the patient's needs for oxygen and delivers fractional inspired oxygen that varies with ventilatory demand.parasympathetic nervous system see parasympathetic nervous system" >parasympathetic nervous system.peripheral nervous system the portion of the nervous system consisting of the nerves and ganglia outside the brain and spinal cord.personal system in the general systems framework and theory of goal attainment, the unified self, a complex whole that is rational, conscious, and feeling and that sets goals and decides on the means of achieving them.pituitary portal system hypothalamo-hypophysial portal system.portal system an arrangement by which blood collected from one set of capillaries passes through a large vessel or vessels and another set of capillaries before returning to the systemic circulation, as in the pituitary gland (the hypothalamo-hypophysial portal system) or the liver (the hepatic portal circulation).renin-angiotensin-aldosterone system see renin-angiotensin-aldosterone system.respiratory system the group of specialized organs whose specific function is to provide for the transfer of oxygen from the air to the blood and of waste carbon dioxide from the blood to the air. The organs of the system include the nose, the pharynx, the larynx, the trachea, the bronchi, and the lungs. See also respiration and Plates 7 and 8.reticular activating system see reticular activating system.reticuloendothelial system see reticuloendothelial system.safety system see safety system." >safety system.SI system see SI units.skeletal system see skeletal system.social system in the general systems framework and theory of goal attainment, an organized boundary system of social roles, behaviors, and practices developed to maintain balance for growth, development, and performance, which involves an exchange of energy and information between the person and the environment for regulation and control of stressors.support system in the omaha system, the circle of friends, family, and associates that provide love, care, and need gratification; it may include church, school, workplace, or other groupings.sympathetic nervous system see sympathetic nervous system.Unified Medical Language system see unified medical language system.Unified Nursing Language system see unified nursing language system.unit dose system a method of delivery of patient medications directly to the patient care unit. Following review by a nurse, a copy of the physician's original order is sent to the pharmacy, where the pharmacist reviews it again. The pharmacist then fills the order and delivers the medication to the patient care unit, usually in a 24-hour supply. Each patient has an individual supply of medications prepared and labeled by the pharmacist.urinary system the system formed in the body by the kidneys, ureters, urinary bladder, and urethra, the organs concerned in the production and excretion of urine.urogenital system genitourinary system.vascular system circulatory system.vasomotor system the part of the nervous system that controls the caliber of the blood vessels.

al·i·men·ta·ry sys·tem

[TA] the digestive tract from the mouth to the anus with all its associated glands and organs. Synonym(s): systema digestorium [TA], alimentary apparatus, apparatus digestorius, digestive apparatus, digestive system, systema alimentarium

digestive system

n. The digestive tract and digestive glands regarded as an integrated system responsible for the ingestion, digestion, and absorption of food.

di·ges·tive sys·tem

(di-jes'tiv sis'tĕm) The digestive tract from the mouth to the anus with all its associated glands and organs.

THE DIGESTIVE SYSTEM

digestive system

The alimentary canal (oral cavity, pharynx, esophagus, stomach, small and large intestines) and the accessory organs (teeth, tongue, salivary glands, liver, and pancreas). See: illustration; digestion

digestive system

The PHARYNX, OESOPHAGUS, STOMACH and intestines and the associated glands that secrete digestive ENZYMES.

digestive system

the ALIMENTARY CANAL or gut, together with associated digestive glands (see DIGESTION). Taking mammals as an example, the main areas of digestive activity are:

MOUTH cavity: starch digestion occurs during chewing, a process which increases the surface area of food, using teeth modified for particular foodstuffs and functions. The pH in the mouth ranges from slightly acid to slightly alkaline.
STOMACH: the site of the start of protein digestion in acid conditions with strong muscular churning movements. Bones are partly digested.
SMALL INTESTINE: the main area of digestion for all types of food, with enzymes secreted by pancreas and intestinal glands. Strong alkaline conditions.

Patient discussion about digestive system

Q. It feels like I cannot breath properly, my chest feels heavy and Whenever i eats something it gets even more. If i don't eat much i get gases problem. I am regularly having constipation if i don't take proper medicine. Mostly i have been diagnosed as Digestive System problem, stomach problem, dryness in body and no Problem with my breathing system just cause of gases which causes my chest to remain heavy. I am worried because i never gets my real power in breathing and my digestive system is not getting well. Kindly suggest.A. I take milk regularly have no problem with it nor with any specific food or i have never tried to look at this aspect. However i do feel stomach problem with foods heavy for stomach like burger or too much oily things..

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The Human Digestive System

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Digestive system

Anatomy

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Nerve and endocrine tissue

Physiology

Motility

Digestion

Absorption

Digestive System

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digestive system

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al·i·men·ta·ry sys·tem

digestive system

di·ges·tive sys·tem

digestive system

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Patient discussion about digestive system

digestive system

Synonyms for digestive system

noun the system that makes food absorbable into the body

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