The Human 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.
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.
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 also 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 also 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.
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 also 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. 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 also 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.
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.
Part II: Alimentary System
The alimentary system is responsible for the breakdown of food into its component parts, and for the absorption of the products into the body. The system consists of a tube running from the mouth to the anus that is variously known as the alimentary tract, digestive tract, gastrointestinal tract, or gut. The salivary glands, liver, gall bladder, and pancreas are distinct organs, but they are intimately associated with the alimentary tract and pass juices into it that are required for digestion.
The inner lining of the gastrointestinal tract (the mucosa) is covered by a layer of cells, the epithelium that performs the separate processes of secretion and absorption. The movement of gut contents from one region to the next is achieved by two layers of smooth muscle (circular and longitudinal) that lie outside the mucosa and that contract and relax in co-ordinated patterns. Between the mucosa and muscle layers lies the submucosa, which is rich in blood vessels and connective tissue. Different regions of the digestive tract are concerned with storage, secretion, the processes of food digestion, absorption, and the elimination of waste products. All regions of the gut have a capacity for the renewal of mucosal cells, and for protection against toxic or damaging agents. The various functions of the gut are co-ordinated by neurons, hormones, and local (paracrine) regulatory molecules.
The conversion of food into a form suitable for digestion is helped by cooking, which may also destroy toxins and microorganisms. The initial steps of digestion occur in the mouth where the enzyme amylase, which is present in saliva, breaks down starch. Mixing of saliva and food is aided by mastication or chewing, which also prepares an appropriately-sized ‘bolus’ of food for swallowing. The secretion of saliva is prompted by the presence of food in the mouth. Slightly acidic solutions are strong salivary stimulants, which might explain why a twist of lemon is perceived as a valuable addition to aperitifs.
The process of swallowing involves raising the larynx to close off the respiratory tract. The progression of a bolus of food down the oesophagus is aided by a muscular reflex, peristalsis, consisting of a wave of relaxation to accommodate the bolus followed by a wave of contraction pushing it ahead. Peristaltic movements are co-ordinated by neurons within the oesophagus and connecting it to the brain. The lower part of the oesophagus is separated from the stomach by a sphincter, the lower oesophageal sphincter, which relaxes to allow food to pass through. This sphincter normally prevents acid from the stomach entering the oesophagus. Failure of this mechanism is one cause of the sensation known as heartburn, and if persistent leads to chronic inflammation: oesophagitis.
The adjective ‘gastric’ applies to all things pertaining to the stomach. The stomach stores food prior to delivery to the small intestine, initiates the digestion of protein, and secretes hydrochloric acid, which destroys many microorganisms. Hydrochloric acid in gastric juice was identified by the physician-chemist William Prout in the 1820s. Important early observations on human gastric digestion were made at about the same time by William Beaumont on his patient, the Canadian trapper Alexis St Martin, who had survived a gun-shot wound leaving him with a permanent hole, or fistula, connecting the stomach with the exterior of the upper left side of the abdomen. Beaumont recorded over 100 separate experiments in which he directly observed gastric function in this subject concluding, in one experiment, ‘… I am confident, generally speaking, that venison is the most digestible of any diet …’
Proteins in the stomach are broken down to polypeptides by the enzyme pepsin, which works best in acidic environments and is produced by ‘chief’, or ‘zymogen’, cells in the gastric mucosa. Separate specialized cells, the parietal cells, secrete hydrochloric acid. The concentration of hydrogen ions in gastric juice is about a million times higher than in blood. The gastric epithelium therefore maintains one of the steepest concentration gradients of an electrolyte in the body. The secretion of acid against this gradient requires energy. It is achieved by a protein known as the proton pump (or, more precisely, the H+/K+ATPase) that exchanges intracellular hydrogen ions for extracellular potassium using energy provided by the breakdown of ATP (adenosine 5?-triphosphate).
The Russian physiologist I. P. Pavlov identified three phases in the control of acid secretion during digestion — the cephalic, gastric, and intestinal phases. The thought, smell, or taste of food stimulate acid secretion in the cephalic phase. In humans up to 50% of maximum acid secretion by the stomach can be evoked by this kind of stimulation. The gastric phase is initiated by the presence of food in the stomach, and the intestinal phase by food in the intestine. At the cellular level, acid secretion is controlled by acetylcholine released from mucosal nerve endings, by the gastric hormone gastrin, and by the local regulator histamine released from cells adjacent to parietal cells. Parietal cells, which also secrete a protein (intrinsic factor) essential for the absorption of vitamin B12, are lost by an autoimmune process in the condition of pernicious anaemia — which is characterized by a failure to produce acid and intrinsic factor, leading to vitamin B12 deficiency.
The stomach converts food to a sludge-like consistency (chyme) suitable for further digestion in the small intestine. The rate of emptying of chyme from the stomach varies with the composition of the meal. Fat-rich meals empty more slowly than carbohydrate-rich meals. Indigestible solids empty more slowly than liquids or semi-solid meals. The rate of emptying is determined by pressure differences between the stomach and duodenum, by the resistance to flow across the muscular band (the pyloric sphincter) separating the two organs, and by a pumping action of the last part of the stomach. Gastric emptying is regulated by signals arising from the duodenum, which therefore itself determines the rate at which it receives chyme.
The small intestine is the primary site of digestion and absorption. It is a tube approximately 20 feet long consisting of three regions; the duodenum, jejunum, and ileum. Gastric acid entering the duodenum is neutralized by bicarbonate secreted by the pancreas, which also secretes a wide variety of digestive enzymes. The major classes of pancreatic enzymes are proteases, which convert protein to polypeptides, peptides, and then amino acids; amylase, which completes the breakdown of starch; and lipase, which converts fats (triglyceride) to glycerol and fatty acids. The digestion of fat also requires bile salts, delivered to the duodenum in bile from the liver, via the gall bladder. The final stages of digestion are completed both within epithelial cells of the small intestine and by enzymes on their surface; for example peptides may be converted to amino acids within these cells, and sucrose is converted to glucose and fructose by a membrane-bound enzyme, sucrase-isomaltase.
The lining of the small intestine is folded into finger-like projections, the villi (each 0.5- 0.8 mm long), and deeper glands, the crypts. The immensely increased surface area provided by the villi aids absorption. Substances move between the gut lumen and the blood both by passing through epithelial cells (the transcellular route) and by passing between them (the paracellular route). Water also moves by these routes along osmotic gradients. Absorption of the products of digestion is mediated by a series of specific ‘transport proteins’. Amino acids and peptides, sugars, and inorganic ions are often moved from the lumen into intestinal cells against a concentration gradient. The energy required for this transport is provided by gradients of sodium or hydrogen ions. The sodium gradient in particular is generated by sodium pumps located on the surface membrane of epithelial cells facing the bloodstream, which lower intracellular sodium. The conditions are thereby created for sodium in the intestinal lumen to move down its concentration gradient into the cells carrying nutrients with it.
Nutrient digestion and absorption is largely completed in the jejunum. In the ileum, there is further absorption of water, electrolytes, remaining nutrients, and also bile salts which are returned to the liver for re-use. The residue then passes to the large intestine, or colon, for the final steps of water and electrolyte absorption, and for storage of the waste products prior to their discharge when socially appropriate (defecation).
Approximately 9 litres of fluid enter the human small intestine each day, some from ingested food and liquids, and more from the secretions of the salivary glands, stomach, pancreas, liver, and the small intestine itself. The jejunum and ileum each account for the reabsorption of about 45% of fluid and sodium chloride. The remainder is delivered to the colon, where all but about 100 ml is reabsorbed. The colon has the capacity to absorb up to 1.5 litres of fluid per day. When greater volumes arrive from the small intestine the excess is lost in faeces as diarrhoea. Although the small intestine is a net absorptive organ, the crypt cells secrete water and sodium chloride. This secretion may be stimulated by toxins generated by microorganisms; one example is cholera toxin, which is responsible for the secretory diarrhoea of cholera. Watery diarrhoea therefore happens when intestinal secretions overwhelm the capacity of the small and large intestines to absorb water and electrolytes. Absorption may be aided by oral administration of solutions consisting of sodium chloride and glucose which engage multiple transport processes. This treatment, oral rehydration therapy, has proved valuable in treating patients with infectious diarrhoea, particularly in Third World countries where access to medical services is limited.
Within the gut there is a rich diversity of microorganisms, many of which are beneficial although some are potentially pathogenic. The colon typically contains very large numbers of microorganisms: approximately 1013 individual organisms, and up to 200 different types. The small intestine and stomach are usually relatively free of microorganisms. An important exception is Helicobacter pylori which is found in the stomach in approximately 50% of people.
Many microorganisms within the gastrointestinal tract are able to convert the otherwise indigestible components of food, particularly plant cell walls, into forms suitable for absorption. In ruminant species (cow, sheep) a modified part of the stomach functions as a fermentation chamber where microorganisms digest the non-starch polysaccharides which make up plant fibre into short chain fatty acids which are readily absorbed. In other species (e.g. horse, elephant), an expanded region of the first part of the large intestine, the caecum, serves a similar function.
Protection and renewal
Many substances present in the gut lumen are potentially damaging, such as gastric acid, ingested noxious molecules, and microorganisms. To counteract these forces, the gut has an elaborate range of protective mechanisms. Gastric acid is resisted by special properties of the surface membrane of mucosal cells, tight connections between cells, good blood flow, and the local production of bicarbonate and mucus gel that lies on the epithelial surface. Breakdown of these mechanisms may lead to the formation of a peptic ulcer. The protective barrier is reduced by aspirin and other non-steroidal anti-inflammatory drugs; this is an important side-effect which limits the use of these compounds. Drugs that inhibit acid secretion are widely used. Some (the proton pump inhibitors, e.g. omeprazole) block the pump that transports acid into the stomach, others block the site at which histamine acts on parietal cells (the H2 receptor antagonists: cimetidine, ranitidine). The presence of Helicobacter pylori in the stomach is associated with peptic ulcer disease, and also with cancer of the stomach. Its recognition and its elimination by antibiotic therapy has provided a major advance in the management of peptic ulcer in the 1990s.
The gastrointestinal tract is well endowed with cells of the immune system, which are important in protection against pathogenic microorganisms and antigens. Malfunction of this system is a factor in inflammatory bowel diseases (Crohn's disease and ulcerative colitis). In addition some components of food may trigger an immune response, for example in coeliac disease there is an intolerance to the protein component of wheat, gliadin.
Epithelial cells of the alimentary tract are subject to continuous wear and tear and so must be regularly replaced. In the small intestine, epithelial cells are generated in the crypts, then migrate up the villi and are lost at the tip. This process takes about three days. During migration cells differentiate into particular types. Cell renewal in the mucosa occurs similarly throughout the gut. Damage to the DNA within dividing cells may disrupt mechanisms that regulate this process, leading to accumulation of mutated forms of genes and development of tumours, particularly in the colon and stomach, which are common sites for cancer.
Nerves and hormones
The gut possesses its own nervous system which can function independently of the central nervous system. The gut and brain do engage each other in two-way communication, but, with exceptions such as swallowing and defecation, the functions of the alimentary system are not under voluntary control. Moreover, the normal processes of digestion do not involve consciousness, even though expressions of the sensations attributed to digestion are commonplace (gut feelings, etc.).
The main nerve trunks linking the gastrointestinal tract and the central nervous system are known as the vagus and splanchnic nerves. In both cases, separate nerve fibres communicate from the gut to the central nervous system, and in the opposite direction. Splanchnic nerve fibres communicating to the central nervous system respond to noxious stimuli, leading to perceptions of pain or discomfort. The sensitivity of these nerves can be modified, for example by inflammation, so that otherwise innocuous stimuli may be perceived as painful. Nerve fibres running from the central nervous system to the gut are part of the autonomic nervous system. In general, alimentary processes are activated by the ‘parasympathetic’ component of this system via the vagus nerves, and are quietened by the ‘sympathetic’ component via the splanchic nerve. Both vagus and splanchnic nerves influence digestion via neurons located within the gut wall. However, because gut neurons can also function independently of the remainder of the autonomic nervous system, they are often considered to represent a third division of this system, the ‘enteric’ component.
The control of digestion depends on interactions between enteric neurons and a system of hormones produced by, and acting on, the gut. The pancreas-stimulating hormone, secretin, was the first hormone to be discovered (by W. M. Bayliss and E. H. Starling in London in 1902). At the turn of the twentieth century, ideas of how digestion might be controlled were dominated by Pavlov who emphasized the role of the nerves supplying the gut. However, Bayliss and Starling observed that acid in the small intestine of an anaesthetized dog stimulated a flow of pancreatic juice even after all nerves to the intestine had been cut. They reasoned that a messenger molecule might be secreted by the intestine into the bloodstream and conveyed by this route to the pancreas. They then found that such a substance could be recovered by extraction from the intestinal mucosa. They called the active factor secretin, and they showed that it stimulated a flow of pancreatic juice when injected into the bloodstream. The word hormone (from the Greek: to rouse or set in motion) was later introduced by Starling in recognition of this novel mechanism of action.
The gut hormones are produced by specialized epithelial cells, the gut endocrine cells, each with a characteristic distribution. Endocrine cells in the stomach, including the gastrin or ‘G’ cells, are mainly responsible for regulating acid secretion. Endocrine cells in the duodenum and jejunum produce secretin, which stimulates water and bicarbonate secretion by the pancreas, and cholecystokinin, which stimulates pancreatic enzyme secretion and gall bladder contraction, and which inhibits gastric emptying and food intake. Hormones produced in the ileum and colon (peptide YY, neurotensin, glucagon-like peptides-I and -II) mediate a phenomenon sometimes called the ‘ileal brake’, by which functions occurring in upper regions of the gut are inhibited, including food intake.
Digestion and fasting
The time taken to digest a meal depends on its composition. Fat-rich meals take longer to digest than those rich in protein or carbohydrate. There is considerable variation between individuals, but representative times to complete the progression from mouth to anus are about 55 hours in UK men, and 72 hours in women. Gastric digestion is completed in 2-3 hours, and small intestinal digestion in about 6 hours, so that the time spent in the colon is around 50-60 hours.
During fasting, or between meals, the gastrointestinal tract is not completely quiescent. Cell debris and microorganisms continue to accumulate during fasting, necessitating a mechanism to maintain the health of the gut. Approximately 12 hours after the last meal, strong waves of contraction start in the stomach and then progress the full length of the gut carrying accumulated debris forwards. These contractions are sometimes called house-keeping movements, or more accurately the ‘migrating myoelectric complex’. They start every 90 minutes, and take approximately 90 min to move the full length of the gut; as one finishes in the colon the next starts in the stomach. They cease on feeding.