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.
Stomach
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 intestines
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.
Illustration
Graham Dockray
Bibliography
The British Digestive Foundation (PO Box 251, Edgware, Middlesex, HA8 6HG) can provide information on a range of diseases of the alimentary tract.
Johnson, L. R. (1997). Gastrointestinal physiology, (5th edn), Mosby Year Book Inc., St Louis, Missouri.
See also
appendix;
bile;
constipation;
defecation;
diarrhoea;
faeces;
gall bladder;
gastrin;
hernia;
indigestion;
liver;
pancreas;
saliva;
vomiting.
