Taste messages are sent via these cranial nerves to the brain. Recognition of a sub-specialty is generally related to dedicated fellowship training offered within the subspecialty or, alternatively, to surgical pathologists with a special interest and extensive experience in gastrointestinal pathology. Digestive tube sensory reception In human sensory reception effects of aging In human aging: This storage capacity allows the body to eat only a few times each day and to ingest more food than it can process at one time. Villus heights of deodenum intestinal mucosa.
What Is the Main Function of the Digestive System?
This response is known as the conditioned salivary reflex. Saliva dissolves some of the chewed food and acts as a lubricant, facilitating passage through the subsequent portions of the digestive tract. Saliva also contains a starch-digesting enzyme called amylase ptyalin , which initiates the process of enzymatic hydrolysis; it splits starch a polysaccharide containing many sugar molecules bound in a continuous chain into molecules of the double sugar maltose.
Many carnivores, such as dogs and cats, have no amylase in their saliva; therefore, their natural diet contains very little starch. Substances must be in solution for the taste buds to be stimulated; saliva provides the solvent for food materials. The composition of saliva varies, but its principal components are water, inorganic ions similar to those commonly found in blood plasma , and a number of organic constituents , including salivary proteins, free amino acids, and the enzymes lysozyme and amylase.
Although saliva is slightly acidic, the bicarbonates and phosphates contained within it serve as buffers and maintain the pH, or hydrogen ion concentration , of saliva relatively constant under ordinary conditions.
The concentrations of bicarbonate, chloride, potassium , and sodium in saliva are directly related to the rate of their flow. There is also a direct relation between bicarbonate concentration and the partial pressure of carbon dioxide in the blood.
The concentration of chloride in the blood varies from 5 millimoles per litre at low flow rates to 70 millimoles per litre when the flow rate is high. The sodium concentrations in similar circumstances vary from 5 millimoles per litre to millimoles per litre. The concentration of potassium in the blood is often higher than that in the blood plasma, up to 20 millimoles per litre, which accounts for the sharp and metallic taste of saliva when flow is brisk.
The constant flow of saliva keeps the oral cavity and teeth moist and comparatively free from food residues, sloughed epithelial cells, and foreign particles.
By removing material that may serve as culture media, saliva inhibits the growth of bacteria. Saliva serves a protective function, for the enzyme lysozyme has the ability to lyse, or dissolve, certain bacteria.
The secretion of saliva also provides a mechanism whereby certain organic and inorganic substances can be excreted from the body, including mercury, lead, potassium iodide, bromide, morphine, ethyl alcohol , and certain antibiotics such as penicillin, streptomycin, and chlortetracycline.
Although saliva is not essential to life, its absence results in a number of inconveniences, including dryness of the oral mucous membrane, poor oral hygiene because of bacterial overgrowth, a greatly diminished sense of taste, and difficulties with speech. We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind. Your contribution may be further edited by our staff, and its publication is subject to our final approval.
Unfortunately, our editorial approach may not be able to accommodate all contributions. Our editors will review what you've submitted, and if it meets our criteria, we'll add it to the article. Please note that our editors may make some formatting changes or correct spelling or grammatical errors, and may also contact you if any clarifications are needed. Salivary glands Food is tasted and mixed with saliva that is secreted by several sets of glands. Previous page The teeth. Page 3 of Learn More in these related Britannica articles: A number of alterations, often causing more or less distress, occur in the physical condition and functions of the gastrointestinal tract during pregnancy.
As the embryo folds off, the endoderm is rolled in as the foregut and hindgut. Continued growth progressively closes both the midbody and the midgut.
The esophagus remains as a simple, straight tube. The stomach grows faster on its dorsal side, thereby forming…. Loss of teeth, which is often seen in elderly people, is more apt to be the result of long-term neglect than a result of aging itself.
The loss of teeth and incidence of oral disease increase with age, but, as programs of water…. In either event, pregnancy complicates their problems because the gastrointestinal disturbances that often…. Some brain cells may also participate as hunger receptors. This is especially true of cells in the lower parts of the brain such as the hypothalamus where some cells have been found to….
More About Human digestive system 5 references found in Britannica articles Assorted References embryological development In prenatal development: Digestive tube sensory reception In human sensory reception effects of aging In human aging: Digestive system pregnancy In pregnancy: The major mechanisms of diarrhea are increased permeability, hypersecretion, and osmosis.
Disorders of motility are often secondary. In healthy animals, water and electrolytes continuously transfer across the intestinal mucosa. Secretions from blood to gut and absorptions from gut to blood occur simultaneously. In clinically healthy animals, absorption exceeds secretion, ie, there is net absorption. If the amount exuded exceeds the absorptive capacity of the intestines, diarrhea results.
The size of the material that leaks through the mucosa varies, depending on the magnitude of the increase in pore size. Large increases in pore size permit exudation of plasma protein, resulting in protein-losing enteropathies eg, lymphangiectasia in dogs, paratuberculosis in cattle, nematode infections. Greater increases in pore size result in the loss of RBCs, producing hemorrhagic diarrhea eg, hemorrhagic gastroenteritis, parvovirus infection, severe hookworm infection.
Hypersecretion is a net intestinal loss of fluid and electrolytes that is independent of changes in permeability, absorptive capacity, or exogenously generated osmotic gradients. Enterotoxic colibacillosis is an example of diarrheal disease due to intestinal hypersecretion; enterotoxigenic Escherichia coli produce enterotoxin that stimulates the crypt epithelium to secrete fluid beyond the absorptive capacity of the intestines.
The villi, along with their digestive and absorptive capabilities, remain intact. The fluid secreted is isotonic, alkaline, and free of exudates. The intact villi are beneficial because a fluid administered PO that contains glucose, amino acids, and sodium is absorbed, even with hypersecretion.
Osmotic diarrhea is seen when inadequate absorption results in a collection of solutes in the gut lumen, which cause water to be retained by their osmotic activity. It develops in any condition that results in nutrient malabsorption or maldigestion or when an animal ingests a large amount of osmotically active substances that are not absorbed, eg, an overeating puppy.
Malabsorption see Malassimilation Syndromes in Large Animals and see Diseases of the Stomach and Intestines in Small Animals is failure of digestion and absorption due to some defect in the villous digestive and absorptive cells, which are mature cells that cover the villi.
Several epitheliotropic viruses directly infect and destroy the villous absorptive epithelial cells or their precursors, eg, coronavirus, transmissible gastroenteritis virus of piglets, and rotavirus of calves.
Feline panleukopenia virus and canine parvovirus destroy the crypt epithelium, which results in failure of renewal of villous absorptive cells and collapse of the villi; regeneration is a longer process after parvoviral infection than after viral infections of villous tip epithelium eg, coronavirus, rotavirus. Intestinal malabsorption also may be caused by any defect that impairs absorptive capacity, such as diffuse inflammatory disorders eg, lymphocytic-plasmacytic enteritis, eosinophilic enteritis or neoplasia eg, lymphosarcoma.
Other examples of malabsorption include defects of pancreatic secretion that result in maldigestion. Rarely, because of failure to digest lactose which, in large amounts, has a hyperosmotic effect , neonatal farm animals or pups may have diarrhea while they are being fed milk. Reduced secretion of digestive enzymes at the surface of villous tip cells is characteristic of epitheliotropic viral infections recognized in farm animals. The ability of the GI tract to digest food depends on its motor and secretory functions and, in herbivores, on the activity of the microflora of the forestomachs of ruminants, or of the cecum and colon of horses and pigs.
The flora of ruminants can digest cellulose; ferment carbohydrates to volatile fatty acids; and convert nitrogenous substances to ammonia, amino acids, and protein. In certain circumstances, the activity of the flora can be suppressed to the point that digestion becomes abnormal or ceases. Incorrect diet, prolonged starvation or inappetence, and hyperacidity as occurs in engorgement on grain all impair microbial digestion.
The bacteria, yeasts, and protozoa also may be adversely affected by the oral administration of drugs that are antimicrobial or that drastically alter the pH of rumen contents. Signs of GI disease include excessive salivation, diarrhea, constipation or scant feces, vomiting, regurgitation, GI tract hemorrhage, abdominal pain and distention, tenesmus, shock and dehydration, and suboptimal performance.
The location and nature of the lesions that cause malfunction often can be determined by recognition and analysis of the clinical findings. In addition, abnormalities of prehension, mastication, and swallowing usually are associated with diseases of the oral mucosa, teeth, mandible or other bony structures of the head, pharynx, or esophagus. Vomiting is most common in single-stomached animals and usually is due to gastroenteritis or nonalimentary disease eg, uremia, pyometra, endocrine disease.
The vomitus in a dog or cat with a bleeding lesion gastric ulcer or neoplasm may contain frank blood or have the appearance of coffee grounds. Horses and rabbits do not vomit. Regurgitation may signify disease of the oropharynx or esophagus and is not accompanied by the premonitory signs seen with vomiting.
Large-volume, fluid diarrhea usually is associated with hypersecretion eg, in enterotoxigenic colibacillosis in newborn calves or with malabsorptive osmotic effects.
Blood and fibrinous casts in the feces indicate a hemorrhagic, fibrinonecrotic enteritis of the small or large intestine, eg, bovine viral diarrhea, coccidiosis, salmonellosis, or swine dysentery. Black, tarry feces melena indicate hemorrhage in the stomach or upper part of the small intestine. Tenesmus of GI origin usually is associated with inflammatory disease of the rectum and anus.
Small amounts of soft feces may indicate a partial obstruction of the intestines. Abdominal distention can result from accumulation of gas, fluid, or ingesta, usually due to hypomotility functional obstruction, adynamic paralytic ileus or to a physical obstruction eg, foreign body or intussusception.
Distention may, of course, result from something as direct as overeating. A sudden onset of severe abdominal distention in an adult ruminant usually is due to ruminal tympany. Ballottement and succussion may reveal fluid-splashing sounds when the rumen or bowel is filled with fluid.
Varying degrees of dehydration and acid-base and electrolyte imbalance, which may lead to shock, are seen when large quantities of fluid are lost eg, in diarrhea or sequestered in intestinal obstruction or in gastric or abomasal volvulus. Abdominal pain is due to stretching or inflammation of the serosal surfaces of abdominal viscera or the peritoneum; it may be acute or subacute, and its manifestation varies among species. In horses, acute abdominal pain is common see Colic in Horses.
Subacute pain is more common in cattle and is characterized by reluctance to move and by grunting with each respiration or deep palpation of the abdomen. Abdominal pain in dogs and cats may be acute or subacute and is characterized by whining, meowing, and abnormal postures eg, outstretched forelimbs, the sternum on the floor, and the hindlimbs raised.
Abdominal pain may be difficult to localize to a particular viscus or organ within the abdomen.