Intestine
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In anatomy, the intestine is the portion of the alimentary canal extending from the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine (or colon). In humans, the small intestine is further subivided into the duodenum, jejunum and ileum while the large intestine is subdivided into the cecum, colon and rectum.
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[edit] Structure and Function
The intestine shares a general structure with the whole gut, and is composed of several layers. The lumen is the cavity where digested material passes through and from where nutrients are absorbed. Along the whole length of the gut in the glandular epithelium are goblet cells. These secrete mucus which lubricates the passage of food along and protects it from digestive enzymes. Villi are vaginations of the mucosa and increase the overall surface area of the intestine while also containing a lacteal, which is connected to the lymph system and aids in the removal of lipids and tissue fluid from the blood supply. Micro villi are present on the epithelium of a villus and further increase the surface area over which absorption can take place. The next layer is the muscualris mucosa which is a layer of smooth muscle that aids in the action of continued peristalsis along the gut. The submucosa contains nerves, blood vessels and elastic fibre with collagen that stretches with inceased capacity but maintains the shape of the intestine. Surrounding this is the muscularis externa which is comprised of longitudinal and smooth muscle that again helps with continued peristalsis and the movement of digested material out of and along the gut. Lastly there is the serosa which is made up of loose connective tissue and coated in mucus so as to prevent friction damage from the intestine rubbing against other tissue. Holding all this in place are the mesenteries which suspend the intestine in the abdominal cavity and stop it being disturbed when a person is physically active.
The large intestine hosts several kinds of bacteria that deal with molecules the human body is not able to breakdown itself. This is an example of symbiosis. These bacteria also account for the production of gases inside our intestine (this gas is released as flatulence when removed through the anus). However the large intestine is mainly concerned with the absorption of water from digested material (which is regulated by the hypothalamus), as well as any nutrients that may have escaped primary digestion in the Ileum.
[edit] Absorption of glucose in the ileum
Initially nutrients will diffuse passively from the lumen of the ileum via the epithelial cells and into the blood stream. However, certain molecules like glucose will passively diffuse in mass quantity some time after a meal and cause a change in concentration gradient. There will be a higher concentration of glucose in the blood (blood sugar level) than in the ileum which means that passive diffusion is no longer possible. Active uptake would be a waste of energy so another system is used to transport the left over glucose from the lumen into the blood stream.
This system is called facilitated diffusion, whereby a glucose molecule will associate with a sodium ion and approach a transporter protein in the cell membrane of an epithelial cell. The protein will let the sodium ion through which "pulls" the glucose molecule into the cell. Once inside the cell they dissociate and the glucose molecule is free to passively diffuse from the cell into the blood stream (this is because the blood flowing past the cell will have a low blood sugar level in relation to the cell cytoplasm, so the glucose molecule will diffuse passively into the blood stream).
[edit] Diseases
- Gastroenteritis is inflammation of the intestines and is the most common disease of the intestines. It can arise as the result of food poisoning.
- Ileus is a blockage of the intestines.
- Ileitis is an inflammation of the ileum.
- Colitis is an inflammation of the large intestine.
- Appendicitis is inflammation of the vermiform appendix located at the cecum. This is a potentially fatal disease if left untreated; most cases of appendicitis will require surgical intervention.
- Coeliac disease is a common form of malabsorption, affecting up to 1% of people of northern European descent. Allergy to gluten proteins, found in wheat, barley and rye, causes villous atrophy in the small intestine. life-long dietary avoidance of these foodstuffs in a gluten-free diet is the only treatment.
- Crohn's disease and ulcerative colitis are examples of inflammatory bowel disease. While Crohn's can affect the entire gastrointestinal tract, ulcerative colitis is limited to the large intestine. Crohn's disease is widely regarded as an autoimmune disease. Although ulcerative colitis is often treated as though it were an autoimmune disease, there is no consensus that it actually is such. (See List of autoimmune diseases).
[edit] Disorders
- Irritable bowel syndrome is the most common functional disorder of the intestines, and specifically the bowel. Functional constipation and chronic functional abdominal pain are other disorders of the intestines that have physiological causes, but do not have identifiable structural, chemical, or infectious pathologies. They are aberrations of normal bowel function but not diseases.
- Diverticular disease is a condition that is very common in older people in industrialized countries. It usually affects the large intestine but has been known to affect the small intestine as well. Diverticular disease occurs when pouches form on the intestinal wall. Once the pouches become inflamed it is known as Diverticulitis, or Diverticular disease.
- Bowel twist (or similarly, bowel strangulation) is a comparatively rare event (usually developing sometime after major bowel surgery). It is, however, hard to diagnose correctly, and if left uncorrected can lead to bowel infarction and death. (The singer Maurice Gibb is understood to have died from this.)
[edit] See also
- Inflammatory bowel disease (or "IBD")
- Diarrhea
- Constipation
| Upper gastrointestinal tract
Mouth | Pharynx (nasopharynx, oropharynx, hypopharynx) | Esophagus | Crop | Stomach (rugae, gastric pits, cardia, pylorus) Lower gastrointestinal tract Small intestine (duodenum, jejunum, ileum) | Vermiform appendix Large intestine: Cecum | Colon (ascending colon, transverse colon, descending colon, sigmoid colon) | Rectum (Houston valve, rectal ampulla, pectinate line) | Anal canal (anal valves, anal sinuses, anal columns) Anus: Sphincter ani internus muscle | Sphincter ani externus muscle Enteric nervous system: Meissner's plexus | Auerbach's plexus Enteroendocrine cells: G cells | Enterochromaffin cells | Enterochromaffin-like cell GALT: Peyer's patches | M cells parietal cells | chief cells | goblet cells | Brunner's glands | Paneth cells | enterocytes intestinal villus | crypts of Lieberkühn | circular folds | taenia coli | haustra | epiploic appendix |
| Endocrine system - edit |
|---|
| Adrenal gland | Corpus luteum | Hypothalamus | Kidney | Ovaries | Pancreas | Parathyroid gland | Pineal gland | Pituitary gland | Testes | Thyroid gland |
