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Mass Movement Colon; Mass movement occur to move the contents of large intestine over long distances, occur 1-3 me s per day. Water absorption occur in distal colon, making the fecal contents semisolid. Mass movement propels the fecal contents into rectum where they are stored until defecation occurs. Colon; Apical membrane contains Na and K channels which responsible for Na absorption and K secretion, induced by aldosterone which lead to increase in Na absorption and to increase in K secretion. Figure (15)electrolyte transport in the colon The colon mucosa is simple columnar epithelium except in the anal canal where it is stratified squamous epithelium. There are no circular folds and a reduced number of villi. The mucosa is thicker, with a larger number of globet cells. The colon mucosa is simple columnar epithelium except in the anal canal where it is stratified squamous epithelium. There are no circular folds and a reduced number of villi. The mucosa is thicker, with a larger number of globet cells. Defecation: As the rectum fills with feces, the smooth muscle wall of rectum contracts and the internal anal sphincter relaxes, called rectosphincteric reflex. The smooth muscle of rectum contract to create pressure, and feces are forced out through anal canal. Gastrocolic reflex: Distension of the stomach by food increases the motility of the colon and increases the frequency of mass movement in large intestine. This reflex is called the gastrocolic reflex, has afferent limb in the stomach which is mediated by parasympathetic nervous system. The efferent limb of the reflex produces increased motility of colon and mediated by CCK (cholecystokinin) and gastrin. hormone increased motility of colon . As well as the afferent signals entering the spinal cord initiate other effects ,such as taking a deep breath, closure of the glottis ,and contraction of the abdominal muscles to force the fecal contents of the colon downward ,at the same time causing the pelvic floor to extend downward to pull out ward on the anus to evaginate the feces Secretion of the large intestine: Mucus which is regulated by direct tactile and by local nervous reflexes to mucous cells. Stimulation of pelvic nerve which carry parasyrspathetic to distal two third of large intestine increase secretion of mucus and mol ity (ever y 30 minut es). Mucus protect the intestinal wall against excoriation, from the great amount of bacterial activity and provide the adherent medium for holding fecal matter together Absorption in large intestine: The colon ( proximal half) absorbs 2-3 L of water per day, Na and Cl- ions, secretes K ions, and bicarbonate ions to neutralize the acidic end products of bacterial action in colon .. In colon the hormone control regulate Na and K absorption is aldosterone. The function of distal half of colon is storage. Bacteria: Intestinal bacterial flora is harmless unless it enter the blood. The function of bacteria are: 1-Ul ize vitami n C, B 12, synt hesi s vitami n K and B plex. 2- Brown color of stool due to metabolism of bile pigment. 3- Acidity of stool due to formation of organic acid from carbohydrate by bacteria 4- Play role in cholesterol metabolism. 5- Formation of toxic substances (histamine and tyramine). 6- Formation of indole, in addition to sulfide gives the odder of stool. 7- Formation of ammonia. The antibodies which are produced by lymphocyte cells (proliferate and differentiate by intestinal antigens) coat the mucous and prevent bacteria from penetrating intestinal wall. Bacterial action in the colon: numerous bacteria such as colon bacilli are present in the absorbing colon. The substances formed as result of bacterial activity are vitamin K, vitamin B12, thiamin, riboflavin and various gases that contribute to flatus in colon especially carbon dioxide, hydrogen gas, N2 and methane. The gases produced due to breakdown of undigested nutrients that reach the colon. bile pigment, sloughed epithelial cells and undigested roughage of food. The brown color of feces is caused by sterobilin and urobilin which are derivatives of bilirubin. The odor is caused by products of bacterial action The function of colon 1-Storage of fecal materials. 2- Absorption of water and electrolytes. Constipation- longer than usual time between bowel movements and there is difficulty, pain, or discomfort when having a bowel movement. Each person’s GI tract responds differently to food, digestion, absorption, and transport. Diarrhea- characterized by frequent loose, watery stools. Indicating that the intestinal contents have moved too rapidly through the intestines for fluid absorption to take place. This can be a symptom of a variety of medical conditions..
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