LARGE INTESTINE Dr. Zahoor Ali Shaikh DR. ABDELRAHMAN MUSTAFA Department of Basic Medical Sciences Division of Physiology Faculty of Medicine Almaarefa Colleges

LARGE INTESTINE Large Intestine consist of cecum, appendix, colon [ascending colon, transverse colon and descending colon, end part of descending colon forms sigmoid colon] and rectum. Large Intestine does the function of water and electrolyte absorption and works as storage organ [primary function of large intestine is to store feces]. 2

LARGE INTESTINE The colon normally receives 500ml of Chyme from the Small Intestine each day. The contents coming to the colon consist of - Indigestible food residues e.g. cellulose, Unabsorbed Biliary components,Fluid. -Colon absorbs water and salt, what remains behind is to be eliminated is know as feces. 3

LARGE INTESTINE MOTILITY Three major types Haustral Contraction or Segmentation Peristaltic wave Mass movement [strong peristaltic waves] 4

MOTILITY IN LARGE INTESTINE  Haustral Contraction [Segmentation Contraction] Function: help to mix the contents of colon and expose contents to mucosa to facilitate absorption. D escription :They occur less frequently may be after every 30mins [they are like segmentation contraction in small intestine but in small intestine they occur /min]. Control: Haustral contraction are largely controlled by locally mediated reflexes involving the intrinsic plexus. 5

MOTILITY IN LARGE INTESTINE  Peristaltic wave Function :They propel the contents towards the rectum.  Mass movement [strong peristaltic waves] Function :They move the material from one portion of intestine to another. Description: They occur 3-4 times per day, generally after meals and increase in motility moves the feces forward in few seconds. When material reaches the rectum, rectal distention initiates the defecation reflex. 6

MOTILITY IN LARGE INTESTINE  Mass movement [strong peristaltic waves] Control: Gastro-colic Reflex – when food enters the stomach, mass movements are triggered in the colon by gastro-colic reflex also. It is mediated from stomach to the colon by gastrin and extrinsic autonomic nerves. It pushes the colonic contents into rectum triggering the defecation reflex. 7

LARGE INTESTINE SECRETION Large intestine secretes alkaline NaHCO 3, mucus solution. Its function is to protect large intestine from mechanical and chemical injury. Mucus provides lubrication to facilitate the passage of feces. 8

LARGE INTESTINE DIGESTION  DIGESTION There are no digestive enzymes secreted, therefore, no digestion takes place in large intestine. 9

LARGE INTESTINE ABSORPTION  ABSORPTION Na+ is actively transported and water follows along the osmotic gradient. Secretion of K+ and HCO 3. Due to absorptive capacity, some drugs are given per rectum especially in children e.g. anesthetics, steroids. There are no villi in Large Intestine. 10

FECES About 500ml of material entering the colon per day from the small intestine, colon absorbs about 350ml, leaving 150g of feces to be eliminated per day. Feces contains 100g of water and 50g of solid [undigested cellulose, bilirubin [stercobilinogen], bacteria, unabsorbed food residue. 11

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LARGE INTESTINE BACTERIA They are commensals, which have no effect on host [they are not pathogen which cause disease]. Bacteria present are E.coli, bacteroides – fragilis. Some bacteria synthesize vitamin K, vitamin B- complex, folic acid. 13

DEFECATION REFLEX Feces are eliminated by defecation reflex.  How this reflex works? Stimulus When mass movements of colon move the feces into the rectum. Distention of rectum initiates the reflex. Receptors :Stretch receptors in the wall of rectum Afferent : parasympathetic nerves Center: to the spinal cord ( S2,S3,S4), Efferent parasympathetic nerves causes contraction of smooth muscle of rectum and sigmoid colon and relaxation of internal sphincter. 14

DEFECATION REFLEX If external anal sphincter [which is skeletal muscle is also relaxed defecation occurs]. External anal sphincter is under voluntary control, therefore, can prevent defecation despite defecation reflex. When defecation occurs, it is assisted by voluntary straining movements that involve contraction of abdominal muscles and forceful expiration against closed glottis which increases intra-abdominal pressure. 15

CONSTIPATION When more water is absorbed from the feces, they become hard and dry. Normally frequency of passing stool vary. It maybe once a day, or after every meal or once every 2 or 3 days. CAUSES OF CONSTIPATION Decreased colonic motility due to low bulk diet, aging, emotion, anxiety. Colonic spasm, tumor in colon. Injury to nerve pathway. 16

INTESTINAL GAS OR FLATUS It is derived from two sources 1. Swallowed air [up to 500ml of air may be swallowed during a meal]. 2. Gas produced by bacterial fermentation in the colon. Most gas in the colon is due to result of bacterial activity, but the quantity and the nature of gas produced depend on the type of food eaten and colonic bacteria. 17

REFERENCESREFERENCES Human physiology by Lauralee Sherwood, seventh edition Text book physiology by Guyton &Hall,11 th edition Text book of physiology by Linda.s contanzo,third edition 18