LECTURE - 9 Dr. Zahoor Ali Shaikh 1

 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

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 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. 4

 We will discuss 1. Motility 2. Secretion 3. Digestion 4. Absorption MOTILITY IN LARGE INTESTINE  Haustral Contraction or Segmentation  Peristaltic wave  Mass movement [strong peristaltic waves] 5

 Haustral Contraction [Segmentation Contraction]  They help to mix the contents of colon and expose contents to mucosa to facilitate absorption.  They occur less frequently may be after every 30mins [they are like segmentation contraction in small intestine but in small intestine they occur /min].  Haustral contraction are largely controlled by locally mediated reflexes involving the intrinsic plexus. 6

 Peristaltic wave  They propel the contents towards the rectum.  Mass movement [strong peristaltic waves]  They move the material from one portion of intestine to another.  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. 7

 Mass movement [strong peristaltic waves]  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. 8

 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. 9

 DIGESTION  There are no digestive enzymes secreted, therefore, no digestion takes place in large intestine. 10

 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. 11

 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. 12

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 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. 14

 Feces are eliminated by defecation reflex.  How this reflex works?  When mass movements of colon move the feces into the rectum. Distention of rectum initiates the reflex.  Stretch receptors in the wall of rectum send impulses to the spinal cord ( S2,S3,S4), parasympathetic nerves causes contraction of smooth muscle of rectum and sigmoid colon and relaxation of internal sphincter. 15

 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. 16

 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. 17

 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. 18

 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. 19

 Food such as beans, contain carbohydrate that human can not digest but can be attacked by gas producing bacteria.  Gases produced are Hydrogen, Hydrogen Sulphide, Nitrogen, Carbon dioxide and Methane. 20

 Amount of gas per day passed is about 200ml.  The smell is largely due to sulphides.  Gas passing through the luminal contents give rise to gurgling sounds known as BORBORGYMI. 21

 We will discuss the following hormones: - Gastrin - Secretin - CCK - Motilin - Somatostatin - GIP - VIP 22

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 Produced by G-cells in the stomach.  Stimulates the release of HCL and Pepsinogen in the stomach.  Increases gastric motility.  Increases ileal motility.  Relaxes Ileocecal Sphincter.  Induces mass movements in colon [because to help the contents moving through GIT on arrival of new meal in the stomach]. 24

 Secretin is released from small intestine when stomach acid contents come to duodenum.  Functions  It inhibits gastric emptying to delay the acid contents of stomach to enter in the duodenum.  It inhibits gastric secretion.  It acts on pancreatic duct to produce large volume of watery, NaHCO 3 secretion.  It stimulates the secretion of NaHCO 3 rich bile in the liver by acting on the bile ducts. 25

 CCK is released from the duodenum in response mainly to fat, to a lesser extent to protein products.  CCK causes (i). Inhibition of Gastric motility and secretion (ii). Stimulates pancreatic Acinar cells to secrete pancreatic enzymes [amylase, lipase, Trypsinogen, Chymotrypsinogen]. (iii). It causes contraction of gall-bladder and relaxation of sphincter of Oddi. 26

 It is polypeptide and secreted by entrochromaffin cells and Mo cells in the stomach, small intestine and colon.  It causes contraction of smooth muscles in the stomach and intestine.  Its level increases during inter-digestive state and controls GIT motility, Migrating motility complex (MMC) between the meals. 27

 It is secreted by D-Cells in pancreatic islets and by similar D-cells in GIT mucosa.  Somatostatin inhibits secretion of Gastrin, VIP, GIP, Secretin and Motilin.  Somatostatin secretion is stimulated by acid in the lumen of intestine.  It acts probably in a paracrine fashion. 28

 GIP is released from the duodenum.  GIP causes release of insulin.  Insulin causes uptake and storage of glucose.  GIP – before it was called Gastric Inhibitory Peptide [this role is minimum]. 29

 VIP is found in nerves in the GIT.  It stimulates intestinal secretion of electrolytes and water.  Other action – relaxation of intestinal smooth muscle including sphincters.  Inhibition of gastric acid secretion.  It potentiates the action of acetylcholine in salivary glands.  VIP is also found in brain, blood, autonomic nerves. 30

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32 REMINDER REGARDING GIT ENZYMES

33 REMINDER REGARDING ABSORPTION IN INTESTINE

 Large Intestine  Motility, Secretion, Digestion, Absorption  Feces  Intestinal Bacteria  Defecation Reflex  Flatus  Constipation  GIT Hormones 34

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