Large Intestine: Absorption, Fermentation, and Bacterial Action, Summaries of Physiology

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Lecture series

Gastrointestinal tract

Professor Shraddha Singh,

Department of Physiology,

KGMU, Lucknow

STRUCTURE OF LARGE INTESTINE

CROSS SECTION OF LARGE INTESTINE

CHARACTERSTICS OF LARGE INTESTINE

• Haustra - Ring like contractions of the circular muscle divide the colon into pockets called haustra
• Teniae coli- Three seperate longitudinal ribbons of smooth muscle on otside of ascending,transverse,descending & sigmoid colons.they are visible below the serosa
• Epiploic appendages - small pouches of peritonium filled with fat ,situated along the colon

RATE OF SECRETION OF MUCUS

• Direct tactile stimuli
• Local nervous reflex
• Stimulation of pelvic nerves from spinal cord which carry parasympathetic innervation Increased mucus production Increased bowel movements

2.Formation of feces

• Around 1500ml of chyme/day from ileum is moved to caecum,of which maximum amount is absorbed in proximal colon.only 100ml of fluid & 1-5meq of sodium,chloride is excreted in feces
• Distal colon act as storage of feces called as storage colon
• LI has tight junction between epithelium cells prevents back diffusion of ions back .LI secretes HCO3,chloride ions.Bicarbonates neutralises acidic products of bacterial action .Absorption of sodium & chloride ions creates an osmotic gradient which in turn causes the absorption of water

3.FERMENTATION

• Bacteria mainly anaerobic & yeast break down substrates mainly starch,fibers,protein to give energy COMPOSITION OF FECES 3/4 water,1/4 solid Solid - 30% dead bacteria
• • 10 - 20% fat
• • 10 - 20% inorganic matter
• • 2 - 3% Protein
• • 30%undigested roughoge
• Stercobilin,urobilin to give colour to feces & urine
• Others-indole,skatole,mercaptans,hydrogen sulphide etc

3.BACTERIAL ACTION IN Large Intestine

• Coliform bacilli is present which causes digestion of small amount of cellulose
• Substances formed by bacterias are vit-k,vit B12,thiamine,riboflavin gases(co2,H2S,CH4)
• Vit b12 helps in iron absorption
• Vit k is used in blood coagulation

SUBSTRATES Bacterial +yeast enzymes breakdown Breakdown products Acetic acid,propionic acid,butyric acid,carboxylic acid,phenolic acid,amines,ammonia Fate of substrates Absorbed/Eliminated in feces/Rectal gases/ Utilized by microflora

CALCIUM ABSORPTION IN INTESTINE

• Calcium entering in GIT is about 1 gm/day of which 40% is absorbed
• Absorbed mainly in duodenum and jejunum Absorption takes place by 3 steps: a)Passive diffusion at enterocyte through ca channel b)Inside the cell binds with ca-binding protein calbindin D(CaBP ) & seqestrated in ER ,Golgi apparatus c) At basolateral membrane ca transported to blood via ca-atpase pump d)Active form of vit D stimulates uptake,increase CaBP &Ca-ATPase pump