Physio Gi 9,10.


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Physio Gi 9,10.

  1. 1. Gastrointestinal physiology Digestion & absorption Dr.M.A.M.Shaikhani.
  2. 2. <ul><li>Digestion </li></ul><ul><li>is breakdown of the important nutrients ,fat,carbohydrates & proteins into smaller fragments so that it can be easily absorbed </li></ul><ul><li>Is basically a process of hydrolysis i.e incorporating hydroxyl & hydrogen ions of water into these nutrients so that the proteolytic,lipolytic & carbohydrolytic enzymes can act easily to digest their substrate. </li></ul><ul><li>Carbohydrate digestion: </li></ul><ul><li>Salivary ptyalin digest 20% & pancreatic amylase the other 80% of complex starches into maltose & glucose polymers which are further digested by intestinal brash border(IBB) enzymes maltase & dextrinase into glucose ,also sucrose is digested into fructose & glucose by (IBB) enzyme sucrase & lactos by (IBB) enzyme lactase into glucose & galactose. </li></ul><ul><li>These sugars are the final common products of carbohydrate digestion which are easily absorbed. </li></ul>
  3. 3. Carbohydrate (CHO) Digestion
  4. 4. Carbohydrate (CHO) Digestion
  5. 5. <ul><li>Digestion of proteins: </li></ul><ul><li>The proteins are first digested by stomach pepsin into peptones,proteases & polypeptides which are further digested by pancreatic trypsin,chemotrypsin &carboxypolypeptidase into amino acids & polypeptides which are further digested by peptidases on the (IBB) into amino acids which are easily absorbed. </li></ul><ul><li>Digestion of fats: </li></ul><ul><li>The fats are digested in the upper SI by the aid of bile which through the formation of water soluble complexes called micelles as these bile salts have 2 ends a hydrophilic one & a hydrophobic one, so they accumulate at lipid- water interfaces & thus emulsify fats and facilitate their hydrolysis.micelles causes agitation & detergent action on the fats leading to its emulsification preparing the fats to be easily digested by pancreatic lipase into monoglycerides & fatty acids which are easily absorbed. </li></ul><ul><li>Sulfation of bile acids in the liver in the major way of their intestinal excretion since the sulfated bile salts can not be absorbed to reciculate in the enterohepatic circulation. </li></ul>
  6. 6. Digestion and Absorption of Fat <ul><li>Emulsification by bile salts </li></ul><ul><li>Pancreatic lipase </li></ul><ul><li>Micelles are water-soluble </li></ul>
  7. 7. Digestion and Absorption of Fat <ul><li>Monoglycerides and FFA enter cells by diffusion </li></ul><ul><li>Triglyceride synthesis </li></ul><ul><li>Add protein </li></ul><ul><li>Chylomicrons </li></ul><ul><li>To lacteal (lymph) </li></ul>
  8. 8. Absorption: The majority of absorption occurs in the SI . in the stomach only alcohol & some drugs as aspirin are absorbed. large intestine only water &electrolytes are absorbed. Absorption takes place in the villi & microvilli which increase the surface area of SI by 600 times to make the absorption process very efficient. Water & electrolyte absorption: Either by simple diffusion as water or by active secretion as Na+ & HCO3-. Absorption of H2O is by simple diffusion following the law of osmosis to cause isosmotic equilibrium when ions & nutrients are absorbed. Absorption of Na+ is by active transport from inside the epithelial cells into the intercellular spaces requiring energy supplied by ATPase carrier enzyme in the cell membrane,part of Na+ is absorbed with Cl- while the other is either with K+ or H+ ions transported in the opposite direction in exchange for Na+ helped by aldosterone when there is need for Na+ in the body as in dehydration,
  9. 9. <ul><li>when Na+ is absorbed H+ is secreted into the SI lumen in exchange of Na+ , excess H+ combines with excess HCO3- to form carbonic acid which dissociates into H2O & CO2 excreted by the lungs. </li></ul><ul><li>In the ileum & large intestine there is secretion of HCO3- in exchange of CL- by exchange proteins in luminal membrane of the epithelial cells to provide alkaline medium to neutralize acid formed by the bacteria. </li></ul><ul><li>Absorption of carbohydrates& proteins: </li></ul><ul><li>Glucose is the most rapidly absorbed than other carbohydrates digestion end products; fructose, manose,xylose& arabinose with the exception of galactose which is slightly more absorbed than glucose but glucose is present in very much higher concentrations. </li></ul><ul><li>Mechanism of glucose & galactose absorption is Na+-cotransport (secondary active transport of glucose & galactose) while that of fructose is different:dose not require energy,is by fascilated diffusion & fructose is partly converted to glucose inside the epithelial cells before entering the portal blood. </li></ul><ul><li>Absorption of amino acids is not by Na cotransport but by secondary active transport of amino acids & peptides requiring special membrane transport protein of which there are at least five. </li></ul>
  10. 10. <ul><li>Absorption of fats: </li></ul><ul><li>Bile salts & pancreatic lipase are the most important factors in fat absorption. </li></ul><ul><li>The bile salts facilitate fat absorption by reducing the surface tension of fat particles &by forming water soluble complexes called micelles in which fatty acids & glycerol also participate. </li></ul><ul><li>The fats digestion end products; fatty acids & monoglycerides are recombined after entering the epithelial cells by the smooth endoplasmic reticulum to form triglycerides & chylmicrones again which are easily absorbed by the lymphatics of the villi called the lacteals to the lymphatic system & not by the portal blood,while some short chained fatty acids are absorbed directly into the portal blood rather than lymphatics and this can be used in fat malabsorbtion </li></ul>