is breakdown of the important nutrients ,fat,carbohydrates & proteins into smaller fragments so that it can be easily absorbed
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.
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.
These sugars are the final common products of carbohydrate digestion which are easily absorbed.
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.
Digestion of fats:
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.
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.
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,
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.
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.
Absorption of carbohydrates& proteins:
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.
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.
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.
Bile salts & pancreatic lipase are the most important factors in fat absorption.
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.
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