Gastrointestinal absorption: 1.There are many folds called valvulae connivents which increase surface area of absorption of mucosa about 3 folds located on the intire surface of S.I.from the point at which CBD empties into the duodenum down to the ileocecal valve .

2.Villi which project about abouit 1 millimeter from the surface of mucosa , the presence of villi enhance absorptive surface about 10 folds.

3.Each intestinal epithelial cells is characterized by brush border consisting of microvilli this increase surface area about another 20 fold .

so by combination of the folds , villi µvilli the absorbtive surface area increase to about 1000 folds.

Absorption of water : Water is transported through the intestinal membrane by diffusion this diffusion obey the law of osmosis , when the chyme is dilute , water absorbed through the intestinal mucosa in to the blood of villi by osmosis , the water can be transported in the opposite direction from the plasma in to the chyme , when hyperosmtoic solution are discharged from stomach into the duodenum , water transferred by osmosis to make the chyme isomotic with plasma.

Absorption of ions: Active transporte of Na+: 20-30 gram of Na is secreted daily in to the intestinal secretion , in addition , the normasl person eat 5- 8 gram/ day sodium , so the S.I. must absorb 25 35 gram /day sodium which equal to about 1/7 th of all sodium present in the body , normally less than 0.5% of intestinal sodium is lost in feces each day .the sodium also plays an important role in the absorption of suger & amino acids , the basic mechanism of Na+ absorption from the intestine are the same as those for absorption of Na+ from the renal tubules&G.B.The motive power for Na+ absorption is provided by active transport of Na+ from inside the epithelial cells in to the paracellular spaces , it requires energy, the energy process is catalized by ATPase enzyme in the cell membrane

This active transport reduces the Na+ concentration inside the cell to about 50 mEq/liter), but the Na+ concentration in the chyme is normally about 142mEq/L sothat Na+ moves from the chyme through the brush border of epithelial cell in to the cytoplasm of epithelial cell &replaces the Na+ that is actively transported, the next step in the transport process is osmosis of water in to the paracellular spaces , this is caused by the the elevated concentration of ions in the paracellular spaces.

Aldosteron greatly enhance sodium absorption : When a person becomes dehydrated , large amount of aldosterone are secreted from adrenal gland within 1-3 hours , the excess aldosterone enhance the enzyme &transport mechanism of Na+ absorption by epithelial cells of intestine . The increased Na+ absorption lead to increase in absorption of Cl – ion , water & some other substances , this effect of aldosterone is important to prevent loss of water & electrolytes.

- Absorption of bicarbonate in the duodenum & jejunum: : large quantity of bicarbonate must be reabsorbed from the upper S.I , because of the secretion of large amount of bicarbonate through the pancreatic &bile secretion this absorption is in indirect way as the fallowing : When Na+ ion are absorbed , moderate amount of H+ ion are secreted into the lumen of the gut in exchange with some of Na+, these H+ ion combined with bicarbonate ion to form carbonic acid, which then dissociated into water &carbone dioxide , the water remain as part of the chyme in the intestine , but CO2 is absorbed to the blood , then expired through the lung .

Absorption of other ions : Calcium actively absorbed in the duodenum , controled by the needed of Ca ion in the body .parathyroid hormone activate vitamine D enhances Ca absorption .

Iron ion actively absorbed in S.I.the absorption controlled by the body neeed to iron, especially for Hb formation .

Potassium , magnesium, &phosphate : Actively absorbed .

Absorption of nutrient: Absorption of carbohydrate: All CHO are absorbed in the form of monosaccharide &only small fractions absorbed as disaccharides , glucose absorption is the most absorption of monosaccharides (80%), the remainder (20%) are galactose & fractose, because glucose is the final digestion products of most CHO. In food .

Glucose is basically transported by Na+- co transport mechanism: In the absence of Na+transport through the intestinal membrane , virtually , no glucose can be absorbed , because absorption of glucose occur with active transport of sodium.

There are 2 stages in the transport of sodium through the intestinal enterocyte: 1 st is active transport of Na+ through the basolateral membrane into the paracellular spaces , this leade to decrease the sodium inside the cells , then cause diffusion of Na+ through the brush border of the enterocytes from intestinal lumen to inside of these cells by facilitated diffusion .the sodium 1 st combined with transporter protein , but this will not transport Na+ to inside the cell until it also combined with other substance such as glucose , therefore,both sodium &glucose are transported together to the interior of the cell. Inside the enterocytes there are other transport protein & enzyme cause facilitated diffusion of the glucose outward through the basolaterl membrane of enterocytes into the paracellular spaces.

The digestion of disaccarides & trisaccharides at the brush border enhance glucose transport. When disach. &trisach. Composed of glucose come in contact with the brush border , the digestive enzyme that attached to the membrane of the brush microvilli cause hydrolysis of disach.&trisacch. into glucose , this digestion increase the concentration of glucose in the area adjacent to the absorptive membrane of enterocytes , the high concentation of glucose then plays an additional role in enhancing glucose transport to interior of enterocytes&then all the way through the enterocytes into the paracelluilar spaces .

Absorption of other monosaccharides Galactose transported by the same mechanism as glucose .

Fractose by facilitated diffusion , most of fractose converted to glucose all the way through the enterocyts.

Absorption of proteins : Most of protein are absorbed through the S.I.enterocyts in the form of di&tripeptides &free amino acids by the same way that sodium co- transport of glucose occurs that is called co transport or 2 ndry active transport of the amino acids or peptides .

A few a.a .are transported by facillitated diffusion in the same way that fractose is transported.

Absorptoion of fat : The end products of fat digestion ( monoglycerids &free fatty acids ), become dissolved in the central lipid portion of bile acid micells , in this form , the monog. & free f.a. are carried to the surface of microvilli in the brush border , penetrating among the moving , agitating microvilli , here both monog. &f.a diffuses immediately through the enterocyts cell membrane to the interior of the enterocyts because these lipids are soluble in the enterocyts membrane as in the micells , this leave the bile acid micells in the chyme & absorbe more monog. & f.a . , thus the micells perform a ferrying function , which is highly important for fat absorption . In the absence of bile acid only 40- 50% is normally absorbed & about 97& of fat absorbed of abundasnce bile acid micells.

Small quantities of the undigested tri & diglycerids (which are highly soluble in the lipid mkembrane of enterocytes are normally absorbed because bile acid micells will not dissolve them & will not ferry them to the enterocyte membrane , after entering the enterocytes, the f.a. & monoglycerids are taken up by the SER & here they recombined to form new triglyceride , a few of monog. Are digested into glycerol & f.a.by intracellular lipase , then these free f.a. are reconstituted by SER into triglyceride by using new glycerol that synthesis required energy from ATP& complex of enzyme.

Formation of chyloimicrone : The reconstited trglycerids aggrigate 1 st within the E.R. & then in the golgi apparatus into globuls that contain absorbed cholesterol, absorbed phospholipid , a small amount of newly synthesized cholesterol & , phospholipid then phspholipid arrangev themselves in these globules in which the polar portion located on the surface , this provide an electrically charged surface that make these globule missible with the fluid of the cell, in additition several types of apoproteins also synthesized by E.R. coat part of the surface of globule , in this form the globuls released from golgi apparatus & excreted into the basolateral spaces & pass to the lymph these globuls then called chylomicrons

Transport of chylomicrons in the lymph: When the chylomicrons wend their way into the central lacteals of the villi they are propelled along with the lymph by the lymphatic pump upward through the thoracic duct to be emptied into the great vein of the neck , between 80- 90% of alll fat absorbed from the gut is absorbed in this manner &transported to the blood by way of the thoracic lymph in the form of chylomicrone. .