Secretion of small intestine: Extensive array of compound mucous secreting glands located in the 1 st few centimeters of duodenum, these glands secrete mainly alkaline mucous in response to : 1- tectile or irritating stimulus of the overlying mucosa 2- vagal stimulation 3- gastrointestinal hormones especially secretin The function of mucous secreted by the glands is to protect the duodenal wall from digestion by gastric juice., their rappid & intense response to irritating stimuli is especially geared to this purpose, in addition, the secretin stimulated secretion by glands contains a large excess of bicarbonate ion which add to the bicarbonate ion from pancreatic secretion &bile in neutralizing acid entering duodenum from stomach. bruner´s gland is inhibited by sympathetic stimulation, therefore, such stimulation is likely to leave duodenal bulb unprotected &is perhaps one one of the factors that cause this area to be site for peptivc ulcer in abou 50% of cases

Secretion of intestinal digestive juice by crypts of leiberkuhn: crypts of leiberkuhn are small pits located on the entire surface of small intestine between the villi, the intestinal surface of both the crypt &villi are covered by an epithelium composed of 2 types of cell : amoderate number of goblet cells which secrete mucus that lubricate &protect the intestinal surface, & largre number of enterocytes which in the crypts secrete large quantity of water &electrolytes &over the surface of the villi reabsorbe water &electrolytes,the intestinal secretions are formed by the enterocytes of the crypt at rate about 1800ml/ day, have slightly alkaline PH(7.5- 8), this circulation of fluid from the crypt to the villi supplies a watery veichle for absorption of substances from the crypt as it come in contact with villi which is the primary function of S.I.in addition to the watery secretion, the goblet cells in the epithelium secrete moderate amount of mucus which provide its usual function of lubrication &protection of intestinal mucosa.

Mechanism of secretion of watery fluid: The exact mechanism that cause the marked secretion of watery fluid by the crypts of leiberkun is not known, it is believed to involve at least 2 active secretory processees:1- active secretion of bicarbonate ion &2-active secretion of chloride ion. Finally all these ions together osmotic movement of water. Enzyme in the secretion of S.I.” When secretion of S.I.are collected without cellular debris they have almost no enzyme, howevere, the the enterocytes of the mucosa, especoially those that cover the villi, contain digestive enzyme that digest specific food substances while they are being absorbed through the epithelium.. These enzymes are 1-several peptidase (for splitting peptides in to amino acids ) 2- some enzymes for splitting disacharides in to monosacharides 3- intestinal lipase splitt neutral fat in to glycerole &fatty acids. Most if not all of these enzymes are mainly in the brush border, yhey are believed to hydrolysis of food on the outside surface of the microvilli before absorption of the end products.

Regulation of small intestinal secretion : 1- local stimuli: the various local nervous reflexes in response to the presence of chyme in the intestine especially reflexes intiated by tactile or irritative stimuli &by increase enteric nervous activity associated with gastrointestinal movements, therefore,the greater amount of chyme the greater secretion. 2- hormonal secretions:some of hormones that promote GITsecretion also increase S.I. secretion, especially secretin &cholecystokinin in general, the local enteric reflex mechanism play the dominant role.

Secretion of large intestine : 1-mucus secretion :the mucosa of the large intestine has many crypts of lieberkun but unlike that of S.I.there are no villi,also the epithelial cells contain no enzymes, they consist mainly of mucous cells that secrete only mucus, therefore, the great secretion in the large intestine is mucus, this mucus containe large amount of bicarbonate ion caused by active transporte through other epithelial cells that lie between epithelial cells that secret mucus. The rate of secretion regulated by: 1- tactile stimulation 2- local nervous reflex 3- stimulation of pelvic nerves which carry parasympathetic innervation to cause marked increase in mucus scretion. Mucus in large intestine 1- protect the wall against excoriation 2- provide the adherant medium for holding fecal matter together. 3- protect the intestinal wall from bacterial activity 4 it plus the alkalinity of secretion. 2- secretion of water &electrolytes :when the L.I. becomes irritated like in bacterial infection, the mucosa secretes large quantities of water &electrolytes in addition to the normal viscid solution of alkaline mucus this act to dilute the irritating factors &cause rappid movements of fluid results in diarrhea,also washes away irritant facters