In an enterohepatic circulation process bile salts and other substances enter the lumen of the GI tract in the duodenum and are reabsorbed further down the intestine into the blood. The substances are removed from the blood again by liver cells. Intestinal secretions participate in digestion by adding fluid, electrolytes, and enzymes. Enzymes include intestinal amylase, enterokinase, disaccharidases, peptidases, lipases, nucleotidases and nucleosidases. Some of these enzymes are free in the lumen due to the digestion of sloughed mucosal cells.
Intestinal secretion is both passive, due to the concentration of lumen contents, and active.
Active secretion is usually directed at a specific ion with others following for electrical neutrality. Water follows because of osmotic pressure. The GI tract is the largest endocrine gland in the body with the endocrine cells being diffusely scattered in the mucosa over the length of the alimentary canal. Food in the lumen of the gut is the normal stimulus for secretion of GI hormones but nervous system activity, stretch, or chemical stimulation may also cause the release of GI hormones. The hormones enter the blood stream to effect tissues distant from the releasing cells.