Bile salts are steroid or cholesterol derivatives synthesized or recovered from the blood by liver cells. Liver cells also make and secrete lecithin that emulsifies dietary fats. Products of hemoglobin metabolism are excreted in bile salts. 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.