The small intestine is approximately twenty-three feet (7m) in length and can be divided into three regions, the duodenum, the jejunum, and the ileum. It begins at the pyloric-valve of the stomach, ends at the ileocecal valve between the ilium and cecum, and has an average diameter of about one inch. The inner absorptive surface area is increased tremendously by millions of fingerlike projections, villi. which are present throughout, andby plicae circulares which are deep circular folds in the mucosa of the intestine.
Three types of intestinal movements have been described. Peristalsis is a slow, wavelike muscular contraction that pushes chyme through the tube. Rhythmic segmentation involves circular muscle contractions at alternate points around the chyme; this helps mix the chyme and the intestinal juices. Pendular movement involves the alternate contraction of circular and longitudinal muscles in the intestine; this also increases the mixing of chyme and intestinal juices.
Most of the digestion and a majority of the absorption occurs in the small intestine . Acid chyme from the stomach is neutralized in the intestine by pancreatic, liver and intestinal secretions. This occurs such that the ph of the intestine is continually increasing from the duodenum to the ileocecal valve.
Glands of the small intestine (Crypts of Lieberkuhn) are located in its mucosal -layer. The endocrine secretions, enterogastrone, has already been discussed. Other hormones which are indirectly related to digestive activities are secretin, pancreatin, enterocrinin, and cholecystokinin. Chyme entering the small intestine stimulates the secretion of these hormones.
Secretin and pancreatin stimulate the secretion of pancreatic juices,enterocrainin stimulates the secretion of digestive enzymes from the small intestine, and cholecystokinin stimulates both the contraction of the gallbladder and the relaxation of the sphincter of Oddi; this sphincter controls the flow of bile and pancreatic juices from the common bile duct into the duodenum.
The exocrine secretions of the small intestine are digestive in nature. Enterokinase activates pancreatic trypsinogen (inactive trypsin) so that protein digestion may proceed. Trypsin is involved with the final break down of amino acid chains. Weak amylase aids in the break down of complex sugars to disaccharides. Maltage breaks down maltose into glucose.
Sucrase breaks down lactose into glucose and galactose. A final intestinal secretion is released from Brunner's Gland. It is a mucous secretion which protects the intestinal lining from the acid chyme. Besides their digestive and protective qualities, the intestinal juices aid in the transport of undigested material.