The usable food is prepared in the small intestine for its journey to the cells of the body. However, this function can’t be done by the small intestine alone; hence it needs the accessory organs like the liver and the pancreas.
Contrary to popular belief, the stomach is not the major digestive organ but it is the small intestine. Food is partially digested in the stomach but it is in the intestine that digestion is completed and where absorption occurs.
The small intestine can be subdivided into three subdivisions namely the duodenum, the jejunum and the ileum. The duodenum is immovable and its path curves at the head of the pancreas. It has the most features of interest with respect to being the smallest of the three divisions. The hepatopancreatic ampulla is found in the duodenum. It is a bulblike point where the bile duct and pancreatic duct unite. The bile ducts serves by delivering the produced bile from the liver and the pancreatic duct carry the synthesized pancreatic juice from the pancreas. The hepatopancreatic ampulla, through the major duodenal papilla enters the duodenum in a volcano shape entrance. The entry of the two juices, bile and pancreatic juice is being controlled by the sphincter of Oddi.
The jejunum is the second of the three division of the small intestine. It is about 2.5 meters long and connects the duodenum to the ileum. The ileum is the bridge where the small intestine connects to the large intestine at the ileocecal valve.
The small intestine is tailored specifically to the absorption of nutrients. Because of its long length, it offers a huge surface area and together with the three structural modifications namely the villi, microvilli and plicae circulares, it amplifies the absorption power of the surface of the small intestine. As the food travels to the distal end of the small intestine, the absorption power decreases.
The plicae circulares or just simply the circular fold of the small intestine is specifically designed to force the chyme to spiral through the lumen. This action slows the movement of food, allowing the body ample time to absorb nutrients.
The microvilli are very tiny projections of the absorptive cells and give the mucosal surface an appearance called the brush border. Within the membranes of the microvilli an enzyme called the brush border enzyme exists. This enzyme completes the digestion of proteins and carbohydrate in the small intestine.