The main function of small intestine is absorption. The purpose of these wrinkles and projections is to increase surface area for absorption of nutrients. Each villus is covered in microvilli, which increase the surface area manifold.
Each villus contains a lacteal and capillaries. The lacteal absorbs the digested fat into the lymphatic system which will eventually drain into the circulatory system. The capillaries absorb all other digested nutrients.
Foods are break down using enzymes released by the pancreas and bile from the liver. Peristalsis also is at work in this organ, moving food through and mixing it with digestive secretions from the pancreas and liver.
Contents of the small intestine start out semi-solid, and end in a liquid form after passing through the organ. Water, bile, enzymes, and mucous contribute to the change in consistency. Once the nutrients have been absorbed and the leftover-food residue liquid has passed through the small intestine, it then moves on to the large intestine, or colon.
The duodenum is largely responsible for the continuous breaking-down process, with the jejunum and ileum mainly responsible for absorption of nutrients into the bloodstream.
The duodenum begins just beyond the stomach and curves around the head of the pancreas and the entrance of the common bile duct, in a C-shaped formation. At the spot where the stomach and duodenum meet, there is a muscle called the pyloric sphincter which prevents the back flow of material back into the stomach.
The duodenum is responsible for further processing the chyme from the stomach, by secreting enzymes which aid in digestion. Bile and pancreatic juice also enter the duodenum around its midpoint, and by moving the chyme in a shaking kind of motion which is peristalsis, the duodenum mixes the chyme with these enzymes within its lumen and further aiding digestion.
The jejunum is the next portion of the small intestine, and it has a lining which is specialized known as villi in the absorption of carbohydrates and proteins. The proteins have been broken down in the stomach by enzymes called pepsin and acid into amino acids. The carbohydrates are broken down in the duodenum by enzymes from the pancreas and liver into sugars. Fats are broken down in the duodenum by lipase from the pancreas into fatty acids. Amino acid, sugar, fatty acid particles, vitamins, minerals, electrolytes and water are small enough to diffuse into the villi of the jejunum and drop into the blood stream.
The ileum is the last portion of the small intestine, and it is responsible for absorption of fats, and bile salts which are a component of bile. The pores in the ileum are slightly bigger than those in the jejunum and allow vitamin B12, vitamins dissolved in fatty liquids, electrolytes, bile salts and water to soak through the walls and into the blood stream.
Where the ileum joins the large intestine is a valve, called the ileocecal valve, which prevents the back flow of materials into the small intestine. By the time material reaches this point, it has a rather pasty consistency.
Muscular & Digestive System 