Working of the Small Intestine

In the small intestine, proteins are changed into amino acids; fats are changed into fatty acids; and carbohydrates are changed into sugars. These products are soluble and can be absorbed into the bloodstream through the walls of the small intestine.
The main areas of the small intestine are the duodenum, jejunum, and ileum. In humans, the small intestine, which measures from 22 to 25 feet (6.7 to 7.6 meters) long, is the longest part of the digestive tract.
Food remains in the small intestine for several hours. Digestion here is aided by secretions produced by the liver and pancreas, which are connected to the small intestine by ducts.
The pancreas is one of the most important glands in the body. It secretes pancreatic juice, which contains digestive enzymes such as trypsin, which helps digest protein foods; amylase and maltase, which break down carbohydrates; and lipase, which works with bile from the liver to digest fat. Bile is produced in the liver but stored in the gallbladder, a small hollow organ located just under the liver. Bile does not contain enzymes; rather, it consists chiefly of acids, salts, cholesterol, and other substances that work with lipase to break down fats.
The gallbladder, which is prone to numerous disorders, can be surgically removed without serious effect. The liver, however, is essential to life. It has many important functions, such as removing clotting factors for the blood. The liver is an important site of metabolism; proteins and fats are synthesized here, and sugars are converted to glycogen, which is then stored in the liver until it is needed for energy.  

The walls of the hollow organs of the digestive tract—the esophagus, stomach, and small and …
As food is broken down in the small intestine it is further diluted by fluid secreted by glands in the intestinal wall. The lining of the small intestine contains many folds that expand its surface area, allowing increased contact between the wall and food products. The surface area is further increased by thousands of villi, microscopic fingerlike projections of the intestinal wall. Nutrients pass through the cell membranes of the villi and are taken into the blood and lymph, which carry them to the cells, where they are used for energy and growth.
By the time the diluted food products have traveled the length of the small intestine, most of their nutrients have been absorbed into the bloodstream. Some water is absorbed here, though most water is reabsorbed in the large intestine.