Short Notes:

The small intestine lies between the stomach and the large intestine. It comprises three main sections: duodenum, jejunum and ileum. Digestion of food and absorption of nutrients takes place within the small intestine. This leaves mainly waste matter to pass into the large intestine.

When the chyme leaves the stomach, it enters the duodenum (first section of small intestine). In the duodenum, digestive juices from the liver and pancreas are added to the chyme. From the duodenum, the chyme passes into the jejunum (second section of small intestine) and then into the ileum (third and last section of small intestine). Digestive enzymes are produced by glands (crypts of Lieberkuhn) in the small intestine wall. These enzymes complete the breakdown of peptides into amino acids, and carbohydrates into simple sugars after the stomach has done its part of the digestive process.

Absorbing nutrients

We need to absorb the digested food. The absorption process is achieved by the tiny projections on the inner surface of the small intestine called 'villi'. Each villus is about 0.5 millimetres long and contains blood capillaries and a lymph vessel (a drainage tube, also called a 'lacteal'). The outer edge of each villus is covered with thousands of smaller projections called microvilli. The folds, villi and microvilli, increase the surface area of the small intestine - providing about 600 times greater surface for absorption of nutrients than a flat surface would provide. The walls of each villus are very thin, so that nutrient molecules can pass through them, mainly by the process of diffusion. Amino acids pass from the villus into the blood capillaries, and are carried away in the bloodstream. Fatty acids pass into the lacteal, and are carried away into the lymphatic system. Water and minerals are also absorbed into bloodstream - water-soluble vitamins enter the bloodstream and fat-soluble vitamins enter the lymphatic system.

After the absorption of nutrients in the small intestine, only waste matter and water is left. This enters the large intestine.