Small Intestine

The small intestine is the part of the gastrointestinal tract following the stomach, and is where the majority of digestion takes place. Although the small intestine is much longer than the large intestine, it is referred to as such due to its comparatively smaller diameter. On average, the diameter of the small intestine of an adult human measures approximately 2.5-3 cm, and the large intestine measures about 7.6 cm in diameter. In humans over 5 years old it is approximately 23 ft. The small intestine is divided into three structural parts: Duodenum 9.8 inch in length, Jejunum 8.2 ft, Ileum 11.5 ft.

The small intestine is covered in wrinkles called plicae circulares. From the plicae circulares project microscopic finger-like pieces of tissue called villi (Latin for shaggy hair). Each villus is covered in microvilli, which increase the surface area manyfold. Each villus contains a lacteal and capillaries. The function of the plicae circulares, the villi and the microvilli is to increase the amount of surface area available for secretion of enzymes and absorption of nutrients.

Food from the stomach is allowed into the duodenum by a muscle called pyloric sphincter. It is then pushed through the small intestine by a process of muscular-wavelike contractions called peristalsis. Most of the chemical digestion takes place in the small intestine. Chemical breakdown begun in the stomach is further broken down in the small intestine. Thus proteins are degraded into peptides by proteolytic enzymes such as trypsin and chymotrypsin. Then peptides are further broken down into amino acids. Lipids (fats) are broken down into fatty acids and glycerol by the pancreatic lipase. And carbohydrates such as starch are degraded into glucose (a simple form of sugar) by the pancreatic amylase.

The digested food passes now into the blood vessels in the wall of the intestine. This process is called absorption. The thousands of finger-like outgrowths called villi (s. vellus) in the inner walls of the small intestine increase the surface area for absorption of the digested food. Each villus has a network of thin and small blood vessels close to its surface. The surface of the villi absorbs the digested food materials. The absorbed substances are transported via the blood vessels to different organs of the body. Absorption of the majority of nutrients takes place in the jejunum, with the following notable exceptions. Iron is absorbed in the duodenum. Vitamin B12 and bile salts are absorbed in the terminal ileum. Water and lipids are absorbed by passive diffusion throughout. Sodium is absorbed by active transport and glucose and amino acid co-transport. Fructose is absorbed by facilitated diffusion.