The small intestine is the longest section of the digestive tract, with an average length of about 6 metres. Although only 2.5 cm in diameter - which is why it is called the small intestine - its surface area for absorption covers the size of a tennis court. This is due to the numerous folds on its surface, covered in tiny projections known as villi, which in turn are covered in even tinier projections known as microvilli.
Large quantities of nutrients and water can be absorbed in the small intestine. Daily, it is capable of absorbing: several kilograms of carbohydrate; up to 1 kg of fat; 500gms protein; and 20 litres of water.
The surface cells of the small intestine are highly specialised for digestion and absorption of nutrients. Almost all the body's nutrient absorption occurs in the small intestine, along its three sub-divisions: the duodenum Þ jejunum Þ ileum. Sites for absorption of specific nutrients (eg: iron, vitamin.B12) are located in these divisions, but most absorption occurs in the jejunum (middle section). The specialised cells contain digestive enzymes, carrier proteins and other secretions. Blood vessels transport nutrients away from the intestine to the liver in the first instance.
Other features include:
Intestinal Crypts - these secrete enzymes, hormones and mucus
Peyer's Patches - lymph nodes preventing bacteria from entering the bloodstream
Brunner's Glands - these produce an alkaline mucus which protects the intestinal wall from gastric acid