Small Intestine is most part of digestive system. It is involved in the absorption of food content. Anatomy of small intestine is as follows:
Anatomy of small intestine
Start from pylorus and ends at ileocecal junction. The small intestine is 7 meters long in male and 6.5 meters in females.
Main function of small intestine is movement and chemical treatment of food in alkaline medium and absorption of the nutrients.
Small intestine is divided into three parts:
- Duodenum (25-30cm)
- Jejunum (2/5th part)
- Ileum (3/5th part)
It is first part of small intestine. It is developed from foregut. It is C shaped or horse-shoe shaped.
Duodenum is divided into 4 parts:
- Superior part
- Descending part
- Inferior part (horizontal part)
- Ascending part
Starting part of duodenum is also known as duodenum bulb because this particular part show much more resemblance to stomach. It has longitudinal fold while rest of duodenum has circular folds.
Jejunum and Ileum
There is no manifest boundary between jejunum and ileum.
Jejunum is larger in diameter; its walls are thicker and richer in vessels. Also, inner mucosal lining has numerous prominent folds that circle the lumen (plicae circulares).
In approx. 2% cases, Meckel’s diverticulum is present on the ileum at the distance of 1cm from its end. The diverticulum is ruminant of the embryonic vitello intestinal duct. It is 5 to 7 centimeters in length.
Structure of small intestine
Innermost layer is mucous coat in which intestinal villi are present, 1mm in length. The villi have blood and lymphatic supply and concerned with absorption of food. The number of villi is greater in jejunum.
To increase the absorption surface, circular folds (plicae circulares) is present. It is formed by tunica mucosa and submucosal layer. It is absent on initial part of duodenum and at the end of ileum.
In comparison between jejunum and ileum, circular folds are highest and closely packed in jejunum and lower and distantly packed in ileum.
Mucosal layer also has small simple intestinal gland for secretion of intestinal juice. The small intestine also has lymphatic apparatus, which renders harmful substances and microorganisms, known as Peyer’s Patches.
Peyer’s patches are arranged along the antimesenteric border of ileum. Neither villi or intestinal glands are present on patches. The number of Peyer’s patches are 20 to 30.
2nd layer of small intestine is tunica muscularis. It is composed of two layers of smooth muscles. Inner layer is circular muscle layer and outer is longitudinal muscle layer. Inner circular muscle layer is more developed than the outer layer.
Some scientists have opinion that the inner circular layer has spiral muscle fibers which promote peristaltic movement.
Third and outermost layer is tunica serosa (serous coat). It invests the small intestine except for a narrow strip on the posterior surface between two layers of mesentery where nerves, blood vessels and lymphatic vessels enters the intestine.