Small Intestine Anatomy

The small intestine is a part of the digestive system. The digestive system consists of the stomach, small intestine and large intestine. The small intestine is the part that connects the stomach and the large intestine. This is the main region where maximum amount of absorption of food takes place. The small intestine is so named not because it is small, it measures on an average five meters (sixteen feet), when uncoiled. This part of the gastrointestinal tract is called the small intestine because it is only 2.5 - 3 cm in diameter. The small intestine is divided into three parts - the duodenum, jejunum and ileum. Given below are details regarding the small intestine anatomy.

Anatomy and Physiology of the Small Intestine

Duodenum
The duodenum is the first section of the region of the small intestine that precedes the jejunum and the ileum. This is the shortest part of the intestine where most of the initial biochemical reactions take place. The duodenum is around 25 - 30 cm in length and it connects the stomach to the jejunum. It begins with a structure called the duodenal bulb and ends at the ligament of Treitz. The duodenum has the primary function of aiding in breakdown of food in the small intestine with the help of enzymes. Brunner's glands, which are a part of the glandular epithelium, are present in the duodenum and are responsible for secreting mucus. The duodenum also has the function of regulating the rate of emptying the stomach with the help of hormones, thus, preventing the occurrence of dumping syndrome. The duodenum is also responsible for secreting hormones like secretin and cholcystekinin. The duodenal lining consists of a very thin layer of cells which form the muscularis mucosae.

Jejenum
The jejunum is the middle section of the small intestine that is present between the duodenum and the ileum. The transition from the duodenum and the jejunum is called the ligament of Treitz. The jejunum is around 2.5m in length. The jejunum has a pH of around 7 to 8, that is, it is either neutral or slightly alkaline in nature. This kind of pH is brought about with the help of the bile, which is carried by the bile duct coming from the gallbladder. The pH in this region is maintained at 7 to 8 because, 7 to 8 is the optimum pH for the functioning of the enzymes in this region. The jejunum and the ileum are suspended in the abdomen by the mesentery, which gives them space for bowel movement. It is wrapped by smooth muscle that helps to move food forward, which is known as peristalsis.

The microscopic structure of the jejunum consists of mucous membrane, which contains projections known as villi, which are a characteristic feature of small intestine anatomy. These villi are folded and convoluted, which eventually increase their surface area many fold, thus, increasing the amount of surface area available for absorption. The transport of nutrients across these villi is in the form of both active and passive transport. The villi in the jejunum are longer than the villi in the ileum.

Ileum
The ileum is the last part of the small intestine, which follows the jejunum. It eventually gives rise to the cecum, from which it is separated by the ileocecal valve. The ileum is the longest part of the small intestine, measuring around 2 - 4m. It is also responsible for performing most of the small intestine function. This region has a pH that is neutral to alkaline. The main function of the ileum is to absorb vitamin B12 and bile salts, along with whatever is not absorbed by the jejunum. The wall of the ileum is also made up of many villi, which as in the jejunum, increase the surface area for absorption of digested food and adsorption of enzymes.

The small intestine anatomy is quite different from the large intestine anatomy, as their functions are different. However, the intestine is the main part that is responsible for absorption of digested food. If there is any kind of deviation from this anatomy and functioning of the small intestine, it could lead to various small intestine problems, which could be difficult to deal with.