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Small Intestine

What is the Small Intestine?

The small intestine is a hollow tube spanning from the end of the stomach i.e. the pyloric sphincter to the beginning of the large intestine. It is the longest par of the alimentary canal and measures approximately 21 feet in length in the living body.

Where is the Small Intestine situated?
The small intestine is situated beneath the liver and gall bladder. Behind the coils of the small intestine, there are the kidneys, ureters and the major blood vessel called the inferior vena cava. Near the junction of the small and the large intestine is present the appendix. It is a rudimentary organ in humans i.e. it is not of any use in the human body.

How can we describe the Small Intestine?
The small intestine can be divided into three parts for the purpose of study. These are the:

  • Denum : This is a C shaped portion of the intestine arising from the pyloric sphincter. It is about 25 cm. in length and joins the stomach to the jejunum.
  • Jejunum : This is a coil of intestine about eight feet in length. It begins from the duodenum and ends merging with the ileum. It lies in the upper part of the peritoneal cavity, that cavity which holds all the abdominal organs. It is thicker and wider in diameter than the ileum. This is due to permanent folding of the mucus membrane layer on the inside.
  • Ileum : The ileum is also a coil of intestine arising from the jejunum. It is around 12 feet in length and ends at a junction with the large intestine called as the ileo-caecal junction. The ileum is thinner walled than the jejunum. It has also got some amount of lymphoid tissue, that tissue which protects against infection.

The walls of the small intestine from the inside to the outside are made of :

  • Mucosa : This layer is the most important layer as it contains tiny projections called villi, which assist the absorption of food. There are thousands of projections over each villi called as microvilli. This adds up to the surface area of the intestine. The mucus producing cells are arranged in between the villi. There are also cells that produce enzymes in between the villi of the jejunum, which aid in complete digestion and absorption of food.
  • Submucosa : This layer contains the nerves, blood vessels and lymph vessels passing through it.
  • Muscle layers : These provide the contractile movements of the small intestine. This layer has circularly arranged muscle fibres enveloped by lengthwise-arranged fibres.
  • The serosa : A tough layer surrounds the intestine finally giving it protection and stability.

A structure called as the mesentery is attached to one border of the small intestine. This is a two layered fold of peritoneum that attaches part of the intestine to the abdominal wall at the back. Due to this that part of the intestine with the mesentery is very mobile.

Functions of the Small Intestine

The main function of the small intestine is digestion and absorption. There are two types of contractions seen in the small intestine. These are the segmentation contractions and the peristaltic contractions.

  • Segmentation contractions are also called mixing contractions. These are concentric contractions involving rings of 1 cm. each. This movement helps to cut the chyme at the rate of 8 to 12 times per minute. His causes thorough mixing of the food with the enzymes in the intestine and thus assists in digestion.
  • Peristaltic contractions are propulsive movements that move the chyme forward through the intestinal tract. As the bolus of food enters the small intestine, the muscular wall behind it squeezes to push it forward into the next segment, which is relaxed. This movement is controlled by the nervous system along with the hormones and enzymes present in the intestine.

Assimilation or absorption of the nutrients present in food occurs in the small intestine. After absorption the nutrients are passed into the blood stream and finally reach every cell in the body.