Anatomy of Small Intestine
Anatomically the small intestine extends from the pylorus to the ileocaecal junction. It is about 6 m long and divided into three parts: duodenum, jejunum and ileum. The proximal fixed part which is about 10 inches long is called duodenum. The remaining long part which freely mobile is divided into two parts- jejunum and ileum.The duodenum is principally retroperitoneal and fixed. It receives chyme from the stomach, bile from gallbladder, and pancreatic juice from the pancreas. The fixation of duodenum is essential because mobile duodenum will cause twisting and kinking of common bile duct and pancreatic ducts which open into it. It is intimately related with pancreas and extra hepatic biliary apparatus. Hence , clinically it is considered as a separate entity.
The jejunum and ileum are principally concerned with the digestion and absorption of digested food: hence these two parts together form the small intestine proper. The following description of the small intestine is confined only to the jejunum and ileum.
Ileum part of small intestine
The small intestine proper extends from the duodenojejunal flexure to the ileocaecal junction. Its upper two-fifth forms the jejunum and its lower three-fifth forms the ileum. However, there is no definite line of demarcation. The jejunum and ileum are suspended from the posterior abdominal wall by a large fold of peritoneum called the mesentery of small intestine; hence, the small intestine enjoys a considerable mobility . The structure of the small intestine corresponds to its functional requirements.
The wall of the small intestine consists of four layers
1- Mucosa
2- Submucosa
3- Muscle layer
4- Serosa
a) Mucosa
The mucosa presents the following three relevant features-
Large surface area
1- Plicae circulares
2- Villi
3- Microvilli
Intestinal glands
In between the bases of villi, the epithelium is invaginated in the lamina propria to form intestinal glands. They secrete digestive enzymes and mucous.
Lymphatic follicle
The lamina propria of the mucous membrane contain two types of lymphatic follicles-
1- Solitary lymph follicles
2- Aggregated lymph follicles
b) Submucosa
It is made up of loose areolar tissue and contain blood vessels, lymph vessels and nerve plexus.
c) Muscle layer
It is made up of outer longitudinal and inner circular layers of smooth muscle.
d) Serosa
It is formed the visceral peritoneal and is lined by the simple squamous epithelium.
Arterial supply
The jejunum and ileum are supplied by the jejunal and ill branches of the superior mesenteric artery. They arise from the left side of the superior mesenteric artery and enter the mesenteric to reach the intestine.
Venous drainage
The veins correspond to the branches of superior mesenteric artery and drain into the portal vein, which carries the products of protein and carbohydrates to the liver.
Nerve supply
The small intestine is supplied by both sympathetic and parasympathetic nerve fibres. The sympathetic supply is derived fromT10-T11 spinal segments through splenchnic nerves and superior mesenteric plexus . The parasympathetic supply is derived from the vagus nerves through the coeliac and superior mesenteric plexuses.
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