Small Intestine PDF

# Small Intestine ## By Eesha Zuvaina and Samiya Alam ### Small Intestine The small intestine is divided into three sections: the duodenum, jejunum and ileum. These sections can extend up to six meters in length. All three parts are covered with the greater omentum anteriorly. The duodenum has...

# Small Intestine ## By Eesha Zuvaina and Samiya Alam ### Small Intestine The small intestine is divided into three sections: the duodenum, jejunum and ileum. These sections can extend up to six meters in length. All three parts are covered with the greater omentum anteriorly. The duodenum has both intraperitoneal and retroperitoneal parts, while the jejunum and ileum are entirely intraperitoneal organs. The small intestine is the main site for the final stages of food digestion and its absorption, and its gross and microanatomy are adjusted to that function. ### Duodenum The duodenum is the first part of the small intestine. It extends from the pyloric sphincter of the stomach, wraps around the head of the pancreas in a C-shape and ends at the duodenojejunal flexure. This flexure is attached to the posterior abdominal wall by a peritoneal fold called the suspensory muscle (ligament) of duodenum, also called the ligament of Treitz. The duodenum has four parts: superior (duodenal bulb/ampulla), descending, horizontal and ascending. The superior part (duodenal bulb/ampulla) is the only intraperitoneal part, as the hepatoduodenal ligament and greater omentum attach to it. The descending part of the duodenum has an opening called the major duodenal papilla (tubercle of Vater). The papilla contains the hepatopancreatic sphincter (sphincter of Oddi, Glissons' sphincter) which regulates the emptying of the bile from the hepatopancreatic ampulla. ### Jejunum The jejunum is the second part of the small intestine. It begins at the duodenojejunal flexure and is found in the upper left quadrant of the abdomen. The jejunum is entirely intraperitoneal as the mesentery proper attaches it to the posterior abdominal wall. There is no clear line of demarcation between the jejunum and ileum, but there are some anatomical and histological differences that distinguish them. The jejunum represents the proximal two-fifths of the jejunum-ileum continuum. The wall of the jejunum is thicker and its lumen is wider than in the ileum. The jejunum contains more prominent circular folds of Kerckring. ### Ileum The ileum is the last and longest part of the small intestine. It is found in the lower right quadrant of the abdomen, although the terminal ileum can extend into the pelvic cavity. The ileum terminates at the ileal orifice (ileocecal junction) where the cecum of the large intestine begins. At the ileocecal junction, the lamina muscularis of the ileum protrudes into the lumen of the cecum forming a structure called the ileocecal fold. These muscular fibers form a muscular ring within the fold called the ileocecal sphincter which controls the emptying of ileal content into the large intestine. ### Function The main functions of the small intestine are secretion and absorption. The epithelial cells of the small intestine secrete enzymes which digest chyme into the smallest particles, making them available for absorption. Concurrently the duodenum functions to mix food with bile and pancreatic enzymes to continue the digestion of carbohydrates, fats, and proteins. Concerning absorption, carbohydrates and proteins are absorbed in the duodenum and jejunum respectively. The jejunum also functions to absorb most fats. The ileum function involves absorption of vitamin B12, bile salts and all digestion products which were not absorbed in the duodenum and jejunum. All three small intestine segments absorb water and electrolytes.