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Large intestine

Large intestine

The large intestine is the most distal organ of the digestive system. It extends from the ileocecal valve to the anus, with an overall length of 1.5 meters in the average adult. It occupies a curve, which forms a border around the loops of the small intestine that is located centrally within the abdomen. The large intestine functions to absorb fluids and salts from the indigestible residue of the liquid chyme received from the small intestine. This converts it into feces that are stored temporarily to accumulate until defecation occurs. The large intestine consists of the cecum, the appendix, the colon, the rectum and the anal canal.

Cecum

The cecum is the first part of the large intestine. This large blind-ending pouch lies in the right iliac fossa, inferior to the ileocecal valve, where it is continuous proximally with the ileum of the small intestine and distally with the ascending colon. Though not suspended by mesentery, the cecum is considered an intraperitoneal structure due to its mobility. The appendix is a worm-like narrow tube that arises from the posteromedial end of the cecum. Despite its variable positioning, it usually lies in the retrocecal region. The appendix contains masses of lymphoid tissue, but has no vital function in humans.

Colon

The colon encircles the small intestine, and consists of four parts: the ascending, transverse, descending, and sigmoid colon. 

Ascending Colon

The ascending colon extends superiorly from the cecum, on the right side of the abdominal cavity, to the inferior surface of the right lobe of liver, where it makes a sharp turn forwards and to the left at the right colic flexure (hepatic flexure). 

Transverse Colon

The transverse colon is the longest and most mobile part of the large intestine. It extends across the abdomen from the right colic flexure to the left colic flexure (splenic flexure), which is located below the spleen. 

Descending Colon

The descending colon extends between the left colic flexure and the left iliac fossa, where it meets with the sigmoid colon. During its descent, it passes the lateral border of the left kidney. 

Sigmoid Colon

The sigmoid colon is the final part of the colon. It is an S-shaped structure that joins the descending colon with the rectum.  The ascending and descending segments are considered secondary retroperitoneal structures, while the transverse and sigmoid segments are considered intraperitoneal.

Rectum

The rectum is the fixed terminal portion of the large intestine. It extends from the rectosigmoid junction, at the level of the third sacral vertebra (S3), and ends at the beginning of the anal canal. The anal canal passes inferiorly and posteriorly from the rectum at the anorectal junction and terminates at the anal verge.

Neurovasculature

The arterial blood supply of the cecum, appendix, and ascending colon are somewhat variable, but all originate from the superior mesenteric artery via the ileocolic and right colic arteries. Their venous drainage follows that of their arteries. The transverse colon is primarily supplied by the middle colic artery. However, its distal one-third is supplied by the left colic artery via the marginal artery of the colon. The venous blood of the transverse colon is drained by the superior mesenteric and inferior mesenteric veins. The descending colon receives its arterial supply from the left colic and superior sigmoid arteries that branch from the inferior mesenteric artery. Its blood is drained by the inferior mesenteric vein. The sigmoid colon is supplied by the sigmoid arteries, and is drained by the inferior mesenteric vein as well. The rectum and anal canal are supplied by the superior, inferior, and middle rectal arteries. Their venous drainage is similar to their corresponding arteries. The large intestine is innervated by the autonomic system via both parasympathetic and sympathetic nerve fibers.

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