Colon (anatomy) - Anatomy

Anatomy

The location of the parts of the colon is either in the abdominal cavity (intraperitoneal) or behind it in the retroperitoneum. Retroperitoneal organs in general do not have a complete covering of peritoneum, so they are fixed in location. Intraperitoneal organs are completely surrounded by peritoneum and are therefore mobile.

Of the colon, the ascending colon, descending colon and rectum are retroperitoneal, while the caecum, appendix, transverse colon and sigmoid colon are intraperitoneal. This is important as it affects which organs can be easily accessed during surgery, such as a laparotomy.

The haustra of the colon are the small pouches caused by sacculation, which give the colon its segmented appearance. The taenia coli runs the length of the large intestine. Because the taenia coli is shorter than the intestine, the colon becomes sacculated between the taenia, forming the haustra.

Arterial supply to the colon comes from branches of the superior mesenteric artery (SMA) and inferior mesenteric artery (IMA). Flow between these two systems communicates via a "marginal artery" that runs parallel to the colon for its entire length. Historically, it has been believed that the arc of Riolan, or the meandering mesenteric artery (of Moskowitz), is a variable vessel connecting the proximal SMA to the proximal IMA that can be extremely important if either vessel is occluded. However, recent studies conducted with improved imaging technology have questioned the actual existence of this vessel, with some experts calling for the abolition of the terms from future medical literature.

Venous drainage usually mirrors colonic arterial supply, with the inferior mesenteric vein draining into the splenic vein, and the superior mesenteric vein joining the splenic vein to form the hepatic portal vein that then enters the liver.

Lymphatic drainage from the entire colon and proximal two-thirds of the rectum is to the paraaortic lymph nodes that then drain into the cisterna chyli. The lymph from the remaining rectum and anus can either follow the same route, or drain to the internal iliac and superficial inguinal nodes. The pectinate line only roughly marks this transition.

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