The large intestine is within the alimentary tract where water is absorbed from indigestible contents. The large intestine includes the cecum, appendix, entire colon, rectum, and anal canal. It begins at the terminal ileum with the cecum. Unlike the small intestine, it has a shorter length but a much larger lumen. It is distinguished further from the small intestine by the presence of omental appendices, haustra, and teniae coli.
The cecum is the proximal blind pouch of the ascending colon, lying at the level of the ileocecal junction. The terminal ileum contents open into the cecum on the medial wall, and the ileocecal valve guards this opening. The appendix is a thin cylindrical organ with a blind attachment to the cecum. The base of the appendix lies on the posteromedial wall of the cecum about 1 to 2 centimeters below the ileocecal junction. The tip of the appendix frequently floats in the peritoneal cavity and can be located in a retrocecal position. It has a short triangular mesentery called the mesoappendix.
The cecum is continuous with the second part of the large intestine: the ascending colon. The ascending colon runs superiorly on the right side of the abdomen from the right iliac fossa to the right lobe of the liver. At this point, it makes a left turn at the right colic flexure (hepatic flexure). It is found in a deep, vertically-oriented grove lined with parietal peritoneum known as the right paracolic gutter. The transverse colon is the third, most mobile, and longest part of the large intestine. It is found between the right and left colic flexures. The left colic flexure is less mobile than the right and is attached to the diaphragm through the phrenicocolic ligament. The transverse colon is attached to a mesentery, the transverse mesocolon, which has its root along the inferior border of the pancreas. The transverse colon continues to become the descending colon found within the left paracolic gutter between the left colic flexure superiorly and the left iliac fossa inferiorly. It terminates into the sigmoid colon, which is the fifth part of the large intestine. The sigmoid colon links the descending colon to the rectum. It is an S-shaped loop of varying length and becomes the rectum at the level of S3.
The rectum occupies the concavity of the sacrococcygeal cavity. It is fixed, primarily retroperitoneal, and subperitoneal in location. It transitions to the anal canal at the level of the puborectal sling which is formed by the fibers of the levator ani muscles. The rectum has an expanded middle segment called the ampulla. Anterior to the rectum are the rectovesical pouch, prostate, bladder, urethra, and seminal vesicles in males. In females, it is the recto-uterine pouch, cervix, uterus, and vagina.
The key functions of the colon include the following:
Embryologically, the colon develops from the midgut (cecum to the distal transverse colon), the hindgut (distal transverse colon to the dentate line in the anorectum), and the proctodeum (below the dentate line).
The blood supply to the colon is provided by the superior mesenteric artery (SMA) and the inferior mesenteric artery (IMA). Communication between these two vessels happens via the marginal artery which runs parallel to the length of the entire colon. The branches supplying specific portions of the bowel are as follows:
Venous drainage usually accompanies arterial colonic supply. Ultimately, the IMA drains into the splenic vein, while the SMA joins the splenic vein to form the hepatic portal vein. Finally, lymphatics of the large intestine drain into the lymph nodes associated with the main vessels of arterial supply.
The midgut-derived ascending colon and proximal two-thirds of the transverse colon receive parasympathetic, sympathetic, and sensory nerve supply from the superior mesenteric plexus.
The hindgut-derived structures, which include the distal one-third of the transverse colon, descending, and sigmoid colon, receive parasympathetic, sympathetic, and sensory nerve innervation from the inferior mesenteric plexus.
The appendix, transverse colon, and sigmoid colon all have a mesentery and are thus considered to be intraperitoneal in location. The cecum also is located intraperitoneally, but it lacks a mesentery. It is considered to be an intraperitoneal organ; however, as it is covered on all sides by peritoneum, the ascending colon, descending colon, rectum, and anal canal are retroperitoneal structures.
The marginal artery of Drummond is a large collateral that supplies the colon and is of importance when there is occlusion of one of the major vessels supplying blood to the colon. There are several disorders that can affect the colon, including diverticular disease, colon cancer, bowel obstruction, lower gastrointestinal (GI) bleeding from polyps and AV malformations, strictures, and peristalsis.
Of note when discussing the large intestine is the greater omentum. It is an apron-like covering composed of four layers of peritoneum that are fused together. From the stomach, two layers of omentum descend and then roll upward to attach to the transverse colon. Also, as previously mentioned, the intestines have particular relations and attachments to the peritoneum and mesenteries, respectively.