The large intestine, otherwise known as the colon, is the terminal part of the gastrointestinal tract, before excretion of the nutritional intake occurs. Its function is to absorb water from the indigestible food matter and to collect and eventually relieve the body of waste matter. Here the sections of the colon will be discussed and at the end several pathological disorders will be mentioned.
Firstly, a brief and general overview of the colon will be noted before each section is described in greater detail. The entire colon has three main characteristics, which are helpful when trying to distinguish it from the rest of the alimentary canal. The first feature is the teniae coli, which are three thickened longitudinal bands of muscle that span the length of the colon, with the exception of the appendix and the rectum. The second feature is called the haustra, which are the sacculations or bulging pouches formed by the contraction of the teniae coli. Lastly, the epiploic appendages which are fatty pouches situated along the part of the peritoneum known as the omentum.
The colon is divided into the cecum with its vermiform appendix, the ascending colon, the transverse colon, the descending colon, the sigmoid colon and the rectum and anal canal. It makes up part of the midgut and the hindgut and is both intraperitoneal and retroperitoneal. In the internal lining the plicae semilunaris are the areas of the mucosa that are raised.
The Cecum and Vermiform Appendix
An intraperitoneal section that is around 7.5 cm in diameter and pouch-like in shape. It is bound to the abdominal wall via the cecal folds of the peritoneum and is a blind structure. It connects the ileum to the large intestine via and invagination known as the ileocecal valve. The blood supply comes from the ileocolic artery and vein and the lymphatic drainage flows to the ileocolic nodes. The superior mesenteric plexus provides parasympathetic fibers from the vagus nerve and sympathetic fibers from the lower thoracic spinal cord segments.
The vermiform appendix is a small 4 inch tubular intraperitoneal structure that is blind and arises from the posteromedial side of the cecum. It is supplied by the appendicular artery that branches from the ileocolic artery.
The Ascending Colon
A retroperitoneal section of the colon that is found on the right side of the posterior abdominal wall and stretches between the cecum and the liver. At the hepatic flexure which is the curvature just below the liver it turns left to form the next section. It is supplied by the superior mesenteric vessels and their branches including the ileocolic artery and vein and the right colic artery and vein. The epicolic and paracolic nodes filter the lymph and the nervous supply comes from the superior mesenteric plexus.
The Transverse Colon
The transverse colon is intraperitoneal, so within the peritoneum sheaths and it extends from the hepatic flexure to the splenic flexure. It is the largest of all the sections of the large intestine and it is the most mobile, although it is attached to the posterior abdominal wall via its own mesentery. The superior mesenteric vessels provide it with the left and right colic arteries and veins and the middle colic artery and vein. Innervation occurs via the superior and inferior mesenteric plexus and the lymphatic drainage goes to the middle colic nodes.
The Descending Colon
The descending colon is retroperitoneal and can be found of the posterior left side of the abdominal wall, extending from the curvature next to the spleen to the sigmoid colon. The inferior mesenteric artery and vein give the left colic artery and vein and also the superior sigmoid artery and veins as the vascular supply. Innervation occurs in the same way as the sigmoid colon.
The Sigmoid Colon
This structure is intraperitoneal and suspended via the sigmoid mesocolon as an s-shaped loop whose length varies. It is the link between the descending colon and the rectum. Its blood supply arrives via the branches of the inferior mesenteric artery and vein, which are known as the left colic artery and vein as well as the superior sigmoid artery and vein. The epicolic and paracolic lymph nodes drain the region, while the lumbar sympathetic trunk and the superior hypogastric plexus innervate it sympathetically. Parasympathetic fibers are contributed by the pelvic splanchnic nerves.
The Rectum and Anal Canal
The rectum is intraperitoneal, but as it descends into the pelvic floor to the anal canal, it becomes retroperitoneal.
The rectum starts at the level of the 3rd sacral vertebra and is a 15 cm muscular tube which connects the colon to the anal canal, where it is fixed and continuous.
There is an endless list of disorders that can directly or secondarily affect the large intestine and since this article’s main focus is the macroanatomy of the organ, only a few common ailments shall be mentioned and briefly described.
Intestinal obstruction can affect the small or the large intestine. It occurs when an area of the bowels becomes twisted, blocked by food or stops working altogether. Symptoms include pain, vomiting, abdominal distension and constipation.
Colitis is the inflammation of the walls of the colon. It can be caused by a bacterial infection or by inflammatory bowel disease (IBS).
Diverticulosis occurs when certains areas of the colon’s muscular wall become weak and the intestinal lining protrudes, forming little pouches, which can be subject to bleeding or inflammation.
Crohn’s disease is an autoimmune inflammatory disease of the bowels which limits the ability of the alimentary canal to absorb nutrients.