Anatomy

The colon is about six feet long and has four parts namely the ascending colon, transverse colon, the descending colon, and the sigmoid colon. Beyond the sigmoid colon is the rectum and the anus. The colon from cecum to the mid-transverse colon is also known as the right colon. The remainder is known as the left colon.

The ascending colon, on the right side of the abdomen, is about 12.5 cm long. It is the part of the colon from the cecum to the hepatic flexure (hepatic means liver). The transverse colon extends from the hepatic flexure to the splenic flexure(near the spleen). The descending colon extends from the splenic flexure to the beginning of the sigmoid colon. The sigmoid colon starts after the descending colon and ends before the rectum. The name sigmoid means S-shaped.

The rectum is about eight inches and connects the sigmoid colon with the anal canal. The anal canal is 2.5 - 4 centimeters long. It's situated between the rectum and anus.

Physiology

The functions of the Colon are absorption of water and minerals and the formation and elimination of feces. The small intestine absorbs the nutrients from the food and pours the leftover sludge into the cecum. This sludgy waste then moves from the cecum to the colon for further processing. The colon absorbs water from the sludge while transporting it toward the rectum.

The colon stores the waste material until it is time for it to be evacuated. The colon moves the waste material through by involuntary wavelike contractions, made possible by smooth muscles within the colon wall, a process which is referred to as peristalsis.

The urge to defecate is signaled by the propulsion of feces from the sigmoid colon to rectum. Distention of the rectum causes relaxation of the internal anal sphincter (involuntary sphincter). For defecation to proceed, the external anal sphincter must voluntarily relax. Defecation is facilitated by squatting or sitting and by increasing intra-abdominal pressure.