The colon or large intestine is anatomically continuous with the small intestine through the ileo-cecal valve, which serves to keep food from flowing backward from the large intestine to the small intestine. The first part of the large intestine is called the cecum which is connected to the appendix.
The second part of the large intestine is the colon which is divided anatomically into 4 parts. These are the ascending colon and the transverse colon and the descending colon and finally the sigmoid colon. After that comes the rectum and the anal canal.
The main function of the large intestine is as a reservoir for waste products of the digesting process. In addition it has a role in absorption of electrolytes such as sodium and potassium and the absorption of water.
Special cells in the epithelium of the large intestine are called columnar absorptive cells are specialized in absorption of electrolytes and water. The absorption of these electrolytes is processed with the help of pumps that use ATPase enzymes to drive the transport of these ion against their concentration gradient.
The large intestine secretes large amount of mucous into its inner lumen. This mucous in turn help lubricate the intestinal inner surface which helps to propel the waste products and undigested food outside the body.
This mucous is secreted into the lumen using specialized cells that are called goblet cells. These cells due to its special function are more abundant in the large intestine than in the small intestine. The ratio of absorptive columnar cells to goblet cells is approximately 4 to 1 in the large intestine. The number of goblet cells become greater upon the end of the large intestine.
The propulsion of food in the large intestine as well as in the small intestine is done using a mechanism that is called peristalsis, which is the concurrent contraction of the the colon and its propulsion toward the end of the colon. This movement propels the food and helps in its expulsion from the body.
Special medications which are called prokinetic drugs help in accelerating the movement of the peristalsis and thus helping in evacuation of the bowel contents faster.
On the contrary, other medications can slow down the peristalsis movement and thus slow down the emptying of the bowel contents. This type of medication makes the colon do segmentation movements thus maintaining the bowel content for longer periods. This is especially useful in people with diarrhea.
A condition that affects often the colon due to segmentation is called polyps. These are inward protrusions of the colon mucosa. They are benign and unless bigger than 2 cm are harmless. If they are bigger than 2 cm they can become malignant and therefore excision is required. Their symptoms are diarrhea and rectal bleeding.
Another condition that is similar to polyps is called diverticulitis. They arise also as a result of segmentation movements but they protrude outward the lumen. A complication that can arise as a result is diverticulitis which is an inflammation of the diverticulitis.
Diarrhea can be caused as result of irritation of the large intestine or due to bacteria. On the other hand constipation can be caused due to lack of peristalsis which can happen due to neuronal defect.