Anatomy and physiology of the colon

Anatomy

The colon is a hollow organ (or bowel), placed in the abdominal area, about a meter and a half long, which begins at the level of the ileo-cecal valve, terminal part of the small intestine, and ends with the rectum and the anal canal. It is made up of different parts: blind, ascending colon, transverse colon, descending colon and sigma.

The colon wall is formed, from the inside to the outside, by several layers: the mucosa, the submucosa, the muscular and the serosa.

The mucosa is essentially composed of two types of cells:

• epithelial, cylindrical, which have the function of reabsorbing water and salts.

They present, on their outer surface, the one that looks towards the lumen (the channel through which nutrients and feces pass), a series of invaginations, called crypts, which have the purpose of increasing the absorbent surface;

• muciparous goblets, which have the function of secreting a mucous and viscid substance into the lumen, so as to lubricate the same and facilitate the passage of the faeces.

The submucosa is located immediately below the mucosa and is very rich in vascular, lymphatic and nerve fiber structures that regulate peristalsis (propulsive bowel movements that favor the progression of feces towards the rectum).

The muscle is formed by two layers of musculature: an inner one, with a transversal course, and an outer one, with a longitudinal course. They give the viscera a characteristic sacculated appearance.

The serosa, also called peritoneum, is instead a global external covering of the whole colon and also of all the other abdominal organs and viscera.

Physiology

A very important function of the colon is to absorb water and electrolytes (salts) in considerable quantities: it has been calculated that the volume of liquid that flows from the terminal part of the small intestine (ileum) into the ascending colon is 800-1800 ml per day, of which only 40-400 ml are emitted with feces.

The colon is also endowed with a secretory activity, represented mainly by the production of mucus and immunoglobulins (antibodies), which would exert respectively a lubricating role and a protective action from the immune point of view on the whole mucosa of the large intestine.

The main function remains, however, to advance its content, and it is realized, in particular, through two types of contractions: the segmentary ones, which manifest themselves as annular, constant movements, capable of causing the fragmentation of the colic content, and propulsive (peristaltic) ones, which appear intermittently, often as a reflex, mostly after food ingestion, aimed at advancing the previously fragmented material.

The arrival of the faeces in the rectum, stretching the walls of the bowel, determines the beginning of the reflex to defecation, which involves the passage of the feces in the anal canal and their elimination with the evacuation, through the voluntary control of the defecation.