Functions of the large bowel: an overview

The large intestine has been studied rather less than other portions of the alimentary canal for several reasons: a) considerable variations occur among species in the anatomy of the large bowel and in the absorptive contributions of the hindgut to homeostasis; b) in most species, the transit of colonic contents and the interactions between mucosa and contents are more complex than those occurring in the small bowel; and c) the existence of a rich microbial flora, which is of considerable importance ecologically, complicates the experimental approaches to colonic function and the interpretation of results. The colon possesses efficient mechanisms for sodium and chloride absorption, and an exchange of bicarbonate for luminal chloride is important. Absorptive function in the large bowel must also encompass the activities of faecal enzymes. By modifying faecal substrates, of both endogenous and exogenous origin, the flora facilitates and modifies absorption by the colon; in some species these events are important nutritionally. On the other hand, under pathophysiological conditions, the colon can secrete electrolytes and water. Storage and transit in the colon are also complex, relative to these phenomena in other areas of the gut. A major portion of the total mouth-to-anus transit time occurs in the colon, where to-and-fro movement of the contents is noticeable. Perhaps these complex movements facilitate absorption, by allowing optimum contact between contents and the mucosa. The colon also delivers material to the rectum in a manner whereby the distal bowel can prepare stools for convenient evacuation. Despite these difficulties, research is advancing our comprehension of the colon, its multiple and diverse functions and the possibilities for alterations of function leading to disease.