Constipation and Disorders of Pelvic Floor Function

Function Colonic functions include the absorption of water and electrolytes, storage of intraluminal contents until elimination is socially convenient, and nutrient salvage from bacterial metabolism of carbohydrates that are not absorbed in the small intestine. The colon absorbs all but 100 mL of fluid and 1 mEq of sodium and chloride from approximately 1, 500 mL of chyme received over 24 hours. Absorptive capacity can increase to 5 to 6 L of fluid and 800 to 1, 000 mEq of sodium and chloride daily. In healthy subjects, the average mouth-to-cecum transit time is approximately 6 hours, and average regional transit times through the right, left, and sigmoid colon are about 12 hours each, with an average total colonic transit time of 36 hours. (The physiology of defecation is discussed in the section on “Disorders of Pelvic Floor Function.”) Regional Differences in Colonic Motor Function The right colon is a reservoir that mixes and stores contents and absorbs fluid and electrolytes. The left colon is primarily a conduit, whereas the rectum and anal canal are responsible for continence and defecation. The ileocolic sphincter regulates the intermittent transfer of ileal contents into the colon, a process that normalizes in response to augmented storage capacity in the residual transverse and descending colon within 6 months after right hemicolectomy.