Constipation and Disorders of Pelvic Floor Function

Colonic functions include water and electrolyte absorption, 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 and 800 to 1,000 mEq of sodium and chloride daily. In healthy subjects, the average mouth-tocecum transit time is approximately 6 hours, and the average regional transit time through the right, left, and sigmoid colon is about 12 hours each, yielding an average total colonic transit time of 36 hours. (The physiology of defecation is discussed in the section on “Disorders of Pelvic Floor Function.”).