Large intestine

The large intestine is distinguished from the small intestine by three external features: teniae coli (three longitudinal bands of outer smooth muscle that gather the bowel wall into pouches), haustra (the outpouchings or sacculations created between the teniae), and epiploic appendages (small fat-filled peritoneal pouches projecting from the surface). Internally, the large intestine lacks the villi present in the small intestine, reflecting its primary role in water absorption and waste storage rather than nutrient digestion.

The large intestine serves several key functions: (1) water and electrolyte absorption — recovering approximately 1.5 liters of fluid daily from the liquid chyme arriving from the small intestine, (2) vitamin synthesis — the colonic microbiome produces vitamin K and some B vitamins, (3) stool formation — compacting and storing fecal matter until defecation, and (4) fermentation — gut bacteria ferment undigested carbohydrates (dietary fiber), producing short-chain fatty acids that nourish colonic epithelial cells.

The vermiform appendix is a narrow, blind-ended tube attached to the cecum (the first segment of the large intestine). Once considered vestigial, the appendix is now recognized as a lymphoid organ and a reservoir of commensal gut bacteria that may help repopulate the colon after diarrheal illness. The appendix is clinically important because appendicitis — inflammation from obstruction — causes acute abdominal pain and is the most common reason for emergency abdominal surgery.