The large intestine is about 1.5 meters long and frames the abdomen from the right lower quadrant down to the pelvis. It turns liquid chyme into formed stool.
Parts
It starts at the cecum, where the appendix hangs, then runs as the ascending, transverse, descending and sigmoid colon to the rectum and anal canal.
Water and salts
The colon absorbs much of the remaining water along with sodium and chloride. Fast transit leaves stool loose; slow transit makes it hard and dry.
Gut bacteria
Trillions of bacteria ferment fiber into short-chain fatty acids that feed colon cells, and they make vitamin K. Antibiotics can upset this balance and let Clostridioides difficile overgrow.
Defecation
Stool stretching the rectum triggers the urge to defecate. The internal anal sphincter relaxes by reflex, while the external sphincter is under voluntary control.
At the bedside
Bright red blood usually comes from the lower colon or rectum, while black stool points to bleeding higher in the gut. The consistency of stoma output depends on where along the bowel the stoma sits.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).