The large intestine is wider and shorter than the small intestine (approximately 1.5 metres, or 5 feet, in length as compared with 6.7 to 7.6 metres, or 22 to 25 feet, in length for the small intestine) and has a smooth inner wall. In the proximal, or upper, half of the large intestine, enzymes from the small intestine complete the digestive process, and bacteria produce B vitamins (B₁₂, thiamin, and riboflavin) as well as vitamin K. The primary function of the large intestine, however, is absorption of water and electrolytes from digestive residues (a process that usually takes 24 to 30 hours) and storage of fecal matter until it can be expelled. Churning movements of the large intestine gradually expose digestive residue to the absorbing walls. A progressive and more vigorous type of movement known as the gastrocolic reflex, which occurs only two or three times daily, propels the material toward the anus.

Common afflictions of the large intestine include inflammation, such as colitis (ulcerative colitis); diverticulosis (diverticulum); and abnormal growths, such as benign or malignant tumours.