colonrectum

Colon and rectum, collectively referred to here as the colon-rectum, form the distal part of the digestive tract and constitute the large intestine. They are continuous with the ileum at the ileocecal valve and extend from there to the anal canal. The colon consists of the ascending, transverse, descending, and sigmoid segments; the rectum is the final, straight portion that ends at the anal canal.

The colon primarily reabsorbs water and electrolytes and concentrates feces, while the rectum acts as a temporary storage site before defecation. The mucosa secretes mucus to aid passage of stool, and the resident microbiota contribute to digestion and host health. The rectum contains anal cushions and is closely involved in the defecation reflex, which coordinates stool evacuation with voluntary relaxation of the external anal sphincter.

Blood supply to the colon and rectum follows a pattern related to embryologic origin. The superior mesenteric artery supplies the midgut portions of the colon, with branches such as the ileocolic and right colic arteries feeding the proximal sections. The left colic and other branches of the inferior mesenteric artery supply the distal colon and sigmoid. The rectum receives the superior rectal arteries from the inferior mesenteric artery and the middle and inferior rectal arteries from the internal iliac system. Venous drainage mirrors this arrangement, with tributaries feeding into the portal and systemic circulations. Lymphatic drainage follows the arterial routes to regional nodes, including superior and inferior mesenteric nodes and pelvic nodes.

Innervation includes parasympathetic input via the vagus nerve to the proximal colon and pelvic splanchnic nerves (S2–S4) to the distal colon and rectum, with sympathetic fibers from mesenteric plexuses modulating activity. Disorders of the colon-rectum include colorectal cancer, inflammatory bowel disease, diverticular disease, hemorrhoids, and rectal prolapse, with screening and management guided by standard clinical guidelines.