rectum

The rectum is the final straight portion of the large intestine, about 12 to 15 centimeters long, connecting the sigmoid colon to the anal canal. It resides in the pelvic cavity and serves as a temporary reservoir for feces before defecation. The rectum begins where the sigmoid colon ends and extends to the anorectal junction, where it becomes the anal canal.

Anatomy: The rectum is largely retroperitoneal in the pelvis, with peritoneum covering its anterior and lateral aspects in the upper portion. Its mucosa is smooth and glandless, lacking villi. It contains rectal valves that form small pockets (the Houston valves) and a rectal ampulla that accommodates fecal storage. Unlike the colon, the rectum has a continuous outer longitudinal muscle layer rather than taeniae coli.

Blood supply comes from the superior rectal artery (a branch of the inferior mesenteric artery) with additional supply from the middle and inferior rectal arteries (branches of the internal iliac and pudendal arteries). Venous drainage mirrors this pattern via the superior rectal veins to the portal system and the middle/inferior rectal veins to the systemic circulation. Lymphatics drain to inferior mesenteric, internal iliac, and sacral nodes, depending on the region.

Innervation and function: Autonomic input comes from the inferior hypogastric plexus, with parasympathetic fibers from the pelvic splanchnic nerves. Somatic sensation travels through the inferior rectal nerves of the pudendal nerve. The rectum stores feces and initiates the defecation reflex; the anorectal angle, maintained by the puborectalis muscle, helps continence, while the internal and external anal sphincters regulate passage.

Development and clinical relevance: The rectum derives from the hindgut; the upper portion is endodermal, the lower anal canal is ectodermal, separated by the pectinate line. Common conditions include hemorrhoids, rectal cancer, and rectal prolapse.