Rectum

The rectum is the terminal section of the large intestine (colon), situated between the sigmoid colon above and the anus below. It is a muscular tube lined with mucous membrane, located within the pelvis, in front of the sacrum.

The rectum's function centres on storing faeces from the colon and generating sensory signals when its wall stretches. This stretching triggers the urge to defecate, followed by the process of defecation itself, which involves two anal sphincters: an involuntary (internal) and a voluntary (external) one.

Structure

The rectal wall comprises four layers: mucosa (inner lining), submucosa, muscular layers, and outer layer. The inner surface contains longitudinal folds called transverse rectal folds, which help support the contents. The widest section, known as the ampulla, is where most faecal storage occurs. The inner lining is columnar epithelium with folds, changing to a different type of lining at the dentate line near the anus.

Blood supply and innervation

The rectum receives blood from arteries branching from the lower mesenteric and internal pelvic arteries. Venous drainage follows a dual pathway: partly via the portal system, partly directly into the systemic circulation. The upper part receives autonomic innervation, resulting in pain that cannot be precisely localized; the lower part around the anus has somatic sensation providing sharp, well-defined pain signals.

Examination and health

The rectum is readily accessible for examination. Methods include digital rectal examination and instruments with visual capability such as proctoscopy, sigmoidoscopy, and colonoscopy. Common conditions affecting it include haemorrhoids, anal fissure, fistula, proctitis (inflammation of the rectum), and rectal cancer, one of the common forms of colorectal cancer. The rectum is also occasionally used as a route for administering suppositories or enemas when oral administration is not possible.