rectosigmoid

Rectosigmoid refers to the anatomical region where the sigmoid colon ends and the rectum begins, marking the transition from intraperitoneal to retroperitoneal portions of the large bowel. The junction is typically located in the pelvis, near the level of the third sacral vertebra, at or just above the pelvic brim, where peritoneal coverings change from a serosa on the sigmoid colon to adventitia on the rectum.

Anatomically, the rectosigmoid junction is contiguous with the sigmoid colon proximally and the rectum distally, and lies near the margins of the mesorectal fascia that define rectal surgery planes. The arterial supply is from the inferior mesenteric artery via the sigmoid arteries and the superior rectal artery, with collateral flow through the marginal artery. Venous drainage follows the arterial supply to the inferior mesenteric vein and portal circulation. Lymphatic drainage primarily targets the inferior mesenteric lymph nodes, with possible drainage to pelvic (internal iliac) nodes at lower levels.

Innervation is via the autonomic nervous system, with sympathetic fibers from the inferior mesenteric plexus and parasympathetic fibers from the pelvic splanchnic nerves (S2-S4) supplying hindgut structures; visceral pain is conveyed with these fibers and can be referred to the lower abdomen and pelvis.

Clinical relevance: The rectosigmoid area is a common site for colorectal pathology, including colorectal cancer and diverticular disease affecting the sigmoid colon. Surgical management often involves resection with preservation or division of mesorectal planes, such as extended sigmoid resection or low anterior resection with total mesorectal excision, depending on disease extent.