rectum

The rectum has three lateral curves marked internally by the transverse rectal folds (Houston's valves). It lacks taeniae coli, haustra, and appendices epiploicae (unlike the rest of the colon), though it retains a complete longitudinal muscle layer. It is divided into upper 1/3 (covered by peritoneum anteriorly and laterally), middle 1/3 (peritoneum only anteriorly), and lower 1/3 (entirely extraperitoneal), which is clinically important for staging rectal cancer and surgical approach (anterior peritoneal reflection). Blood supply: superior rectal artery (from inferior mesenteric artery, IMA) supplies upper rectum; middle and inferior rectal arteries arise from internal iliac and internal pudendal arteries respectively, supplying the rest. Venous drainage mirrors arterial supply: superior rectal vein drains into the portal system (via IMA to splenic vein), while middle/inferior rectal veins drain into the systemic (caval) system via internal iliac veins. This creates a clinically important portosystemic anastomosis at the rectum, relevant in portal hypertension causing internal hemorrhoids/rectal varices. Lymphatic drainage: upper 2/3 drains to inferior mesenteric nodes; lower 1/3 drains to internal iliac nodes (and also superficial inguinal nodes below the pectinate line at the anal canal). Innervation is autonomic: sympathetic (hypogastric nerves, inhibits motility) and parasympathetic (pelvic splanchnic nerves S2-S4, stimulates defecation reflex). Clinical correlations include rectal cancer (staged by depth of invasion via TNM, with total mesorectal excision as key surgical principle), rectal prolapse, hemorrhoids (internal above pectinate line - visceral innervation, painless; external below - somatic innervation, painful), and the digital rectal exam used to assess prostate, rectal masses, and sphincter tone. The rectum is a common site for polyps and adenocarcinoma, often screened via colonoscopy or sigmoidoscopy.