Mastdarm

Mastdarm (rectum) is the final straight section of the large intestine. In adults it measures about 12 to 15 centimeters and begins at the end of the sigmoid colon, near the level of the third sacral vertebra, and ends at the anal canal at the anorectal junction. Its primary roles are to store feces temporarily and to facilitate defecation by propelling contents toward the anal canal.

Anatomy and histology: The rectum is typically described as having three parts—the upper, middle, and lower rectum—and it ends in the rectal ampulla before becoming the anal canal. The mucosa is mainly lined by simple columnar epithelium, with a gradual transition to stratified squamous epithelium near the anal transition zone. The rectum also contains transverse folds known as the valves of Morgagni, which help maintain fecal continuity during movement.

Vascularization, innervation, and lymphatics: The arterial supply comes from the superior rectal artery (a branch of the inferior mesenteric artery) and from the middle and inferior rectal arteries (branches of the internal iliac system). Venous drainage mirrors this pattern, with the superior rectal veins draining to the portal system via the inferior mesenteric vein and the middle and inferior rectal veins draining to the systemic circulation via the internal iliac and internal pudendal veins. Lymphatics drain to the inferior mesenteric and internal iliac nodes. Innervation arises from the autonomic nervous system through the inferior hypogastric plexus, with parasympathetic pelvic splanchnic fibers promoting defecation and somatic innervation to the external anal sphincter via the pudendal nerve.

Function and clinical relevance: The rectum stores feces and coordinates defecation through voluntary and reflex mechanisms. Common clinical issues include hemorrhoidal disease, rectal prolapse, rectal cancer, inflammatory conditions, and proctitis. Diagnostic approaches include digital examination, endoscopy, and imaging, while treatment ranges from lifestyle measures to surgical procedures such as proctectomy or total mesorectal excision for cancer.