Rectum

The rectum is the last part of the large intestine. It is located within the pelvis and is the continuation of the sigmoid colon after the rectosigmoid junction and continues as the anal canal at the anorectal angle created by puborectalis. 

Gross anatomy

At the level of the S3 vertebral body, the sigmoid colon loses its mesentery and becomes the rectum. As the rectum passes in front of the sacrum, it takes an AP concave shape. It is also is sinuous and takes onwith three alternating bends when viewed anteriorly. Thetaenia coli also flatten and fuse to form an outer longitudinal muscular layer, thus the rectum does not have the distinctive haustra nor epiploic appendages that the rest of the large intestine has. The rectum is approximately 15 cm long. 

If demarcated from a purely gastrointestinal perspective, the rectum extends from the rectosigmoid junction to proximal anorectal sphincter. Its upper third is covered by parietal peritoneum anteriorly and laterally, its mid third only anterior and its lower third is not covered by peritoneum.

The lower part of the rectum is dilated and is called the rectal ampulla and there are three semilunar transverse rectal folds (valves of Houston), which project into the rectal lumen.

The rectum continues on as the anal canal at the level of the coccyx tip, where it takes on an acute inferior angle, the anorectal angle or perineal flexure, as it passes anterior to the sling of the puborectalis muscle where it becomes the anal canal. 

Relations

• anteriorly
  • male: bladder, ureter, prostate, seminal vesicle
  • female:  bladder, ureter, vagina, rectovaginal septum
• posteriorly: presacral fascia, S3-S5, coccyx, median sacral nerves, superior rectal vessels, sacral sympathetic chain
• inferiorly: piriformis muscle
• laterally: sigmoid colon, terminal ileum

Arterial supply

Arterial supply to the rectum is formed from an anastomotic submucous plexus derived principally from superior and inferior rectal arteries, and middle rectal artery (if present).

• superior rectal artery (terminal branch of inferior mesenteric artery) supplies upper two thirds of the rectum
• middle rectal artery (branch of internal iliac artery) supplies mid and lower rectum
• inferior rectal artery (branch of the internal pudendal artery) supplies mid and lower rectum

Venous drainage

Veins corresponding to their named arteries form a rectal venous plexus, consisting of an internal part within the submucosa, and an external part outside the muscle layer. This represents a site of portosystemic anastomoses.

• superior rectal vein drains to the inferior mesenteric vein
• middle rectal vein drains to the internal iliac vein
• inferior rectal vein drains to the internal iliac vein via the internal pudendal vein

Nerve supply

• upper rectum: inferior mesenteric plexus
• middle and lower rectum: superior and inferior hypogastric plexus

Lymphatic drainage

Lymphatics run with superior rectal vessels to the inferior mesenteric group to drain the upper third. The lower two-thirds drain along the middle rectal vessels to the internal iliac group. 

Variant anatomy

• imperforate rectum
• fistula between the rectum and pelvic viscera

Related pathology

• rectal cancer
• rectal foreign bodies