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. 

Gross anatomy

In front of S3 the sigmoid colon loses it mesentery and becomes the rectum. The taenia 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. 

The rectum lies in front of the sacrum and has an AP concave shape. It also is sinuous and takes on three bends when viewed anteriorly. 

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 rectum.

The rectum continues on as the anal canal at the level of the coccyx tip. It takes on an acute angle, the perineal flexure, as it passes through the puborectalis muscle and is the anal canal after this point. 

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

Blood supply

• arterial supply
  • superior rectal artery: terminal branch of inferior mesenteric artery
  • middle rectal artery: branch of internal iliac artery
  • inferior rectal artery: branch of the internal pudendal artery
• venous return: via similarly named veins to both the portal venous system and systemic venous system

Nerve supply

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

Lymphatic supply

Lymphatics run with superior rectal vessels to the inferior mesenteric group to the 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