The Rectum

The rectum is the most distal segment of the large intestine, and has an important role as a temporary store of faeces.

It is continuous proximally with the sigmoid colon, and terminates into the anal canal.

In this article we will discuss the anatomy of the rectum – its structure, anatomical relationships, and clinical relevance.

Anatomical Structure

The rectum begins at the level of the S3 (as a continuation of the sigmoid colon). It is macroscopically distinct from the colon, with an absence of taenia coli, haustra, and omental appendices.

The course of the rectum is marked by two major flexures:

• Sacral flexure – anteroposterior curve with concavity anteriorly (follows the curve of the sacrum and coccyx).
• Anorectal flexure – anteroposterior curve with convexity anteriorly. This flexure is formed by the tone of the puborectalis muscle, and contributes significantly to faecal continence.

There are additionally three lateral flexures (superior, intermediate and inferior), which are formed by transverse folds of the internal rectum wall.

The final segment of the rectum, the ampulla, relaxes to accumulate and temporarily store faeces until defecation occurs. It is continuous with the anal canal; which passes through the pelvic floor to end as the anus.

Peritoneal Coverings

In the superior third of the rectum, the anterior surface and lateral sides are covered by peritoneum. The middle third only has an anterior peritoneal covering, and the lower 1/3 has no peritoneum associated with it.

In males, the reflection of peritoneum from the rectum to the posterior bladder wall forms the rectovesical pouch. In females, the peritoneum reflects to the posterior vagina and cervix, forming the rectouterine pouch (pouch of Douglas). See more about the peritoneal cavity here.

Anatomical Relations

The rectum is located within the pelvic cavity, and is the most posterior of the pelvic viscera. Its anatomical relations are different in men and women:

Anterior		Posterior
Male	Female	Sacrum and coccyx Piriformis Coccygeus Levator ani Sacral plexus
Rectovesical pouch Sigmoid colon Ileum Bladder Prostate Seminal vesicles	Rectouterine pouch Sigmoid colon Ileum Vagina Cervix

Neurovascular Supply

The rectum receives arterial supply through three main arteries:

• Superior rectal artery – terminal continuation of the inferior mesenteric artery.
• Middle rectal artery – branch of the internal iliac artery.
• Inferior rectal artery – branch of the internal pudendal artery.

Venous drainage is via the corresponding superior, middle and inferior rectal veins. The superior rectal vein empties into the portal venous system, whilst the middle and inferior rectal veins empty into the systemic venous system. Anastomoses between the portal and systemic veins are located in the wall of anal canal, making this a site of portocaval anastomosis.

Note: the rectum is also closely anatomically associated with the rectal venous plexus; however this structure is more functionally related to the anal canal.

Innervation

The rectum receives sensory and autonomic innervation.

Sympathetic nervous supply to the rectum is from the lumbar splanchnic nerves and superior and inferior hypogastric plexuses. Parasympathetic supply is from S2-4 via the pelvic splanchnic nerves and inferior hypogastric plexuses. Visceral afferent (sensory) fibres follow the parasympathetic supply.

Lymphatic Drainage

Lymphatic drainage of the rectum is via the pararectal lymph nodes, which drain into the inferior mesenteric nodes.

Additionally, the lymph from the lower aspect of the rectum drains directly into the internal iliac lymph nodes.