The rectum is an organ located in the pelvis which stores faeces. Knowledge of Rectal anatomy is important in understanding the staging of rectal malignancies and it’s potential pathways of spread.
Rectum is the distal part of the large intestine, beginning from the rectosigmoid junction and terminating in a dilatation called the ampulla. It is located anterior to the sacrum and coccyx, within the concavity of the sacrum and posterior to the urinary bladder in male and uterus in female.
It extends from S3 vertebrae and becomes continuous with the anal canal, and follows the concavity of the sacrum.
It has three lateral curvatures. the upper, middle and lower lateral curvatures.
The upper and lower curvatures are convex to the right, the middle curvature is convex to the left.
Upper curvature is anterior to the junction of S3-S4 vertebrae, the middle curvature is anterior to the sacrococcygeal junction and the lower curvature is anterior to the tip of the coccyx.
The rectum is draped by the peritoneum which covers it both anteriorly and laterally in the upper one third, anteriorly in it’s middle third and does not cover it at all in the lower thirds.
Anatomic relations of the rectum
In males, anterior to the rectum lies the urinary bladder, seminal vesicles, prostate and ureters, ampulla of the vas derferens.
In females the uterus and vagina lie anterior to the rectum.
In males the bladder lies anterior to the rectum.
Posterior relations of rectum
Interior of the rectum
The Houston’s valves are located in against the concavities of each lateral curvature of the rectum.
Arterial Supply of the rectum
The rectum is supplied by the superior, middle and inferior rectal arteries.
The superior rectal artery is a continuation of the inferior mesenteric artery. It reaches upto the level of rectosigmoid junction opposite the S3 vertebrae where it divides into left and right branches. The entire mucosa of the rectum is supplied by the superior rectal artery. The superior rectal artery enters the submucosa of the rectum and forms a plexus, from which vessels further arise and anastamose with branches from the inferior rectal artery at the pectinate line.
The Middle rectal artery is a branch of the internal iliac artery and travels downwards along the walls and floor of the pelvis, and then medially to supply the lower part of the rectum. This vessel supplies the outer coating of the rectum.
The inferior rectal artery is a branch of the pudendal artery and travels towards the rectum and within the rectal submucosa and anastomoses with the submucosal arterial plexus at the pectinate line.
Venous drainage of rectum
The rectum is drained by the internal and external venous plexus. The internal venous plexus lies above the pectinate line and is present within the subcutaneous coat. The external venous plexus lies between the skin and subcutaneous part of the anal sphincter.
Superior rectal vein– The internal venous plexus, merge and form veins, which travel through the columns of the pectinate line and emerge from the rectum about 7.5 cm from the anus. These veins continue as internal mesenteric vein and drain into the splenic vein (portal system).
The middle and inferior rectal veins are formed from the external venous plexus and travel along the middle and inferior arteries respectively.
The middle rectal vein drains into the internal iliac vein, and the inferior iliac veins drain into the internal pudendal vein.
Lymphatic drainage of the rectum
Unlike the other structures in the gut, rectal lymphatics are longitudinal.
That is why, when the carcinoma of the rectum spreads along the lymphatics it does not cause as much obstruction as compared to the rest of the GI tract.
The upper half of the rectum, travel along the superior mesentric vessels and drain into the inferior mesentric nodes.
Lymphatics from the lower half of the rectum travel along the middle rectal vessels and drain into the internal iliac nodes.