Rectum is a part of the large intestine that is about 5 inches long and begins as a continuation of the sigmoid colon in front of the third sacral vertebra. It continues downward, in front of the sacrum and coccyx and ends in front of the tip of the coccyx by piercing the pelvic diaphragm and becoming continuous with the anal canal.
During its course from third sacral vertebra to the tip of the coccyx, the rectum deviates to the left a little, but it quickly returns to the midline. On lateral view, the rectum follows the anterior concavity of the sacrum. Near its junction with the anal canal, it bends downward and backward.
Rectal ampulla is the lower dilated part of the rectum.
The puborectalis portion of the levator ani muscle forms a sling at the junction of the rectum and anal canal. The sling pulls this part of the rectum forward producing an angle called the anorectal angle. Therefore the anorectal angle is the angle between anal canal and rectum produced by the puborectalis sling.
The proximal third of rectum is covered by peritoneum on anterior and lateral surfaces. The middle third is covered only on its anterior surface. The lower third is devoid of peritoneal covering.
The rectum has similar outer longitudinal and inner circular smooth muscle layers as the rest of the large intestine. However, the three teniae coli of the sigmoid colon come together so that the longitudinal fibers form a broad band on the anterior and posterior surfaces of the rectum.
The mucous membrane of the rectum forms two or three semicircular folds, which are permanent. These folds are called transverse folds of the rectum and they vary in position.
- In males:
- Sigmoid colon
- Coils of ileum that occupy the rectovesical pouch
- Posterior surface of bladder
- In females:
- Sigmoid colon
- Coils of ileum that occupy the rectouterine pouch (Douglas’ pouch)
- Posterior surface of vagina
- Sacrum and Coccyx
- Sacral plexus and Sympathetic chain
Arterial supply of the rectum comes from three different arteries: Superior, Middle, and Inferior rectal arteries.
- Superior rectal artery: It is the terminal continuation of the inferior mesenteric artery and forms the major blood supply to the mucosa of rectum.
- Middle rectal artery: It is a branch of the internal iliac artery and forms the major blood supply to the muscular coat.
- Inferior rectal artery: It is a branch of the internal pudendal artery. It anastomoses with the middle rectal artery at the anorectal junction.
The veins of the rectum correspond to the arteries. The superior rectal vein drains into the inferior mesenteric vein, which is a tributary of the portal venous system. The middle rectal vein drains into the internal iliac vein. And the inferior rectal vein drains into the pudendal vein.
The union between the superior, middle and inferior rectal veins is an important portal-systemic venous anastomosis.
The lymph vessels originating in the rectum drain into the pararectal nodes and eventually into the inferior mesenteric nodes. From the lower part of the rectum, the lymphatics follow the middle rectal artery to the internal iliac nodes.
Inferior hypogastric plexus supplies both the sympathetic and parasympathetic fibers to the rectum. Rectum is sensitive only to stretch.