Rectum and Anal Canal

A. Rectum

Is the part of the large intestine that extends from the sigmoid colon to the anal canal and follows the curvature of the sacrum and coccyx.
Has a lower dilated part called the ampulla , which lies immediately above the pelvic diaphragm and stores the feces.
Has a peritoneal covering on its anterior, right, and left sides for the proximal third; only on its front for the middle third; and no covering for the distal third.
Has a mucous membrane and a circular muscle layer that forms three permanent transverse folds (Houston's valves) , which appear to support the fecal mass.
Receives blood from the superior, middle, and inferior rectal arteries and the middle sacral artery. (The superior rectal artery pierces the muscular wall and courses in the submucosal layer and anastomoses with branches of the inferior rectal artery. The middle rectal artery supplies the posterior part of the rectum.)
Has venous blood that returns to the portal venous system via the superior rectal vein and to the caval (systemic) system via the middle and inferior rectal veins. (The middle rectal vein drains primarily the muscular layer of the lower part of the rectum and upper part of the anal canal.)
Receives parasympathetic nerve fibers by way of the pelvic splanchnic nerve.

B. Anal canal

Lies below the pelvic diaphragm and ends at the anus.
Is divided into an upper two thirds (visceral portion) , which belongs to the intestine, and a lower one third (somatic portion) , which belongs to the perineum with respect to mucosa, blood supply, and nerve supply.
Has anal columns , which are 5 to 10 longitudinal folds of mucosa in its upper half (each column contains a small artery and a small vein).
Has anal valves , which are crescent-shaped mucosal folds that connect the lower ends of the anal columns.
Has anal sinuses , which are a series of pouch-like recesses at the lower end of the anal column in which the anal glands open.
The internal anal sphincter (a thickening of the circular smooth muscle in the lower part of the rectum) is separated from the external anal sphincter (skeletal muscle that has three parts: subcutaneous, superficial, and deep) by the intermuscular (intersphincteric) groove called Hilton's white line.
Has a point of demarcation between visceral and somatic portions called the pectinate (dentate) line , which is a serrated line following the anal valves and crossing the bases of the anal columns.

The epithelium is columnar or cuboidal above the pectinate line and stratified squamous below it.
Venous drainage above the pectinate line goes into the portal venous system mainly via the superior rectal vein; below the pectinate line, it goes into the caval system via the middle and inferior rectal veins.
The lymphatic vessels drain into the internal iliac nodes above the line and into the superficial inguinal nodes below it.
The sensory innervation above the line is through fibers from the pelvic plexus and thus is of the visceral type; the sensory innervation below it is by somatic nerve fibers of the pudendal nerve (which are very sensitive).
Internal hemorrhoids occur above the pectinate line, and external hemorrhoids occur below it.

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C. Defecation

Is initiated by distention of the rectum , which has filled from the sigmoid colon, and afferent impulses transmitted to the spinal cord by the pelvic splanchnic nerve. The pelvic splanchnic nerve increases peristalsis (contracts smooth muscles in the rectum), whereas the sympathetic nerve causes a decrease in peristalsis, maintains tone in the internal sphincter, and contains vasomotor and sensory (pain) fibers.
Involves the following:

The intra-abdominal pressure is increased by holding the breath and contracting the diaphragm, the abdominal muscles, and the levator ani, thus facilitating the expulsion of feces.
The puborectalis relaxes, which decreases the angle between the ampulla of the rectum and the upper portion of the anal canal, thus aiding defecation.
The smooth muscle in the wall of the rectum contracts, the internal anal sphincter relaxes, and the external anal sphincter relaxes to pass the feces.
After evacuation, the contraction of the puborectalis and the anal sphincters closes the anal canal.

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