Anatomy - Resection rectopexy, laparoscopic

Viewed functionally, by acting as a reservoir for feces the rectum prevents continuous fecal expulsion. The rectum is continuous with the sigmoid colon and its S-shaped a/p and lateral curvature (sacral, anorectal and lateral flexure respectively) resembles that of the sigmoid. Usually, the rectum is divided into three sections:

1. Rectal ampulla (main reservoir, anterior to the sacrum)
2. Anal canal (see below, closure region between the anorectal flexure and cutaneous anal verge)

3. Between 1. and 2. an unnamed section emerging at right angles from 1. and transitioning into 2, also at right angles. Here, the mucous membrane displays characteristic folds (valves of Houston).

The gross relations of the rectum are: Posteriorly sacrum and coccyx, laterally the internal iliac artery and vein and the regional lymph nodes, the sacral plexus, and parts of the autonomic nervous system as well as the ureters and uterine appendages. In women the rectum is related anteriorly to the uterus and vagina, and in men to the bladder and prostate gland/seminal vesicles. It is delimited inferiorly by the pelvic floor.

Three muscles in the wall of the lower rectum acting together close off the rectum from the anal verge:

1. The internal anal sphincter is the continuation of the circular smooth muscle fibers of the rectum, ending in a thickened edge and being innervated by the sympathetic nervous system.

2. The levator ani is a striated muscle innervated by the sacral plexus; it also comprises the puborectalis attached to the pubis. Its fibers arising from the symphysis pubis form a large sling around the anal canal and its anteriorly directed pull creates the anorectal angle.

3. The external anal sphincter is also a striated muscle and attaches to the perineal body (centrum perinei) and coccyx. Its voluntary innervation stems from the pudendal nerve. Its contraction completely closes off the distal end of the anal canal.

The different innervation of the three closing muscles provides additional protection against failure with its subsequent incontinence.

The mucosa of the anal canal contains numerous longitudinal folds (columnae anales) with dense arterial (!) plexus and venous drainage. During sphincteric contraction these plexuses fill up quickly with blood, thereby engorging and apposing the mucosa. Hemorrhoids and venous thrombosis are common vascular complications in this region.

Defecation takes place by relaxation of the anal sphincters and puborectalis (initiated by the striated musculature, emptying of the cavernous bodies) and at the same time by rectal motility and actively increased intraabdominal pressure.