Rectum Anatomy and Physiology
The rectum is the distal portion of the large intestine and lies in the
posterior part of the pelvic cavity. It is continuous with the sigmoid colon at
the rectosigmoid junction in front of the third piece of the sacrum (Fig. 5.7), where there is often an acute angulation. The
rectum curves downwards and forwards, lying first on the anterior surface of
the sacrum and then on the upper surface of the pelvic floor. It deviates to
either side of the midline and these lateral flexures become pronounced when
the organ is distended. The lowest part of the rectum, the ampulla, is its most
dilatable portion. Turning abruptly downwards and backwards, the rectum pierces
the pelvic floor and terminates at the anorectal junction, where it is
continuous with the anal canal (Fig. 5.5). The sharp angulations at the
anorectal and rectosigmoid junctions must be navigated with care during
endoscopy.
Unlike the colon, the rectum is devoid of appendices epiploicae and has
no taeniae, the longitudinal muscle being distributed uniformly around its
circumference. The mucous membrane projects into the rectal lumen as three
shelves, which form the horizontal folds.
The upper two-thirds of the rectum are related to the most inferior
portion of the peritoneal cavity. The upper third of the organ is covered
anteriorly and on both sides by peritoneum, while the middle third, lying
behind the rectouterine or rectovesical pouch, has peritoneum only on its
anterior surface.
Posterior to the upper part of the rectum are the sacrum, coccyx and the
piriformis muscles, while its lower part rests on the levatorani muscles.
Descending behind the organ are the superior rectal vessels (Fig. 5.7), the hypogastric plexus of autonomic nerves
and, on each side of the midline, the sympathetic trunk and sacral plexus.
Lateral to the rectum lie the pelvic plexuses of autonomic nerves and the
ureters.
Anterior to the upper portion of the rectum lie those parts of the
sigmoid colon and ileum that descend into the pelvic cavity. The anterior
relations of the rectal ampulla are of clinical interest because they may be
palpated on rectal examination. In the male, these are the prostate (Fig. 5.5),
the base of the bladder, the seminal vesicles and ampullae of the ductus. These
structures are separated from the rectum by the rectovesical septum. In the
female, the rectal ampulla lies adjacent to the vagina. Therefore, rectal
examination permits palpation of the cervix of the uterus.
Blood supply
The arterial supply to the rectum is derived principally from the
superior rectal artery (Fig. 5.7), the continuation of
the inferior mesenteric artery (p. 182). This supply may be supplemented by
middle rectal branches from the internal iliac arteries. Venous blood drains
into the portal venous system via the superior rectal vein and its
continuation, the inferior mesenteric vein. In addition, middle rectal veins
drain into the internal iliac veins. As the superior and middle rectal veins
interconnect, the wall of the rectum is
a site of portacaval anastomosis (p. 185).
