Rectum: Function, Morphology and More

Definition of the Rectum

The rectum is the terminal portion of the gut tube. The rectum begins in front of the vertebrae as a continuation of the sigmoid colon. It is about 13cm long. 
 

Morphology of the Rectum

It passes downwards, following the curve of the sacrum and coccyx by piercing the pelvic diaphragm and becomes continuous with the anal canal. 
The lower part of the rectum is dilated to form the rectal ampulla. The puborectalis portion of the levator ani muscles form a sling at the junction of the rectum with the anal canal and pulls this part of the bowel forward, producing the anorectal angle.
The rectum is covered by the peritoneum throughout, except at the lower 1/3. The muscular coat of the rectum is arranged as outer longitudinal and inner circular layers of smooth muscles.

Relations

Posteriorly- The rectum is in contact with:

1. Bones – the sacrum and coccyx, 
2. Muscles – piriformis, coccygeus and levator ani, 
3. Nerves – the sacral plexus and sympathetic trunks

Anteriorly the relations vary slightly with sex:

In males –

• The upper 2/3 is related to the sigmoid colon and coils of ileum.
• The lower 1/3 is related to the posterior surface of the urinary bladder, the terminal part of the vas deferens and seminal vesicle on each side and to the prostrate.

In females –

• The relations of the upper 2/3 are the same as in males. 
• The lower 1/3 lies in relation to the posterior part of the vagina.

Function of the Rectum

The rectum’s function is to receive stool from the colon, let you know that there is stool to be evacuated and to hold the stool until evacuation happens.

Blood supply

1. Superior rectal artery (continuation of the inferior mesenteric artery) supplies especially the mucous membrane.
2. Middle rectal artery (small branch of the internal iliac artery). Supplies especially the muscular coat.
3. Inferior rectal artery (branch of internal pudendal artery in the perineum).

Venous Drainage

The veins accompany and correspond to the arteries.  
The superior rectal vein is a tributary of the portal circulation and drains into the inferior mesenteric vein. The middle rectal vein drains into the internal iliac vein while the inferior rectal vein drains into the internal pudendal vein.
NB: The union between the rectal veins form a portal systemic anastomosis.

Lymphatic Drainage

Lymphatics from the rectum drain into the pararectal nodes. Efferents from the pararectal nodes drain into the inferior mesenteric nodes. 
Lymphatics from the lower part of the rectum follow the middle rectal artery to the internal iliac nodes.

Nerve Supply

The rectum is supplied by the sympathetic and parasympathetic nerves from the inferior hypogastric plexuses. The rectum is sensitive only to stretch.

Applied Anatomy:

Cancer of the rectum

This is a common clinical finding and often remains localized to the rectal wall for a considerable time. Through the lymphatic channel however, it can spread into the bowel as the lymphatics accompany the vessels.
Once the malignant tumor has extended beyond the confines of the rectal wall, knowledge of the anatomic relations of the rectum will enable the physician to access the structures and organs likely to be involved.