Rectum - Anatomy & Physiology

Introduction

Exists dorsal to the reproductive organs, bladder and urethra.
The cranial portion of the rectum is attached to the dorsal body wall by a short mesorectum which is a continuation of the mesocolon.
The mesorectum is reflected to continue with the parietal peritoneum of the pelvic cavity and to cover the urogenital organs ventrally.
- This forms the rectogenital pouch.
- Therefore the most distal part of the rectum is retroperitoneal.
  - This distal, retroperitoneal part is directly attached to the vagina in the female and to the urethra in the male.
  - The retroperitoneal space is filled with soft tissue rich in fat.
The anal canal joins the bowel to the exterior.
- This is a short passage derived from the proctodeum (formed by invagination of the surface ectoderm).
- Before joining the anal canal, the rectum becomes dilated to form the rectal ampulla.
At the rectoanal junction, the lumen is constricted by longitudinal folds in the mucosa.
- These are normally pressed together to occlude the lumen.
There are two anal sphincters:
- Internal anal sphincter, formed by thickening of the circular smooth muscle of the gut and under autonomic control.
- External anal sphincter, formed from striated skeletal muscle and under voluntary control.