20: Rectum


The rectum is a hollow tube that is continuous with the sigmoid colon proximally and the anal canal distally. The junction with the sigmoid colon is about 15 cm from the external anal margin and is characterised by circumferential muscle instead of the three taeniae coli. The rectum has three prominent lateral curves (right/left/right from superior to inferior). The internal mucosa is pink to tan, and characterised by three folds which assist in anal continence.
The peritoneum is an important landmark. The upper third of the rectum is covered by peritoneum on its anterior and lateral aspects. The middle third is related to the peritoneum on its anterior aspect only. The lower third is completely retroperitoneal.
The mesorectum is an important surgical and radiological structure. It contains the superior rectal artery and vein, and numerous lymphatics as well as rectal branches of the inferior hypogastric plexus. It is bounded by the mesorectal fascia, a distinct layer which separates the rectal fat from other pelvic fat.


In men, the prostate and seminal glands lie anteriorly, separated by the prostatic fascia. Above the peritoneal reflection (the rectovesical pouch), small bowel may intervene between the rectum and the prostate.
In women, the uterus including the cervix are usually separated from the rectum by the rectouterine pouch or Pouch of Douglas.
The rectum is related laterally with the levator ani muscles and the ischoanal fossa.
Posteriorly, the rectum is in close proximity to the sacrum and the coccyx, the piriformis muscle and the nerves of the lumbosacral plexus.
The anal canal and anococcygeal ligament are inferior.
The sigmoid colon lies anterior and superior, the proximal continuation of the rectum.


Microscopic Structure

The rectum is lined with columnar epithelium, with deep crypts that produce mucous. The muscular coat is arranged in three layers. The rectum rests in the loose connective tissue of the mesorectum.

Neurovascular Supply

Arterial Supply

The arterial supply of the rectum is complex. The superior part receives the majority of blood from the superior rectal artery, a terminal branch of the inferior mesenteric artery which also supplies the sigmoid and descending colon. The middle part receives supply from the middle rectal artery, a branch of the internal iliac artery. The inferior part receives some supply from the inferior rectal artery which is a branch of the internal pudendal artery, itself a terminal branch of the internal iliac artery[1].

Venous Drainage

Venous drainage of the rectum is likewise complex! Blood collects into an internal (between epithelium and muscularis propria) and external venous plexus (outside muscularis propria). The internal plexus mostly drains to the inferior rectal vein and thereby to the portal vein. The external plexus drains to the internal pudendal vein or internal iliac vein and thereby to the inferior vena cava.


Lymphatics from the rectum drain to mesorectal nodes. From here, they may pass superiorly to nodes along the inferior mesenteric veins or to the internal iliac or presacral nodes.


The rectum receives autonomic input from sympathetic (arising from lumbar plexus) and parasympathetic nerves (arising from the S2-S4).


There is significant controversy around rectal contouring for radiation oncology[2]. This stems in part from different contouring techniques (contouring the whole rectum versus the rectal wall) and inclusion of different structures such as the anal canal or sigmoid colon as 'rectum'. There are often different definitions of the rectum for prostate, bladder, gynaecological and primary rectal cancer treatments. Gay et all [2] present an interesting consensus guideline from the RTOG regarding rectal contouring (and other pelvic structures).
The RTOG recommends:

  • For prostate cancer, contouring the rectum from the ischial tuberosity inferiorly to the rectosigmoid junction superiorly. This can be identified from where the bowel curves anteriorly at about the level of the sacroiliac joints.
  • For gynaecological cancers, including the anal canal inferiorly to the anal verge as defined by a marker
  • For rectal and anal cancers, contouring the anal canal, rectum and sigmoid colon until the mesorectum disappears.
  • For the mesorectum, contouring from the point where the mesorectal fat disappears inferiorly to the rectosigmoid junction.

Please see the detailed article by Gay et al from 2012.


1. Standring, S. (2008). Gray's Anatomy. Churchill Livingstone.
2. D, H. A. G. M., D, H. J. B. M., D, E. O. C. M., Sc, W. R. B. D., Naqa Ph D, El, I., A, R. A.-L. B., et al. (2012). Pelvic Normal Tissue Contouring Guidelines for Radiation Therapy: A Radiation Therapy Oncology Group Consensus Panel Atlas. Radiation Oncology Biology, 83(3), e353–e362. doi:10.1016/j.ijrobp.2012.01.023