The Rectum

Original Author: Vicky Theakston
Last Updated: October 28, 2017
Revisions: 27

The rectum is the most distal segment of the large intestine, and has an important role as a temporary store of faeces.

It is continuous proximally with the sigmoid colon, and terminates into the anal canal.

In this article we will discuss the anatomy of the rectum –  its structure, anatomical relationships, and clinical relevance.


Anatomical Structure

The rectum is approximately 15cm long, and begins at the level of the S3 (as a continuation of the sigmoid colon). It is macroscopically distinct from the colon, with an absence of taenia coli, haustra, and omental appendices.

The course of the rectum is marked by two major flexures:

  • Sacral flexure anteroposterior curve with concavity anteriorly (follows the curve of the sacrum and coccyx).
  • Anorectal flexure – anteroposterior curve with convexity anteriorly. This flexure is formed by the tone of the puborectalis muscle, and contributes significantly to faecal continence.

There are additionally three lateral flexures (superior, intermediate and inferior), which are formed by transverse folds of the internal rectum wall.

The final segment of the rectum, the ampulla, relaxes to accumulate and temporarily store faeces until defecation occurs. It is continuous with the anal canal; which passes through the pelvic floor to end as the anus.

Fig 1 - The sacral and anorectal flexures of the rectum.

Fig 1 – The sacral and anorectal flexures of the rectum.

Peritoneal Coverings

In the superior third of the rectum, the anterior surface and lateral sides are covered by peritoneum. The middle third only has an anterior peritoneal covering, and the lower 1/3 has no peritoneum associated with it.

In males, the reflection of peritoneum from the rectum to the posterior bladder wall forms the rectovesical pouch. In females, the peritoneum reflects to the posterior vagina and cervix, forming the rectouterine pouch (pouch of Douglas). See more about the peritoneal cavity here.

Fig 2 - The peritoneal reflections of the rectum in males (A) and females (B).

Fig 2 – The peritoneal reflections of the rectum in males (A) and females (B).


Anatomical Relations

The rectum is located within the pelvic cavity, and is the most posterior of the pelvic viscera. Its anatomical relations are different in men and women:

Anterior

Posterior

Male Female Sacrum and coccyx

Piriformis

Coccygeus

Levator ani

Sacral plexus

Rectovesical pouch

Sigmoid colon

Ileum

Bladder

Prostate

Seminal vesicles

Rectouterine pouch

Sigmoid colon

Ileum

Vagina

Cervix

Fig 1.0 - Sagittal section of the female pelvis, showing the anatomical position of the bladder.

Fig 3 – Sagittal section of the female pelvis, showing the anatomical position of the rectum.


Neurovascular Supply

The rectum receives arterial supply through three main arteries:

Venous drainage is via the corresponding superior, middle and inferior rectal veins. The superior rectal vein empties into the portal venous system, whilst the middle and inferior rectal veins empty into the systemic venous system. Anastamoses between the portal and systemic veins are located in the wall of anal canal, making this a site of portocaval anastomosis.

Note: the rectum is also closely anatomically associated with the rectal venous plexus; however this structure is more functionally related to the anal canal.

Innervation

The rectum receives sensory and autonomic innervation.

Sympathetic nervous supply to the rectum is from the lumbar splanchnic nerves and superior and inferior hypogastric plexuses. Parasympathetic supply is from S2-4 via the pelvic splanchnic nerves and inferior hypogastric plexuses. Visceral afferent (sensory) fibres follow the parasympathetic supply.

Fig 3 - The superior rectal artery, supplying the upper aspect of the rectum.

Fig 4 – The superior rectal artery, supplying the upper aspect of the rectum.


Lymphatic Drainage

Lymphatic drainage of the rectum is via the pararectal lymph nodes, which drain into the inferior mesenteric nodes.

Additionally, the lymph from the lower aspect of the rectum drains directly into the internal iliac lymph nodes.


Clinical Relevance – Digital Rectal Examination

The anterior wall of the rectum has a number of close anatomical structures. These can therefore be palpated digitally via the rectum. Most significant are the prostate and seminal vesicles in males, and cervix in females. Bony structures, such as the sacrum and coccyx, may also be palpated in both sexes.

For a guide on performing a digital rectal examination, visit our sister site TeachMeSurgery.

Fig 5 - Digital rectal examination of the prostate in males.

Fig 5 – Digital rectal examination of the prostate in males.

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Quiz

Question 1 / 10
At which vertebral level does the rectum begin?

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Question 2 / 10
Which flexure of the rectum contributes most significantly to faecal continence?

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Question 3 / 10
Which part of the rectum relaxes to accomodate faeces?

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Question 4 / 10
Which anatomical pouch is formed by a reflection of peritoneum between the rectum and posterior vagina?

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Question 5 / 10
Which structure would be least likely to be damaged in a stab wound to a male rectum?

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Question 6 / 10
Which lymph nodes drain the superior 1/3 rectum?

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Question 7 / 10
What is the lymphatic drainage of the rectum?

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Question 8 / 10
During a digital rectal examination, through which wall of the rectum can the prostate be palpated?

Quiz

Question 9 / 10
Which letter correctly identifies the Rectum?

Quiz

Question 10 / 10
Can you identify the structure labelled A?

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