Ano-Rectal Anatomy  

 

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Ano-Rectal Anatomy

Hemorrhoids

Perianal Abscess

Fistula-in-Ano

Anal Fissures

 

 

Ano-Rectal Anatomy

The pelvic diaphragm, pelvic cavity, and ischioanal fossae are complex structures beyond the scope of this tutorial.  In practice, the general surgeon (in the ambulatory setting), will deal with much more superficial anatomy.  Therefore, a more superficial approach to the anatomy of this area will be presented.

Anal  Canal

The anal canal (approximately 2.3-3.5 cm long) is the terminal part of the large intestine, extending from the upper aspect of the pelvic diaphragm to the anus.  Surrounded by internal and external sphincters, it descends posteroinferiorly between the anococcygeal ligament and the perineal body.

The internal anal sphincter is an involuntary sphincter supplied by parasympathetic fibers which pass through the pelvic splanchnic nerves.  This sphincter relaxes in response to pressure (gas or feces) distending the rectal ampulla.

The external anal sphincter is a voluntary sphincter innervated primarily by S4 through the inferior rectal nerve.

Within the anal canal, the uppermost portion of the canal is dominated by a series of mucosal ridges called anal columns.  These columns contain the terminal branches of the superior rectal artery and vein, and it is here (at their superior ends) that the division of the rectum from the anus is defined.

Coronal view of ano-rectal anatomy

The Pectinate Line

The Pectinate Line (also known as the Dentate or Mucocutaneous Line) is a clinically important landmark due to the fact that it is visible and approximates the level of certain anatomic changes that will be discussed below.  It lies at the inferiormost level of the anal columns and indicates the junction of the superior part of the anal canal (derived from the embryonic hindgut) and the inferior part (derived from the embryonic proctodeum).   It is this difference in embryonic origin which gives rise to differing arterial, venous, and nervous connections (above and below this line)

Blood Supply and Nerves

The superior portion of the anal canal (that is, superior to the pectinate line) is supplied by the superior rectal artery.  Below the pectinate line, the inferior rectal arteries supply the inferiormost part of the anal canal.  Between the two, the middle rectal arteries form anastomoses with each to assist in the blood supply.

A plexus of veins branch around the anal canal.  Above the pectinate line, this internal rectal plexus drains into the superior rectal vein.  Below, the plexus drains into the inferior rectal vein.  Similar to the arterial system, a middle rectal vein exists, which mainly drains the muscularis externa, and anastomoses with the superior and inferior veins.

Although not pictured, the lymphatic drainage differs above and below the pectinate line.  Above it, the lymphatics drain into the internal iliac lymph nodes.  Below, the vessels drain into the superficial inguinal lymph nodes.

Lastly, the anal canal also has differing nervous innervations above and below the line.  Above the pectinate line, the nerve supply is visceral, coming from the inferior hypogastric plexus.  As is it visceral, this part of the anal canal is only sensitive to stretch.  Below the pectinate line, the nerve supply is somatic, receiving its supply from the inferior rectal nerves (branches of the pudendal).  As it is somatically innervated, it is sensitive to pain, temperature, and touch.

Arteries Superior, middle, inferior rectal arteries
Veins Superior, middle, inferior rectal veins
Nerves Inferior hypogastric plexus, rectal nerve plexus, inferior rectal nerve