Anal Canal

Structure/Morphology

The anal canal begins at the anorectal junction, at the level of the pelvic floor. It is 2–5 cm long, with the posterior wall being slightly longer than the anterior, and lies approximately 2–3 cm anterior to the distal tip of the coccyx (Standring, 2016). At rest, the anal canal does not retain its circular cross-sectional appearance and is oval or slit-like in an anteroposterior direction.

Internally, the middle third of the anal canal has approximately six to ten vertically orientated mucosal folds, the anal columns. These often contain the terminal branches of the superior anorectal vessels. The distal ends of the columns form small semilunar folds that extend from each to its neighbors. These are called anal valves. The valves form small pockets between anal wall and valve called anal sinuses. The combination of distal anal columns and it's the anal valves/sinuses between them form a circle around the inner wall of the anal canal called the pectinate line.

The pectinate line defines the junction of the upper two thirds and lower third of the anal canal. It’s an important landmark defining the end of the hindgut and the site of transition of vascular supply and epithelial appearance. Specifically, at this point, the columnar epithelium changes to squamous epithelium and is continuous with that of the skin.

The external wall of the anal canal contains the anal sphincter complex that consists of, from inside to out:

- the internal anal sphincter;

- the longitudinal muscle layer (which fans out to form the conjoint longitudinal muscle);

- the external anal sphincter.

The internal anal sphincter is the terminal portion of the inner circular smooth muscle layer of the large intestine. It has a tight spiral arrangement that shortens and widens when relaxed during defecation. The internal sphincter does not extend beyond the superior two thirds of the anal canal.

External to the circular, the longitudinal, smooth muscle layer of the rectum continues as the conjoining longitudinal muscle of the anal canal.

Finally, superficial to this, the external anal sphincter forms the bulk of the complex. It consists of striated muscle, primarily type 1 slow-twitch fibers, and thus is suited to sustained contractions (Schrøder and Reske-Nielsen, 1983). Its upper portion surrounds the internal sphincter.

Anatomical Relations

The proximal end on the anal canal is continuous with the rectum. It projects posteriorly due to the puborectalis muscle pulling the rectum anteriorly, thus forming the puborectal angle. At its distal end, the epithelium becomes continuous with the skin of the perineum.

The middle one third of the anal canal is firmly attached to the perineal body of the perineum. In males, the urethra sits anterior to the anal canal; however, in females, the vagina sits anterior to the anal canal.

Posterolaterally are the ischioanal fossae, which are filled with adipose tissue and permit distension. Posteriorly, the anococcygeal ligament attaches the external anal sphincter to the coccyx in the midline.

Function

The anal canal provides a conduit for feces as it is excreted from the rectum to the anus. It has internal and external sphincters that provide subconscious and conscious control over the process of defecation. Tonic contraction of the internal anal sphincter, as well as contraction of the external anal sphincter, caused by the conscious awareness of fecal matter passing into the anal canal. It prevents defecation until a socially appropriate time (Koeppen and Stanton, 2009).

Arterial Supply

The upper two thirds of the anal canal (above the pectinate line) is vascularized by the termination of the superior anorectal artery, the inferior anorectal artery, and a small contribution from the median sacral artery. The lower third of the anal canal is supplied by the inferior anorectal arteries, a branch arising from the inferior pudendal arteries.

Venous Drainage

An anorectal venous plexus surrounds the anal canal (Standring, 2016). The plexus drains into the systemic circulation via the inferior and middle anorectal vein (through the internal pudendal and internal iliac veins, respectively), and the portal venous circulation via the superior anorectal vein (through the inferior mesenteric vein).

Due to its drainage into both the system and portal venous system, the rectal venous plexus acts as an area of portacaval anastomosis.

Innervation

Parasympathetic innervation comes from pelvic splanchnic nerves (S2-S4) and are distributed via the inferior hypogastric plexus.

Sympathetic innervation is derived from sacral splanchnic nerves and distributed via the inferior hypogastric plexus. Additionally, the superior rectal plexus supplies innervation to the internal anal sphincter.

A branch of the pudendal nerve (S2-S4), the inferior anal nerve, innervates the external anal sphincter. Below the pectinate line it also provides sensory innervation (Standring, 2016).

Lymphatic Drainage

Lymph from the proximal portion of the anal canal drains via pararectal and sacral lymph nodes into the external and internal iliac lymph nodes. The distal anal canal and external anal sphincter drains via the superficial inguinal lymph nodes (Földi et al., 2012).

List of Clinical Correlates

- Continence

References

Földi, M., Földi, E., Strößenreuther, R. and Kubik, S. (2012) Földi's Textbook of Lymphology: for Physicians and Lymphedema Therapists. Elsevier Health Sciences.

Koeppen, B. M. and Stanton, B. A. (2009) Berne & Levy Physiology, Updated Edition E-Book. Elsevier Health Sciences.

Schrøder, H. D. and Reske-Nielsen, E. (1983) 'Fiber types in the striated urethral and anal sphincters', Acta Neuropathol, 60(3-4), pp. 278-82.

Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series: Elsevier Limited.