What is the anal canal and where is it located?

The anal canal is the terminal 3.8 cm portion of the large intestine, located below the pelvic diaphragm in the anal triangle. It extends from the anorectal junction to the anal orifice, about 4 cm below the coccyx, and is directed downward and backward.

What are the key differences between the upper and lower parts of the anal canal?

The pectinate line divides the anal canal into upper and lower parts with distinct features. The upper part is derived from endoderm, has columnar epithelium, autonomic innervation (insensitive), and drains to the portal venous system. The lower part is derived from ectoderm, has stratified squamous epithelium, somatic innervation (pain-sensitive), and drains to the systemic venous system.

What are the anal sphincters and how do they function?

The anal canal has two main sphincters. The internal anal sphincter is involuntary, made of thickened rectal muscle, and extends to Hilton’s line. The external anal sphincter is voluntary, with deep, superficial, and subcutaneous parts, supplied by somatic nerves for conscious control. Together, they help maintain continence.

What is the clinical significance of the pectinate line?

The pectinate line is crucial in clinical conditions because it marks the transition between different embryological origins, innervation, and vascular supplies. For example, internal hemorrhoids (above the line) are usually painless and bleed, while external hemorrhoids (below the line) are painful but do not bleed.

How is the anal canal supplied with blood and nerves?

Blood and nerve supply differ above and below the pectinate line. Arterial supply includes the superior rectal artery (upper) and inferior rectal artery (lower). Venous drainage involves the superior rectal vein to the portal system and inferior rectal vein to the systemic system. Nerves are autonomic (insensitive) for the upper part and somatic (sensitive, via pudendal nerve) for the lower part.

What are common clinical conditions associated with the anal canal?

Common conditions include hemorrhoids (varices of veins—internal above pectinate line, painless bleeding, and external below, painful), anal fissure (a tear in the anal valve, often in the posterior midline, causing pain due to somatic innervation), and fistula (an abnormal tract from an abscess, typically low-level through the superficial sphincter).

What surgical spaces are related to the anal canal?

Key surgical spaces include the ischiorectal fossae (fat-filled spaces lateral to the canal that allow for expansion), perianal space (below Hilton’s line, containing veins, nerves, and subcutaneous sphincter), and submucous space (above Hilton’s line, containing the internal venous plexus).

Anal Canal – Pectinate Line, Sphincters & Hemorrhoids

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