analcsatorna

Analcsatorna, or the anal canal, is the terminal part of the large intestine. In adults it measures about 3 to 5 cm in length and runs from the anorectal junction, at the level of the levator ani, to the anal verge at the opening of the anus. It sits within the perineum and marks the transition between the rectum and the external anal opening. A key anatomical landmark is the pectinate (dentate) line, which approximately lies a couple of centimeters above the anal verge and divides the canal into regions with different embryologic origins, epithelium, and innervation.

Anatomy and histology

The mucosa of the anal canal changes from simple columnar epithelium above the pectinate line to stratified squamous epithelium below it. The area above the line contains anal crypts and columns, while below it the epithelium becomes non-keratinized stratified squamous. The canal contains internal and external anal sphincters. The internal sphincter is a thickened ring of smooth muscle providing involuntary continence, and the external sphincter is composed of skeletal muscle surrounding the canal, under voluntary control. The anal canal also plays a role in the arrangement of hemorrhoidal cushions, which contribute to continence and spacing.

Blood supply, drainage, and innervation

Arterial supply comes primarily from the superior rectal artery (branch of the inferior mesenteric artery) to the upper canal, with contributions from middle and inferior rectal arteries (branches of the internal iliac and internal pudendal arteries) to the lower portions. Venous drainage mirrors this arrangement: the superior rectal vein drains toward the portal system, while the middle and inferior rectal veins drain toward the systemic circulation via the internal iliac veins. Lymphatics drain to the inferior mesenteric nodes (above the pectinate line) and to the internal iliac nodes (below it). Innervation varies by region: autonomic input to the internal sphincter above the pectinate line and somatic innervation (notably the pudendal nerve) to the external sphincter and surrounding skin.

Function and clinical relevance

The anal canal regulates fecal continence and defecation, aided by the sphincters and hemorrhoidal cushions. Common clinical issues include hemorrhoids, fissures, abscesses, fistulas, and anal cancer (predominantly squamous cell carcinoma near the margin, with other lesions more proximal). Diagnostic procedures include anoscopy, sigmoidoscopy, and endoanal ultrasound for imaging and staging.