Rectum and anal canal Flashcards
Rectum location and structure
-The rectum is the terminal part of the large intestine
-Proximally it is continuous with the sigmoid colon at the level of S3 vertebra (rectosigmoid junction)
2 flexures:
-Sacral flexure - as it follows curve of the sacrum and coccyx
-Anorectal (perineal) flexure - as it perforates pelvic floor with sharp angle maintained by puborectalis
Internal aspects of anorectal canal
-3 transverse rectal folds can be observed
-Dilated part of the rectum is known as the ampulla (between the middle transverse rectal fold and the anorectal line) receives and holds faecal material
-Anal columns are mucosal folds produced by the underlying arterial plexus and protrude into the wall of the anal canal
-Distal ends of anal columns are connected by anal valves
-All anal valves together form the dentate (pectinate) line (black dotted line in diagram)
Epithelial regions of the anal canal
The anal canal has 3 portions based on epithelial lining:
-The colorectal zone contains simple columnar epithelium, same as the rectum and the rest of the GI tract
-Transitional zone represents a transition between the simple columnar epithelium to stratified squamous epithelium
-Squamous zone is lined with stratified non-keratinised squamous epithelium
-Zone D is below the anocutaneous line and is the perianal skin that is lined by stratified keratinised squamous epithelium and continues with the skin
-This knowledge allows differentiation between rectal and anal carcinoma
Blood supply of rectum
The rectum and the anal canal are supplied by three major arteries:
-Superior rectal artery - the continuation of the inferior mesenteric artery
-Middle rectal artery – a branch of the internal iliac artery
-Inferior rectal artery – a branch of the internal pudendal arteries
Venous drainage of rectum
-Blood from the rectum drains through the superior, middle and inferior rectal veins
-Portocaval anastomoses occur between the portal (superior rectal) and systemic veins (middle & inferior rectal) in the wall of the anal canal
Pectinate line
-The pectinate line separates the superior part of the anal canal (visceral; derived from endoderm) from the inferior part (somatic; derived from ectoderm)
-It reflects the differences in histology, arterial supply, innervation, venous and lymphatic drainage
Differences above and below pectinate line
Haemorrhoids
-Dilation of the internal rectal venous plexus causes protrusion into the anal canal and internal haemorrhoids
-They can also be caused by portal hypertension
-External haemorrhoids are thromboses in the veins of the external rectal venous plexus
-Because internal haemorrhoids usually arise from above the pectinate line, they are not painful
-External haemorrhoids can be painful because they are supplied by somatic sensory fibres
Anal sphincters
-Faecal continence and defecation require the lower rectum, external and internal anal sphincters to work together
-Distention and stretching of the rectum (usually due to accumulating faecal material) triggers a reflexive action and relaxes the internal anal sphincter by parasympathetic innervation
-Defecation may occur by conscious contraction of the external anal sphincter
-The puborectalis also maintains faecal continence
Faecal incontinence
-Can arise from damage to the pelvic floor or the sphincters directly
-Atrophy of the sphincter may occur secondary to pudendal nerve damage during delivery
-Pudendal nerve supplies external anal sphincter
Anorectal carcinoma
-Differences in histology and lymphatic drainage above and below the pectinate line affects the type of tumour that occurs and the direction in which they metastasise
-Superior to the pectinate line, the lymphatic vessels drain into the internal iliac lymph nodes
-Inferior to the pectinate line, the lymphatic vessels drain into the superficial inguinal lymph nodes
