Anal Canal Flashcards
What is the anal canal surrounded by to help faecal continence?
internal and external anal sphincters
Where is the internal anal sphincter?
Surrounds upper 2/3 of anal canal
What is internal anal sphincter formed from?
Thickening of the involuntary circular smooth muscle in the bowel wall
Where is the external anal sphincter?
surrounds the lower 2/3 of the anal canal (and so overlaps with the internal sphincter)
Which sphincter is voluntary?
External
What does external anal sphincter blend with superiorly?
the puborectalis muscle of the pelvic floor
What forms the anorectal junction / ring?
formed by the fusion of the internal anal sphincter, external anal sphincter and puborectalis muscle (puborectalis forms sling which creates angle)
What divides the anal canal?
The pectinate line
What does the pectinate line mark?
Different embryological origins
Where is above the pectinate line derived from?
Embyronic hindgut (endoderm)
Where is below the pectinate line derived from?
Ectoderm of the proctodeum
Describe the epithelial lining of the superior aspect of the anal canal?
Columnar epithelium (same as the rectum)
Mucosa is organised into longitudinal folds, known as anal columns.
These are joined together at their inferior ends by anal valves.
Above the anal valves are small pouches which are referred to as anal sinuses – these contain glands that secrete mucus.
What are anal columns?
Mucosa in the superior aspect of the anal canal that has been organised into longitudinal folds
What are the anal columns joined by?
Anal valves
What are found above the anal valves?
Anal sinuses
What do anal sinuses secrete?
Mucus
What do the anal valves collectively form?
an irregular circle – known as the pectinate line
Describe the epithelial lining of the inferior aspect of the anal canal
Lined by non-keratinised stratified squamous epithelium (known as the anal pecten). It is a pale and smooth surface, which transitions at the level of the inter-sphinteric groove to true skin (keratinised stratified squamous).
What happens to the epithelial lining at the inter-sphinteric groove?
Transitions to true skin (keratinised stratified squamous)
What is located laterally to the anal canal?
Ischioanal fossae
Describe the artery supply of above the pectinate line
Superior rectal artery (branch of IMA)
Anastomosing branches from the middle rectal artery.
Describe the artery supply of below the pectinate line
Inferior rectal artery (branch of the internal pudendal artery)
Anastomosing branches from the middle rectal artery.
Describe the venous drainage of above the pectinate line
Superior rectal vein, which empties into the inferior mesenteric vein (portal venous system).
What does the superior rectal vein empty into?
Inferior mesenteric vein
Describe the venous drainage of below the pectinate line
Inferior rectal vein, which empties into the internal pudendal vein (systemic venous system).
Describe the nerve supply of above the pectinate line
Visceral innervation via the inferior hypogastric plexus.
Sensitive to stretch.
Describe the nerve supply of below the pectinate line
Somatic innervation via the inferior rectal nerves (branches of the pudendal nerve)
Sensitive to pain, temperature, touch and pressure.
Describe the lymphatics of above the pectinate line
Internal iliac lymph nodes
Describe the lymphatics of below the pectinate line
Superficial inguinal lymph nodes
What are haemorrhoids?
Pathological haemorrhoids are observed in people who suffer from constipation, prolonged straining when defecating, or raised intra-abdominal pressure (e.g pregnancy, ascites).
They can cause bleeding and itchiness, and depending on the severity, can be managed conservatively or surgically.
