rectum and anal canal Flashcards
What is the rectum?
The terminal part of the large intestine, continuous with the sigmoid colon at the level of S3 vertebra (rectosigmoid junction).
What is the meaning of ‘rectum’ in Latin, and is it accurate?
‘Rectum’ means straight, but the rectum has several flexures
What are the two major flexures of the rectum?
Sacral flexure – follows the curve of the sacrum and coccyx
Anorectal (perineal) flexure – occurs where the rectum perforates the pelvic floor
What maintains the angle of the anorectal flexure?
The puborectalis muscle.
What are the three transverse rectal folds?
Internal rectal folds that help support fecal matter before defecation.
What is the rectal ampulla?
The dilated part of the rectum between the middle transverse rectal fold and the anorectal line, where fecal matter is temporarily stored.
What are anal columns, and what forms them?
Mucosal folds in the anal canal, produced by the underlying arterial plexus.
What connects the distal ends of the anal columns?
Anal valves, which together form the pectinate (dentate) line.
What is the clinical significance of the pectinate (dentate) line?
It is an important landmark, marking the division between visceral (autonomic) and somatic innervation, affecting pain sensation and blood supply.
How is the anal canal epithelium divided?
It is divided into four zones based on epithelial characteristics
what are the different zones in anal canal epithelium
Zone A: Colorectal zone
Zone B: Transitional zone
Zone C: Squamous zone
Zone D: Perianal skin (keratinized squamous)
Which zone in the anal canal is lined with simple columnar epithelium?
Zone A (Colorectal zone) – It has the same epithelium as the rest of the GI tract.
What is the function of the transitional zone (Zone B) in the anal canal?
It serves as a transition between simple columnar and stratified squamous epithelium.
What type of epithelium lines the squamous zone (Zone C) of the anal canal?
Stratified non-keratinized squamous epithelium.
What is Zone D, and how is it different from the other zones?
Zone D is below the anocutaneous line, forming the perianal skin, lined with keratinized squamous epithelium.
Why is the knowledge of anal canal epithelial zones clinically significant?
It helps in differentiating rectal from anal carcinomas, as tumors may arise from different epithelial types.
What are the three major arteries supplying the rectum?
Superior rectal artery – Continuation of the inferior mesenteric artery
Middle rectal artery – Branch of the internal iliac artery
Inferior rectal artery – Branch of the internal pudendal artery
What is a portocaval anastomosis in the rectum?
A venous connection between the portal system (superior rectal vein) and the systemic circulation (middle & inferior rectal veins).
What is the pectinate line?
The pectinate line indicates the junction of the superior (visceral, endoderm-derived) and inferior (somatic, ectoderm-derived) parts of the anal canal.
What does the pectinate line reflect embryologically?
It reflects the transition between two different embryological origins: the endoderm (visceral) and ectoderm (somatic).
What are the main differences reflected by the pectinate line?
Differences in histology, arterial supply, innervation, venous drainage, and lymphatic drainage.
What is the arterial supply above the pectinate line?
Superior rectal artery.
What is the arterial supply below the pectinate line?
Inferior rectal artery.
What is the venous drainage above the pectinate line?
Superior rectal vein, which drains into the portal venous system.
What is the venous drainage below the pectinate line?
Inferior rectal vein, which drains into the caval venous system.
What is the nerve supply above the pectinate line?
Autonomic nerve fibers from the inferior hypogastric plexus, sensitive to stretch.
What is the nerve supply below the pectinate line?
Somatic nerve fibers from the inferior rectal nerves, sensitive to pain, temperature, touch, and pressure.
What is the lymphatic drainage above the pectinate line?
To internal iliac lymph nodes.
What is the lymphatic drainage below the pectinate line?
To superficial inguinal lymph nodes
What causes haemorrhoids?
Dilation of the internal rectal venous plexus causes protrusion into the anal canal, leading to internal haemorrhoids.
What condition can contribute to the development of haemorrhoids?
Portal hypertension can contribute, though it is not necessarily required for haemorrhoid formation.
What are external haemorrhoids?
External haemorrhoids are thromboses in the veins of the external rectal venous plexus.
Why are internal haemorrhoids usually not painful?
They arise above the pectinate line, where the innervation is autonomic and sensitive to stretch, not pain.
Why can external haemorrhoids be painful?
They are supplied by somatic sensory fibers, which are sensitive to pain, temperature, touch, and pressure.
What structures are required for fecal continence and defecation?
The lower rectum, external anal sphincter, and internal anal sphincter work together to maintain continence and facilitate defecation.
What triggers relaxation of the internal anal sphincter?
Distention and stretching of the rectum (due to accumulating fecal material) trigger a reflexive action, relaxing the internal anal sphincter via parasympathetic innervation.
How can defecation be voluntarily deferred?
By conscious contraction of the external anal sphincter.
What is the role of the puborectalis muscle in fecal continence?
It maintains fecal continence by forming a sling around the rectum, helping to maintain the anorectal angle.
What can cause fecal incontinence?
Damage to the pelvic floor muscles, the anal sphincters, or the pudendal nerve.
What nerve supplies the external anal sphincter
The pudendal nerve.
How can damage to the pudendal nerve occur?
can be damaged during childbirth, leading to sphincter atrophy and incontinence.
How does the pectinate line affect anorectal carcinoma?
Differences in histology and lymphatic drainage above and below the pectinate line influence the types of tumors that occur and the direction of metastasis.
Where do lymphatic vessels drain above the pectinate line?
Into the internal iliac lymph nodes.
Where do lymphatic vessels drain below the pectinate line?
Into the superficial inguinal lymph nodes.
