Anus and rectum anatomy Flashcards
What is the rectum and its function?
most distal part of large bowel
faecal storage
What is the anus and its function?
distensible short section terminating in a valved opening
faecal continence
Describe the borders of the rectum
pelvic inlet -> anal canal
colorectal junction -> anorectal junction
Describe the anatomy of the rectum
Cranial rectum covered by visceral peritoneum = serosa
Caudal rectum surrounded by connective tissues (adventitia) = retroperitoneal
Describe the peritoneal pouches of the rectum
Points where the peritoneum ‘turns around’ in pelvic canal
Label the male peritoneal pouches
Label the peritoneal pouches in the female
Describe the mesenteric support of the rectum
Mesorectum = extension of mesocolon
Describe the histology of the rectum
Same as large LI
Mucosa:
- no villi
- columnar epithelium
- longer, taller intestinal glands
- more goblet cells
- lymphoid nodules
Stratum longitudinale:
- forms rectococcygeus
Describe the blood supply to the rectum
Arteries:
- cranial rectal supply = caudal mesenteric artery
- middle + caudal supply = internal pudendal artery
Venous drainage:
- cranial rectal = hepatic portal vein
- middle + caudal = internal iliac (systemic)
What is the pelvic diaphragm?
‘tent’ of muscle supporting the rectum within the pelvic canal
Muscles:
- coccygeus
- levator ani
Sacrotuberous ligament
External + internal anal sphincters
Describe the coccygeus and levator ani muscles in the pelvic diaphragm
Innervation: branches of sacral plexus and pudendal nerve
Function:
- compresses rectum during defacation
- pressing tail against anus
Describe the anus anatomy
Specialised junction between mucosa and integument (mucocutaneous junction)
Surrounded by smooth and striated muscle sphincters
Describe the types of muscles in the anal sphincters
Internal anal sphincter = smooth muscle
- extension of circular muscles of rectum
External = skeletal/striated muscle
Label the anal zones
Describe the features of the columnar zone of the anus
Longitudinal ridges
folds create anal sinuses
columnar epithelium
Describe the features of the intermediate zone of the anus
transition from columnar gut epithelium to stratified skin epithelium
stratified columnar cells
Describe the features of the cutaneous zone of the anus
hairless skin
anal sac ducts open in this region
standard stratified squamous epithelium
Describe the location and function of anal glands
dogs and cats
just cranial to anocutaneous line
fatty secretion
Describe the location of the apocrine and sweat glands in the anus
cats and dogs
around anus in cutraneous zone
Describe the location of circumanal glands
Dogs
all around anus in cutaneous zone:
- subcutaneous
- sebaceous
Describe the location of anal sacs in dogs and cats
= paranal sinuses
within lamina propria/submucosa
lined by apocrine paranal sinus glands
ducts open into anal canal
Describe the normal emptying of the anal sacs
embedded between internal and external anal sphincter muscles
compressed during defaecation -> coats faeces with scent
Describe the lining of the anal sacs
cornified, stratified epithelium
coiled, apocrine tubules within walls
secrete bad smelling secretion
Describe the clinical relevance of anal sac ducts in dogs
prone to occlusion => sac engorgement with secretory material and dertritus
Cannulation of ducts for giving antibiotics e.g., treating anal sacculitis
What is tenesmus?
straining
forced expiration against closed glottis => raises intra-abdominal and intra-thoracic pressure
More force generated with fixed limbs => defaecation stance
What is dyschezia?
painful straining
Describe the process of defaecation
rectum innervated by autonomic NS which iniates reflex contraction upon distension
Smooth muscle contraction + conscious increase of intra-abdominal pressure => defaecation
Describe defaecation behaviour
Large herbivores - go anywhere
small furries - special toilet areas
Carnivores - part of scent/territorial marking
Describe the innervation of the internal anal sphincter
Autonomic
Excitatory supply from sympathetics via hypogastric nerves -> constricts
Inhibitory supply from parasympathetics via pelvic nerves -> relaxes
What maintains continence at rest?
high tone in internal anal sphincter
Describe the innervation of external anal sphincter
somatic (voluntary) by anal branches of pudendal nerves
Low tone normally
Describe the defaecation reflex
rectum distends => anorectal reflex => contraction of rectal wall, relaxation of anal sphincters
How is defaecation controlled?
conscious control increases tone of external anal sphincter, preventing defaecation
What occurs if the urge to defaecation is not acted upon?
some species: reverse peristalsis => faeces returns to colon
If rectal pressure gets too high => anal sphincters relax => defaecation
Why does defaecation often occur after eating?
distension of stomach initiates gastrocolic reflex (gastrin release) => emptying of faecal material from colon into rectum => anorectal reflex
What is colitis and the main clinical sign?
irritation of rectum
=> repeated attempts to defaecate even though rectum is empty
Why do mothers likc the anus of neonates?
encourage defaecation
What are the consequences of anal sac impaction/abscesses?
inflammation of anal sac ducts
impaction of anal sac secretion
secondary infection
abscess ruptures to skin surface
What is anal furunculosis?
immune mediated fistulae
seen in combination with anal sac abscessation
What are perianal gland adenomas?
common anal tumour
testosterone dependent benign masses
Describe rectal prolapse
associated with endoparasites/enteritis (young animals) or perineal hernia (middle ages/older animals)
Incomplete prolapse - mucosa only
Complete prolapse - all layers of rectal wall
What are perineal hernias?
degeneration of the pelvic diaphragm
separation of anal sphincter and levator ani
or separation of levator ani and coccygeus
What is rectovaginal fistula?
communication of rectum with vagina
vulva is common opening to GI and urogenital tract
