Anus and rectum anatomy

Question 1

What is the rectum and its function?

Answer 1

most distal part of large bowel
faecal storage

Question 2

What is the anus and its function?

Answer 2

distensible short section terminating in a valved opening
faecal continence

Question 3

Describe the borders of the rectum

Answer 3

pelvic inlet -> anal canal
colorectal junction -> anorectal junction

Question 4

Describe the anatomy of the rectum

Answer 4

Cranial rectum covered by visceral peritoneum = serosa
Caudal rectum surrounded by connective tissues (adventitia) = retroperitoneal

Question 5

Describe the peritoneal pouches of the rectum

Answer 5

Points where the peritoneum ‘turns around’ in pelvic canal

Question 6

Label the male peritoneal pouches

Answer 6

Question 7

Label the peritoneal pouches in the female

Answer 7

Question 8

Describe the mesenteric support of the rectum

Answer 8

Mesorectum = extension of mesocolon

Question 9

Describe the histology of the rectum

Answer 9

Same as large LI
Mucosa:
- no villi
- columnar epithelium
- longer, taller intestinal glands
- more goblet cells
- lymphoid nodules
Stratum longitudinale:
- forms rectococcygeus

Question 10

Describe the blood supply to the rectum

Answer 10

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)

Question 11

What is the pelvic diaphragm?

Answer 11

‘tent’ of muscle supporting the rectum within the pelvic canal
Muscles:
- coccygeus
- levator ani
Sacrotuberous ligament
External + internal anal sphincters

Question 12

Describe the coccygeus and levator ani muscles in the pelvic diaphragm

Answer 12

Innervation: branches of sacral plexus and pudendal nerve
Function:
- compresses rectum during defacation
- pressing tail against anus

Question 13

Describe the anus anatomy

Answer 13

Specialised junction between mucosa and integument (mucocutaneous junction)
Surrounded by smooth and striated muscle sphincters

Question 14

Describe the types of muscles in the anal sphincters

Answer 14

Internal anal sphincter = smooth muscle
- extension of circular muscles of rectum
External = skeletal/striated muscle

Question 15

Label the anal zones

Answer 15

Question 16

Describe the features of the columnar zone of the anus

Answer 16

Longitudinal ridges
folds create anal sinuses
columnar epithelium

Question 17

Describe the features of the intermediate zone of the anus

Answer 17

transition from columnar gut epithelium to stratified skin epithelium
stratified columnar cells

Question 18

Describe the features of the cutaneous zone of the anus

Answer 18

hairless skin
anal sac ducts open in this region
standard stratified squamous epithelium

Question 19

Describe the location and function of anal glands

Answer 19

dogs and cats
just cranial to anocutaneous line
fatty secretion

Question 20

Describe the location of the apocrine and sweat glands in the anus

Answer 20

cats and dogs
around anus in cutraneous zone

Question 21

Describe the location of circumanal glands

Answer 21

Dogs
all around anus in cutaneous zone:
- subcutaneous
- sebaceous

Question 22

Describe the location of anal sacs in dogs and cats

Answer 22

= paranal sinuses
within lamina propria/submucosa
lined by apocrine paranal sinus glands
ducts open into anal canal

Question 23

Describe the normal emptying of the anal sacs

Answer 23

embedded between internal and external anal sphincter muscles
compressed during defaecation -> coats faeces with scent

Question 24

Describe the lining of the anal sacs

Answer 24

cornified, stratified epithelium
coiled, apocrine tubules within walls
secrete bad smelling secretion

Question 25

Describe the clinical relevance of anal sac ducts in dogs

Answer 25

prone to occlusion => sac engorgement with secretory material and dertritus
Cannulation of ducts for giving antibiotics e.g., treating anal sacculitis

Question 26

What is tenesmus?

Answer 26

straining
forced expiration against closed glottis => raises intra-abdominal and intra-thoracic pressure
More force generated with fixed limbs => defaecation stance

Question 27

What is dyschezia?

Answer 27

painful straining

Question 28

Describe the process of defaecation

Answer 28

rectum innervated by autonomic NS which iniates reflex contraction upon distension
Smooth muscle contraction + conscious increase of intra-abdominal pressure => defaecation

Question 29

Describe defaecation behaviour

Answer 29

Large herbivores - go anywhere
small furries - special toilet areas
Carnivores - part of scent/territorial marking

Question 30

Describe the innervation of the internal anal sphincter

Answer 30

Autonomic
Excitatory supply from sympathetics via hypogastric nerves -> constricts
Inhibitory supply from parasympathetics via pelvic nerves -> relaxes

Question 31

What maintains continence at rest?

Answer 31

high tone in internal anal sphincter

Question 32

Describe the innervation of external anal sphincter

Answer 32

somatic (voluntary) by anal branches of pudendal nerves
Low tone normally

Question 33

Describe the defaecation reflex

Answer 33

rectum distends => anorectal reflex => contraction of rectal wall, relaxation of anal sphincters

Question 34

How is defaecation controlled?

Answer 34

conscious control increases tone of external anal sphincter, preventing defaecation

Question 35

What occurs if the urge to defaecation is not acted upon?

Answer 35

some species: reverse peristalsis => faeces returns to colon
If rectal pressure gets too high => anal sphincters relax => defaecation

Question 36

Why does defaecation often occur after eating?

Answer 36

distension of stomach initiates gastrocolic reflex (gastrin release) => emptying of faecal material from colon into rectum => anorectal reflex

Question 37

What is colitis and the main clinical sign?

Answer 37

irritation of rectum
=> repeated attempts to defaecate even though rectum is empty

Question 38

Why do mothers likc the anus of neonates?

Answer 38

encourage defaecation

Question 39

What are the consequences of anal sac impaction/abscesses?

Answer 39

inflammation of anal sac ducts
impaction of anal sac secretion
secondary infection
abscess ruptures to skin surface

Question 40

What is anal furunculosis?

Answer 40

immune mediated fistulae
seen in combination with anal sac abscessation

Question 41

What are perianal gland adenomas?

Answer 41

common anal tumour
testosterone dependent benign masses

Question 42

Describe rectal prolapse

Answer 42

associated with endoparasites/enteritis (young animals) or perineal hernia (middle ages/older animals)
Incomplete prolapse - mucosa only
Complete prolapse - all layers of rectal wall

Question 43

What are perineal hernias?

Answer 43

degeneration of the pelvic diaphragm
separation of anal sphincter and levator ani
or separation of levator ani and coccygeus

Question 44

What is rectovaginal fistula?

Answer 44

communication of rectum with vagina
vulva is common opening to GI and urogenital tract