25 - Defecation and Anal Anatomy

Question 1

Where does GIT arise from

Answer 1

Endoderm

buccopharyngeal membrane → cloacal membrane

Question 2

Foregut

Answer 2

Celiac artery – Pharynx, stomach, proximal duodenum, liver, pancreas

Question 3

Midgut

Answer 3

Superior mesenteric artery – SI, distal duodenum, ascending + transverse colon

Question 4

Hindgut

Answer 4

Inferior mesenteric artery –sigmoid colon, rectum, urethra

Question 5

Imporforate anus

Answer 5

 Failure of rupture of anal membrane

Question 6

Rectoanal atresia

Answer 6

failure of recanalisation or defective blood supply of the developing part

Question 7

Persistent cloaca

Answer 7

complete failure of development of the urorectal septum. F>M - where the urinary bladder, vagina and rectum open in one cavity.

Question 8

Hirschsprungs disease

Answer 8

 Congenital megacolon due to absence of enteric neurones in the bowel

Question 9

Symptoms of hirschsprung

Answer 9

failure to pass meconium, swollen belly, vomiting bile

Question 10

Small intestine fluid and electrolyte abs

Answer 10

o Receives 7-9L fluid – absorption of water occurs passively - osmotic gradient
o Aldosterone stimulates NaCl and water absorption in the ileum

Question 11

Large intestine fluid and electrolyte abs

Answer 11

o Absorbs 90% of remaining volume

o Secretion of water passively – osmotic gradient – Na+/Cl- actively transported across

Question 12

What are the nerves responsible for continence?

Answer 12

S2-S4 parasympathetic supply = pudendal nerve

Question 13

Nerve for EAS

Answer 13

rectal branch of pudendal nerve (anterior primary rami of 2nd, 3rd and 4th sacral spinal nerves)

Question 14

Internal anal sphincter

Answer 14

enteric nervous system (myenteric plexus) + autonomic NS
 Sympathetic - L1, L2 via hypogastric nerves excitatory
 Parasympathetic - S2-S4 pelvic nerves inhibitory

Question 15

Reservoir continence

Answer 15

Ability of the rectum to retain stool

Question 16

Renal compliance

Answer 16

ability of the rectum to adapt to the imposed stretch

Question 17

• Rectoanal inhibitory reflex

Answer 17

Reflex relaxation of the upper IAS –rectal contents are presented to the anal sensory mucosa

Question 18

What allows defecation

Answer 18

o Relaxation of the EAS and puborectalis muscle, to create a broader anorectal angle

Question 19

Constipation

Answer 19

Infrequent stools (<3 a week )/hard stools (>25% of the time)/incomplete evacuation ( >25% of the time)