Defecation Flashcards
Where is the primitive gut tube derived from
Dorsal part of the yolk sac
What ‘derm’ is the gut tube derived from
Endoderm
Describe how the folding occurs to form the gut
- Somatic mesoderm applied to body wall to give rise to parietal peritoneum
- Visceral mesoderm wraps around gut tube to form mesenteries that suspend gut tube within cavity
In which 2 directions does embryological folding occur to make the gut tube
Cranio-caudal
Lateral
What pockets are formed from the folding of the gut tube
Anterior intestinal portal
Posterior intestinal portal
Name the 3 sections of the primitive gut and their blood supply
Foregut= coeliac artery Midgut= superior mesenteric artery Hindgut= inferior mesenteric artery
What is an imperforate anus
Caused by failure of rupture of anal membrane- inperfect development
What is rectoanal atresia
Failure of recanalisation or defective blood supply to developing part
What is persistent cloaca
Complete failure of development of urorectal septum
What are the symptoms of Hirschsprungs disease
Failing to pass meconium withiin 48 hours Swollen belly Vomiting bile (green)
What is Hirschsprungs disease
Congenital megacolon due to a lack of enteric neurons in the distal portion of the gut. Ganglia derived from neural crest that migrate from the neural folds to the wall of bowel
Name 2 things that can cause water absorption in the small intestine
Passively due to result of osmotic gradient produced by active transport
Aldosterone
What % of the remaining volume of water is absorbed by the large intestine
90
Define diarrhoea and constipation
Diarrhoea–> excessive water secretion in the faeces
Constipation–> decreased water secretion in the faeces
What maintains normal faecal continence (3)
- Anal canal
- Pelvic floor musculature
- Rectum
What is the role of the rectum
Stores or expels stool
What is required by the rectum to either store or expel a stool
Cortical sensory awareness in conjunction with intramural and spinal reflexes
What is the role of anal canal
Maintains faecal continence and controls defecation
Describe the muscle found in the internal anal sphincter
Thickened muscle which is a downward continuation of inner circular muscle coat of rectum
What does the internal anal sphincter surround
The entire anal canal
Is the external or internal anal sphincter voluntary
External
Name the pelvic floor musclesw
Pubococcygeus
Puborectalis
Iliococcygeus
Ishiococcygeus
Describe the structure of the puborectalis
- Passes behind the back of pubic symphysis to form a U shaped loop
What is the function of the puborectalis
Supports EAS assists in creating anorectal angle
Which muscle is the largest of the pelvic floor
Levator ani
What do nerves S2, 3 and 4 effect
Rectum, anus, bladder and urethra
What is the parasympathetic supply that aids continence
S2-S4 (pudendal nerve)
Which branch of the pudendal nerve suplies the external anal sphincter
Inferior rectal branch
What terminal branches does inferior rectal branch into? What do these supply
Perineal nerve
Dorsal nerve of the penis
Dorsal nerve of clitoris
What innervates the internal anal sphincter
Enteric nervous system
What is the sympathetic and parasympathetic innervation of internal anal sphincter? What is the effect
- Symp= L1 and L2 via hypogastric nerve. Excitatory
- Para= S2-S4. Pelvic nerve. Inhibitory
What does continence depend upon
- Awareness of rectal filling
- Sensation of impending defecation
What does continence require
- Contraction of puborectalis
- Maintenance of ano-rectal angle
- Normal rectal sensation
- Sphincter contraction
What does the tonic
contraction of the puborectalis muscle create?
Flap valve
What is the role of the flap valve
Maintains the angle so the rectal wall is pushed downwards when intra-abdominal pressure is increased
Define reservoir continence
Ability of rectum to retain stool
What anatomical adaptations provide a mechanical barrier
Lateral angulations in sigmoid colon
Valves of Houston
Define rectal compliance
Ability of rectum to adapt to imposed stretch
What is the advantage of rectal compliance
Allows rectal contents to be accomodated for and for defecation to be delayed
Describe what is meant by the rectoanal inhibitory reflex
- Extent of relaxation depends on degree of distention
- Large volume rectal distension the IAS relaxation can be prolonged
- Slight distention only particular relaxation occur
What makes defecation possible
- Initiation of reflex
- Defecation reflex (opening of the anus)
- Closure reflex (closure of anus)
What muscles relax during defecation
EAS and puborectalis
How does anorectal angle change during defecation
Becomes broader
What is meant by Valsalva manoeuvre
Holding breath, forcibly, closed glottis
What steps occur in the closure reflex
- Closing reflex of EAS stimulation by traction releases
- Receptor adaptation in the ampulla recti removes inhibitory drive to IAS
- Voluntary contraction of EAS closes anus off
How does IAS behave during defecation
Reflex relaxation in response to rectal distention
How does reflex relaxation of the IAS help distinguish flatus from faeces
The relaxation in response to rectal distention allows for sampling of rectal contents
What are the definitions/ criteria for constipation
- Infrequent stool (<3 a week) or
- Passage of hard stools >25% of the time or
- Sensation of incomplete evacuation >25% of the time
What is meant by neural transit constipation
- Normal transit but patient feels constipated
What is meant by slow transit constipation
Infrequency and slow movement of stool
Bloating, abdominal pain and infrequent urge to defecate
What usually causes disordered defecation
Dysfunction of pelvic floor/ anal sphincters
Structural abnormalities such as rectocoele (bulging of rectum to posterior wall of vagina)
What is passive incontience usually caused to
Structural/ functional lesion of the internal anal sphincter
What is urge incontinence usually caused by
Structural/ functional lesion of external anal sphincter
What can be structurally wrong with the anus
Obstetric sphincter tear
Latrogenic sphincter tear
Radiation damage
Congenital malformations
What can be functionally wrong with the anus
Pudendal neuropathy
What are hypersensitivity of rectal sensation and hyposensitivity of rectal sensation related to
Hyper= faecal incontinence Hypo= constipation
Define hypersensitive rectal sensation
Reduced sensory threshold to volumetric rectal distension
What can cause hypersensitive rectal sensation
Proctitis, long term consequences of pelvic radiotherapy, bowel disorder, IBS
Define hyposensitive rectal sensation
Increased sensory threshold to volumetric rectal distention
Summarise the process of normal defecation
- Sensory perception of stool
- Rectal distension
- Contract diaphragm, abdomen and rectal muscles
- Relax EAS
- Relax puborectalis
Summarise the process of incontinence
- Low resting/ low squeeze sphincter processes
- Weakness of puborectalis
- Neuropathy
- Altered rectal or anal sensation
- Diarrheal conditions
- Diminished rectal capacity
