A. Lecture 26 - regulation of water balance Flashcards
what is the value of plasma osmolality?
280-290 mosmol/Kg H2O (± 3)
what is diuresis?
increased production of urine.
what does ADH increase H2O permeability of?
Cortical collecting duct (2/3 reabsorption)
Medullary collecting duct (1/3 reabsorption)
how much hypotonic fluid reaches collecting ducts daily?
23 litres
in the absence of ADH how much dilute urine can be produced?
approx 20 litres.
in the presence of ADH how much concentrated urine can be produced?
300 ml
what is ADH?
Vasopressin
where is ADH produced?
Synthesised in the supraoptic (SON) and paraventricular nuclei (PVN)
where are the SON and the PVN found?
in the hypothalamus.
what is the half life of ADH?
15 minutes
where is ADH degraded?
liver and kidney
what happens to ADH after synthesis?
moves to neurohypophysis
how does ADH travel to neurohypophysis?
in axons of hypothalamophypophyseal tract
what happens to ADH during the movement?
progressively cleaved.
how is ADH stored in nerve terminals?
with neurophysin.
what causes ADH release?
primary stimulus is plasma osmolality.
how is plasma osmolality sensed?
osmoreceptors.
where are osmoreceptors found?
located near SON
what is the activation threshold for osmoreceptors?
280 mosmol/kg H2O
what happens when osmoreceptors are activated?
small amount of tonic ADH release
how sensitive are osmoreceptors?
Very sensitive (small increase osmolality = big increase in plasma ADH)
how can osmoreceptor thresholds be reset?
hypovolaemia
what is another stimulus for ADH release?
haemodynamics - blood pressure and or volume
how is haemodynamics monitored?
via baroreceptors
how sensitive are baroreceptors compared to osmoreceptors?
less sensitive
how does nausea impact plasma ADH?
plasma ADH increases 100 to 1000 fold
what are 3 other stimuli for ADH release?
Hypoglycaemia (only modest changes)
Hypoxia (via carotid chemoreceptors)
Angiotensin (↑ osmotic response)
what receptor does ADH interact with?
V2
How does ADH activate adenylate cyclase?
via Gs protein
what does the activation of adenylate cyclase produce?
cyclic AMP (cAMP)
what does cAMP activate?
protein kinase A (PKA)
what happens to PKA?
PKA phosphorylates and activates non-functional aquaporin-2 water channels
where does active aquaporin-2 channels insert?
apical (luminal) membrane.
what does the insertion of aquaporin-2 do?
causes water permeability to increase. water is reabsorbed and urine is concentrated.
true or false - concentrating ability of kidney is limited?
true
what is the maximum urine osmolality?
1400 mosmol/kg H2O
what does volume of urine excreted depend on?
Level of circulating ADH
Amount of solute to be excreted
how much solute to be excreted?
800 mosmol/kg H2O solute to be excreted
how do you work out the minimum volume of urine which can be excreted?
solute/max urine
this is over 24 hours
what is micturition?
Micturition = urination (voiding of urine)
how is urine conveyed?
Urine is conveyed from renal pelvis to bladder via ureters
what do ureters do?
Ureters encourage urine flow by peristalsis
how is peristalsis initiated?
by pacemaker cells in the renal pelvis
what happens to the rugae as bladder initially fills?
rugae unfold causing little change in pressure
what happens to pressure as volume of urine increases?
pressure rises
what happens to the stretch receptors in bladder?
activate pelvic nerve afferents
what do the pelvic nerve efferents do?
Pelvic nerve efferents relax internal urethral sphincter
is the micturition reflex under voluntary control?
not in babies (0-2) but in children and adults.
how is the micturition reflex achieved?
Achieved by integration with higher centres (pons) via pudendal nerves
what happens when the pudendal nerves are tonically active?
keep external urethral sphincter closed
what happens with voluntary inhibition of pudendal nerve activity?
relaxes external sphincter.
what does the relaxation of sphincter allowed?
micturition.
