Water balance Flashcards
What is the importance of maintaining water balance?
the kidneys maintain water balance in order to maintain ECF and plasma osmolality constant - plasma osmolarity is determined by the NaCl content relative to the water content of the blood - failure to maintain water balance results in changes in plasma and ECF osmolarity
What mainly determines plasma osmolality?
the NaCl content relative to the water content of the plasma
What effect on osmolarity does increasing the water content have?
increasing water decreases the osmolarity
Where is the majority of our body water lost?
1500 mls are lost through urine a day
does the body have water content receptors?
no, but it does have osmoreceptors which measure the osmolarity of the plasma and adjusts accordingly
where are osmoreceptors situated?
in the hypothalamus
Osmoreceptors are activated when plasma osmolarity reaches the threshold value of what?
280 mosmol/l
What does the activation of osmoreceptors lead to?
their activation causes release of anti-diuretic hormone from nearby neurons of the supra-optic nuclei. - activation also gives rise to the sensation of thirst
when osmolarity falls below 280 mosmol/l, ADH levels become minimal or zero
What does ADH do?
works mainly in the collecting duct - binds to the V2 receptor on the basolateral side to increase cAMP activity which activates protein kinaseA. Overall this leads eventually to the movements of pores to the membrane of the cell which provide channels for water (AQP2)
what happens when there is zero water intake in the body?
osmoreceptors detect increased osmolarity
osmoreceptors indicate increased ADH secretion
ADH causes increased water permeability in collecting duct
H2O reabsorption is increased
When water intake is zero, does urine output become zero?
no. When water intake is zero, the water output will decrease to approx 700ml/day- this is the minimum urinary output. The kidney must continue to filter blood, so urine output cannot reduce to zero.
what happens when you drink lots of water?
change in plasma and ECF osmolarity (lowers osmolarity)
osmoreceptors detect lowered osmolarity
osmoreceptors decrease ADH secretion
collecting ducts decrease water permeability - little or no water reabsorbed in the collecting duct and therefore there is a large volume of dilute urine
what is the determinants of urine osmolarity?
water.
what is dilutional hyponatraemia?
when there is too much fluid intake and the serum becomes too diluted leading to cellular disfunction
in water retention or water depletion, which cells are effected?
All cells - water enters or leaves all cells -
What symptoms are associated with disturbances of water balance?
mainly neurological symptoms resulting from water movement from brain cells
- nausea
- headache
- confusion
- fits/coma
what can cause over hydration?
- high water intake during acute renal failure - when GFR decreases to very low value, kidneys may be unable to excrete the water ingested therefore, total body ater rises
- syndrom of inappropriate ADH secretion (schwartz-Bartter syndrome)
how do we characterize overhydration in the clinic ?
we characterize it by plasma ADH levels higher than expected for plasma osmolarity
What could syndrome of inappropraite ADH be due to?
- pulmonary disease and lung tumors
- brain tumours and infections
what does syndrome of inappropriate ADH do to water balance?
decreased plasma osmolarity
increased urine osmolarity
What is the treatment for Syndrome of inaproppriate ADH?
- fluid restriction
- 3% hypertonic saline infusion via IV
- ADH inhibitor
What is demeclocycline? how does it work?
demeclocycline inhibits ADH function by inhibiting adenylyl cyclase after V2 binding
What can cause under-hydration?
- diabetes insipidus - inadequate release of ADH from posterior pituitary
- nephrogenic diabetes insipidus - where collecting ducts do not respond normally to ADH and become insensitive to ADH - certain drugs may induce this
Explain the treatment of diuresis with thiazide diuretics (hydrochlorothiazide)
the diuretic hydrochlorothiazide can be used to create mild hypovolemia which encourages salt and water uptake in proximal tubule and thus improve nephrogenic diabetes insipidus. It seems paradoxical to treat an extreme diuresis with a diuretic, but the thiazide diuretics will decrease distal convoluted tubule reabsorption of sodium and water, therby causing diuresis. This decreases plasma volume, thus lowering the GFR and enhancing the absorption of sodium and water in the proximal nephron - less fluid reaches the distal nephron, so overall fluid concservation is obtained
How do we determine if diabetes insipidous is central or nephrogenic?
carry out the water deprivation test on patient
- patient should be well and initially well hydrated
- patient then deprived of water for at least 8 ours
- measure urine osmolarity at intervals
- administer ADH at the end of the deprivation period
- measure urine osmolarity during next 3-4 hours - if it improves, then it is central DI… if there is no change it is nephrogenic
Na+ alone contributes to what percent of plasma osmolarity?
50%
