Water balance

Question 1

What is the importance of maintaining water balance?

Answer 1

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

Question 2

What mainly determines plasma osmolality?

Answer 2

the NaCl content relative to the water content of the plasma

Question 3

What effect on osmolarity does increasing the water content have?

Answer 3

increasing water decreases the osmolarity

Question 4

Where is the majority of our body water lost?

Answer 4

1500 mls are lost through urine a day

Question 5

does the body have water content receptors?

Answer 5

no, but it does have osmoreceptors which measure the osmolarity of the plasma and adjusts accordingly

Question 6

where are osmoreceptors situated?

Answer 6

in the hypothalamus

Question 7

Osmoreceptors are activated when plasma osmolarity reaches the threshold value of what?

Answer 7

280 mosmol/l

Question 8

What does the activation of osmoreceptors lead to?

Answer 8

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

Question 9

What does ADH do?

Answer 9

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)

Question 10

what happens when there is zero water intake in the body?

Answer 10

osmoreceptors detect increased osmolarity

osmoreceptors indicate increased ADH secretion

ADH causes increased water permeability in collecting duct

H2O reabsorption is increased

Question 11

When water intake is zero, does urine output become zero?

Answer 11

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.

Question 12

what happens when you drink lots of water?

Answer 12

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

Question 13

what is the determinants of urine osmolarity?

Answer 13

water.

Question 14

what is dilutional hyponatraemia?

Answer 14

when there is too much fluid intake and the serum becomes too diluted leading to cellular disfunction

Question 15

in water retention or water depletion, which cells are effected?

Answer 15

All cells - water enters or leaves all cells -

Question 16

What symptoms are associated with disturbances of water balance?

Answer 16

mainly neurological symptoms resulting from water movement from brain cells

• nausea
• headache
• confusion
• fits/coma

Question 17

what can cause over hydration?

Answer 17

1. 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
2. syndrom of inappropriate ADH secretion (schwartz-Bartter syndrome)

Question 18

how do we characterize overhydration in the clinic ?

Answer 18

we characterize it by plasma ADH levels higher than expected for plasma osmolarity

Question 19

What could syndrome of inappropraite ADH be due to?

Answer 19

• pulmonary disease and lung tumors
• brain tumours and infections

Question 20

what does syndrome of inappropriate ADH do to water balance?

Answer 20

decreased plasma osmolarity

increased urine osmolarity

Question 21

What is the treatment for Syndrome of inaproppriate ADH?

Answer 21

• fluid restriction
• 3% hypertonic saline infusion via IV
• ADH inhibitor

Question 22

What is demeclocycline? how does it work?

Answer 22

demeclocycline inhibits ADH function by inhibiting adenylyl cyclase after V2 binding

Question 23

What can cause under-hydration?

Answer 23

• 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

Question 24

Explain the treatment of diuresis with thiazide diuretics (hydrochlorothiazide)

Answer 24

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

Question 26

How do we determine if diabetes insipidous is central or nephrogenic?

Answer 26

carry out the water deprivation test on patient

1. patient should be well and initially well hydrated
2. patient then deprived of water for at least 8 ours
3. measure urine osmolarity at intervals
4. administer ADH at the end of the deprivation period
5. measure urine osmolarity during next 3-4 hours - if it improves, then it is central DI… if there is no change it is nephrogenic

Question 27

Na+ alone contributes to what percent of plasma osmolarity?

Answer 27

50%