Salt & Water balance Flashcards
What are the daily requirements for water, sodium + potassium?
2-3L water (fluid and food)
100-200mmol Na
60-80mmol K
What are the average losses of water as urine, water as stool and water sensible ?
- 5-2L water as urine
- 2L water stool
- 8L water ‘insensible’ loss
What are the ratios of Na and K between the ICF and ECF?
ICF = 28L
- Na: 12mmol/l
- K: 150mm/l
ECF = 14L
- Na: 140mmol/l
- K: 4mmol/l
- glucose: 5mmol/l
- urea: 5mmol/l
What is sodium essential for?
regulating body water - homeostasis more tightly controlled than any other ion
- mostly extracellular and diffuses into ICF down concentration gradient and is actively pumped out via Na/K ATPase
Major determinant of plasma osmolality
Why is the total body sodium so tightly regulated?
maintain water homeostasis - results significantly outside of the ref range (133-146mmol/L) are likely to be primarily a water problem NOT sodium
Regulated by antidiuretic hormone and aldosterone (RAAS)
How is plasma water volume regulated?
controlled by ADH
- increased plasma osmolality sensed by osmoreceptors in the hypothalamus
=> stimulates thirst and drinking
=> stimulates ADH release from posterior pituitary leading to renal water reabsorption
= corrects osmolality
reverse occurs in fluid excess
How is plasma water volume regulated by aldosterone?
fluid depletion leads to reduced ECF and therefore this causes reduced blood pressure (sensed by arterial baroreceptors)
therefore the sympathetic nervous system activated = vasoconstriction
juxtaglomerular apparatus senses the reduced renal arterial perfusion
this all leads to increased renin => increased angiotensin II => increased aldosterone => increased Na reabsorption in the distal nephron = leading to sodium retention and then water retention
What are the causes of hypernatraemia?
sodium retention in excess of water
- low water intake (renal underperfusion and activation of RAAS)
- primary hyperaldosteronism (Conn’s)
- cushing’s disease
water loss in excess of sodium - diabetes insipidus with inadequate drinking, diarrhoea, vomiting, burns, haemorrhage
artefactual - sudden increase in plasma sodium conc - contamination of sample with IV saline
What is diabetes insipidus due to ?
inability of the pituitary to produce ADH (central DI) or of the kidney to respond to ADH (nephrogenic DI)
What are the clinical symptoms of DI?
polyuria and thirst - the thirst compensates for renal water loss
the plasma sodium will be normal if drinking but if fluids are restricted then it leads to hypernateaemia
and water deprivation test is the way to diagnoes DI
(take serial sodium and osmolality measurements)
What happens in hyponatraemia?
water retention in ECF in excess of sodium
depending on cause patient can be hypo=, eu or hypovolaemic therefore it is essential to assess fluid status
What clinical signs and investigations are you looking for when assessing fluid status?
skin turgor, mucous membranes, blood pressure (lying and standing), pulses, presence of pitting oedema
urine output
serum sodium / osmolality
urine osmolality - measured in lab
What are the causes of hyponatraemia?
Water retention in excess of sodium
- total body sodium is normal or high
=> oedematous state - nephrotic syndrome, cardiac/renal/liver failure
=> syndrome of inappropriate ADH
=> excessive drinking (psychogenic polydisplasia)
Sodium loss in excess of water
- total body sodium is low
=> renal - osmotic diuresis (DKA), diuretic stage of renal failure, diuretic use, hypocortisolism
=> non-renal - diarrhoea, vomit, burns, fistula (can also see hypernatraemia with this)
What should you consider with hyponatraemia with postural hypotension and no polyuria?
consider adrenal failure (Addison's) - plasma osmolality = low - urine osmolality = high - urine sodium = high differential diagnosis = SIADH
What happens in SIADH?
ADH secretion is inappropriate for ECF osmolality or volume status
=> renal water retention => high urine osmolality => hyponatraemia
