Plasma osmolality, thirst and the osmoreceptor loop
Effect and feedback · 275-295 mOsm/kg Thirst Free water
Retained water dilutes plasma and the osmoreceptors switch ADH and thirst off — the negative feedback of the posterior axis. Sodium concentration therefore reports water balance, not sodium content.
Traced from the start
- Supraoptic and paraventricular magnocellular neurones
- Posterior pituitary — vasopressin release
- Collecting duct principal cell (V2 receptor)
- Aquaporin-2 mediated water reabsorption
- Plasma osmolality, thirst and the osmoreceptor loop
Detail
- Set point
- Plasma osmolality is held at 275-295 mOsm/kg; thirst is the more powerful defence, so hypernatraemia is rare in anyone alert with access to water
- SIADH criteria
- Hyponatraemia with low plasma osmolality, urine osmolality above 100 mOsm/kg, urine sodium above 30-40 mmol/L, clinical euvolaemia, and normal thyroid and adrenal function; urate is characteristically low
- Vasopressin deficiency
- Large volumes of dilute urine that concentrate by more than 50% after desmopressin, which does not happen in vasopressin resistance
- Correction limit
- Raise sodium by no more than 8-10 mmol/L in 24 hours (8 in high-risk patients); acute symptomatic hyponatraemia is treated with hypertonic saline boluses
When it goes wrong
Small cell lung carcinoma with sodium 118 mmol/L, urine osmolality 500 mOsm/kg and urine sodium 60 mmol/L
SIADH: fluid restriction first, with tolvaptan or urea if refractory, alongside treatment of the tumour
Polyuria after pituitary surgery with sodium 150 mmol/L and dilute urine
Arginine vasopressin deficiency (formerly central diabetes insipidus): desmopressin, free access to water and daily sodium checks
Correcting chronic hyponatraemia faster than 8-10 mmol/L a day
Osmotic demyelination days later: dysarthria, dysphagia, quadriparesis and a locked-in state
Practise this structure
1 question in the bank tagged Water Balance.