Renal Handling of Water Load

Expansion

After a water load:

1. Plasma osmolality falls, detected by hypothalamic osmoreceptors
2. Antidiuretic hormone secretion is suppressed within minutes
3. Existing aquaporin-2 channels are retrieved from the collecting duct membrane
4. The collecting duct becomes water impermeable
5. The ascending limb continues to remove solute, so dilute urine (down to 50 mOsm/kg) is produced
6. Diuresis peaks at about 60 to 90 minutes

Maximum urine flow is about 15 to 20 litres per day, limited by the solute load available to carry the water out.

Two important qualifications:

• If solute intake is low, as in beer potomania or a tea and toast diet, the capacity to excrete water falls sharply, so hyponatraemia occurs at much lower intakes
• Any stimulus to antidiuretic hormone that overrides osmolality, notably hypovolaemia, pain, nausea, opioids or surgery, prevents this response entirely; this is why postoperative patients given hypotonic fluid develop hyponatraemia