Osmolality Regulation

Expansion

• Sensor: hypothalamic osmoreceptors in the organum vasculosum, responding to changes of only 1 to 2 per cent
• Effectors: antidiuretic hormone release (from about 280 mOsm/kg) and thirst (from about 290 mOsm/kg)
• Normal plasma osmolality: 275 to 295 mOsm/kg

Calculated osmolality = 2 x sodium + urea + glucose, all in mmol/l.

A useful distinction runs through all fluid physiology:

• Osmolality is regulated by water, through antidiuretic hormone and thirst
• Volume is regulated by sodium, through the renin-angiotensin-aldosterone system and natriuretic peptides

The two systems can conflict, and volume always wins. Significant hypovolaemia stimulates antidiuretic hormone release even when the patient is hypo-osmolar, which is why hypovolaemic hyponatraemia persists until volume is restored.

An osmolar gap, the difference between measured and calculated osmolality of more than 10, indicates unmeasured osmoles such as ethanol, methanol, ethylene glycol or mannitol.