Expansion
Unreabsorbed solute in the tubule holds water and drags electrolytes with it
Expansion
When a solute is filtered but not reabsorbed, it retains water osmotically within the tubule. The resulting high flow rate also reduces the time available for sodium reabsorption, so electrolytes are lost as well.
Causes:
- Glucose in uncontrolled diabetes, above the renal threshold
- Mannitol, used deliberately to lower intracranial pressure
- Urea, during recovery from acute kidney injury or with high protein feeding
- Radiocontrast media
- Relief of urinary obstruction, giving a post-obstructive diuresis
Consequences:
- Polyuria and polydipsia
- Hypovolaemia
- Hypernatraemia, since proportionately more water than sodium is lost
- Loss of potassium and magnesium
This is the mechanism of the profound dehydration of diabetic ketoacidosis and hyperosmolar hyperglycaemic state, where total body deficits may reach 6 to 9 litres.
It is distinguished from a water diuresis by the urine osmolality, which is isosmotic or slightly hyperosmotic in osmotic diuresis but very dilute in a water diuresis.