Expansion
Filtered, largely reabsorbed proximally, then secreted in the distal nephron
Expansion
- Filtered freely
- Reabsorbed: about 65 per cent in the proximal tubule and 25 per cent in the thick ascending limb, largely unregulated
- Secreted by principal cells of the distal nephron; this is the regulated step
Factors increasing secretion:
- Aldosterone
- High distal sodium delivery and flow rate, as with diuretics
- Alkalosis, which drives potassium into cells and increases secretion
- High plasma potassium
- Poorly reabsorbed anions in the tubule, such as bicarbonate
Factors decreasing secretion:
- Acidosis
- Aldosterone deficiency or blockade
- Low distal flow
This framework explains most clinical potassium disorders. Loop and thiazide diuretics cause hypokalaemia by raising distal flow and sodium delivery, and by activating aldosterone through volume depletion. Vomiting causes hypokalaemia partly through alkalosis and secondary hyperaldosteronism, not simply through loss in vomit.