Expansion
Proximal 65 per cent, loop 25 per cent, distal 5 per cent, collecting duct 3 per cent
Expansion
| Segment | Sodium reabsorbed | Transporter | Diuretic |
|---|---|---|---|
| Proximal tubule | 65 per cent | Na/H exchanger | Acetazolamide (weak) |
| Thick ascending limb | 25 per cent | NKCC2 | Loop diuretics |
| Distal convoluted tubule | 5 per cent | NCC | Thiazides |
| Collecting duct | 3 per cent | ENaC | Amiloride, spironolactone |
Less than 1 per cent of filtered sodium is normally excreted.
Although the proximal tubule reabsorbs the most, blocking it is a weak diuretic strategy because downstream segments simply reabsorb the extra load. The loop is the most effective target because it handles a large fraction and the distal segments have limited reserve capacity.
Diuretic braking limits the effect of any of them: volume depletion activates the renin-angiotensin system and increases proximal reabsorption, while the distal tubule hypertrophies with chronic loop diuretic use. Combining a loop diuretic with a thiazide, “sequential nephron blockade”, overcomes this.