Diuretics: where each one acts in the nephron — and what it costs the patient
Five classes, five points of attack along one tube: block salt higher up and the collecting duct pays in potassium.
Select any card for the reasoning, what to watch for and an exam tip.
Diuretics (water tablets)
Where each diuretic acts
Each kidney holds about a million nephrons, running from the cortex down into the medulla. Filtrate leaves the blood at the glomerulus and runs through the tubule to the collecting duct. The green boxes show what each part takes back into the blood (at the collecting duct, what it pushes into the urine), and each numbered pin is a class from the columns above, placed where it acts.
- Mannitol is filtered but not reabsorbed, so it holds water in the parts of the tube that let water out — the proximal tubule and the descending limb. A water diuresis, not a salt block.
- Block higher up and more sodium reaches the collecting duct: that is why loops cost the most potassium.
- Acetazolamide leaves the blood a little acid and the urine alkaline — the reason it can cause stones and is used at altitude.
Key points
- Order of play: acetazolamide acts at the proximal tubule, loops at the thick ascending limb, thiazides at the distal tubule, potassium-savers at the collecting duct; mannitol never meets a transporter at all.
- Loops cost potassium and magnesium and send calcium down the urine; thiazides cost potassium but keep calcium, and raise urate and glucose — check the electrolytes, not just the weight.
- Never give a potassium supplement beside a potassium-sparing diuretic, and never add one casually to an ACE inhibitor or ARB: severe hyperkalaemia is the price.
- Low potassium makes digoxin toxic, and loops and thiazides both lower potassium; they also raise lithium levels — so watch both pairings.
- Correct hypotension and hypovolaemia before starting a loop: vigorous diuresis can drop the pressure fast and push the kidneys into injury.
When to refer
Key takeaway
Each diuretic blocks one transporter at one place in the nephron: loops waste potassium and magnesium, thiazides waste potassium but keep calcium, potassium-savers reverse the risk — and the electrolytes are what must be watched.