diuretic

The kidney filters huge amounts of fluid and then reclaims most of the salt and water back into the body. A diuretic tells the kidney to reclaim less salt; because water follows salt, more of both leave as urine. With less fluid in the circulation, swelling subsides and blood pressure falls.

Mechanistically, diuretics block specific salt-handling proteins along the kidney tubule. Loop diuretics such as furosemide inhibit the Na-K-2Cl cotransporter and are the most powerful; thiazides such as hydrochlorothiazide block the sodium-chloride cotransporter in a different segment; potassium-sparing agents such as spironolactone instead block aldosterone's nuclear receptor or sodium channels, avoiding potassium loss.

Chemically, the loop and thiazide diuretics famously grew out of sulfonamide chemistry: the sulfonamide group, first explored as an antibacterial, turned out to inhibit these transporters and carbonic anhydrase, a serendipitous discovery that founded a whole therapeutic class.

An honest caveat: by changing how much salt the body keeps, diuretics also shift potassium, sodium, and other electrolytes, so they require monitoring; the different classes are often combined precisely to balance these effects.