diuretic
A diuretic helps the body bail out excess water, the way opening a drain lowers the level in an overfilled tub. By making the kidneys excrete more salt, water follows it out in the urine. Less fluid in the bloodstream means lower blood volume, which eases swelling and brings blood pressure down.
Diuretics act on different segments of the kidney's filtering tubule, and the site determines their power. Loop diuretics work on the loop of Henle and are the most potent; thiazides act on the distal tubule and are mainstays for hypertension; potassium-sparing diuretics such as spironolactone and amiloride act at the collecting duct and conserve potassium. Most other diuretics cause potassium loss, which is the trade-off for their salt-shedding effect.
They are used both to lower blood pressure and to relieve fluid overload in heart failure, liver and kidney disease, where the body retains water. The main cautions are electrolyte disturbances, especially low potassium and sodium, dehydration with dizziness on standing, and, for some agents, gout flares from raised uric acid. Blood tests for electrolytes and kidney function are routine during diuretic therapy.
A patient with swollen legs and breathlessness from heart failure is given a loop diuretic, and over a few days passes large volumes of urine, loses several kilograms of fluid and breathes more easily.
Loop diuretics offload fluid quickly, the cornerstone of treating acute heart failure congestion.
Because they make urine, diuretics are usually taken in the morning so the extra trips to the toilet do not interrupt sleep.