A diuretic has exactly one idea: stop the tubule from taking sodium back, because water follows sodium out the door. There are only four sodium doors along the tube — so knowing which door a drug jams tells you how much fluid it can pull, what it does to potassium and calcium, and which lab you check before you give it. Tap a segment, or tap a class.
One nephron
Where the filtered sodium is reclaimed
Proximal 65% · thick ascending limb 25% · distal 5% · collecting duct 3% → under 2% of the filtered sodium ever reaches the urine.
Serum K⁺—
Serum Ca²⁺—
Acid–base—
One nephron · 180 L filtered a daytap a segment of the tubule, or a drug class