Diuretics

Free Water Clearance and diuretics for NEET/USMLE/FMGE/PLAB

Free Water Clearance:
• Free water clearance is the amount of solute free water generated by the kidney.

• This solute free water is generated in the thick ascending limb and distal convoluted tubule, which are permeable to solutes but impermeable to water. Thus, this solute free water generated in the lumen is called as positive free water clearance.

• Solutes absorbed in thick ascending limb gives rise to medullary hyperosmolarity which absorbs the free water in collecting duct in response to vasopressin. Thus, here solute free water moves out from the lumen in to the body and this is known as negative free water clearance.

• Carbonic anhydrase inhibitors (acetazolamide) deliver more Na+ and water to thick ascending limb and distal convoluted tubule, where only Na+ is absorbed and water remains in lumen. Thus there is an increase in positive free water clearance.

• Loop diuretics (furosemide) and thiazides (chlorothiazide) inhibit Na+ absorption and hence there is more Na than before in lumen as compared to water. Hence loop diuretics and thiazides decrease positive free water clearance.

• Loops decrease Na+/Cl- absorption in thick ascending limb and thus there is a decrease in medullary hyperosmolarity. This decreases absorption of solute free water from the lumen of collecting duct in to the body. Hence loop diuretics decrease negative free water clearance as well.