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Free water clearance (CH2O)

figure

In the CD water reabsorption, and therefore excretion, is controlled independently of solute transport by the action of ADH. The excretion of water, independent of solutes, is termed free water clearance and is calculated as the difference between urine flow rate (V) and osmolar clearance, as illustrated in the figure.

Free water clearance is thus a positive value for a dilute urine (Uosm less than Posm) and a negative value for a concentrated urine (Uosm greater than Posm). Stated another way, the negative CH2O is the amount of water which must be added to a concentrated urine, and the positive CH2O is the amount of water which must be removed from a dilute urine, to make the urine isosmotic with plasma.