Vasopressin & water clearance

Urine

Water clearance

Urine = water + solute

Water = urine - solute

Renal Clearance

Renal Clearance

GFR is the volume per minute (ml/min) of plasma filtered by nephrons.  The normal value is 125 ml/min.

For creatinine (Cr) and inulin, the amount filtered by the nephrons is the amount in the urine.  This is because these substances are freely filtered from the plasma in Bowman's capsule* but are not reabsorbed from the filtrate and not secreted* from blood into filtrate.

So for Creatine (or inulin), the amount filtered = the amount in urine.
Definition of amount = volume x concentration.

amount filtered (f) = Vf x Cf
amount in urine (u) = Vu x Cu.

Vf x Cf =  Vu x Cu      V can also mean V/min

normal values =
125 ml/min x 1 mg/dL = 1 ml/min x 125 mg/dL

rearrange to:

Vf = Vu x Cu  = 1 ml/min x 125 mg/dL  = 125 ml/min = GFR
          Cf                      1mg/dL 


*creatinine is filtered to a small extent in the proximal tubules.  This elevates the Cf and lowers the estimation of Vf which is the eGFR.

water mechanisms in kidney


free water clearance

Urine Volume (total clearance) = H2O clearance + solute clearance

H2O clearance = Urine vol - solute clearance


H2O clearance = Vu - (Uosm x Vu)/Posm