Question

Understand how renal handling of a solute can be determined by calculating clearance for that solute and comparing clearance to GFR

Answer 1

Clearance is defined as volume of plasma that is cleared of a particular substance per unit time.

Renal handling of solute can be determined from clearance of that substance and comparing it with the GFR : -

1) For a solute which is freely filtered at glomerulus, if the clearance of solute is more than the GFR then this solute must be secreted into the tubular lumen.

E.g. PAH (paraminohippuric acid ) - it is completely secreted activily in urine hence its clearance is more than GFR.

clearance >>> GFR = secretion

2) For a solute which is freely filtered at glomerulus, if the clearance of solute is Less than that of GFR, then this substance is Reabsorbed from the tubular lumen into blood.

E.g. Glucose

clearance < GFR = Reabsorption

3) If a subatance is neither secreted nor Reabsorbed, then GFR is equal to Clearance.

E.g. Inulin, neither secreted nor Reabsorbed.

4) Creatinine is very slightly secreted in PCT but not Reabsorbed and hence its clearance is approximately equal to GFR.