Expansion
The volume of plasma completely cleared of a substance per unit time
Expansion
Clearance = (urine concentration x urine flow) / plasma concentration
Interpretation:
- Clearance = GFR (about 125 ml/min): the substance is freely filtered and neither reabsorbed nor secreted. Example: inulin, the gold standard
- Clearance < GFR: net reabsorption. Example: sodium, urea, and glucose (clearance near zero below the transport maximum)
- Clearance > GFR: net secretion. Example: para-aminohippurate (PAH), cleared almost completely in one pass, so it measures renal plasma flow (about 625 ml/min)
Creatinine clearance is the practical substitute for inulin. It slightly overestimates GFR because a small amount is secreted by the proximal tubule, and this error grows as GFR falls.
Filtration fraction = GFR / renal plasma flow, normally about 20 per cent. It rises when the efferent arteriole constricts, which is why angiotensin II raises filtration fraction, and it explains the rise in peritubular oncotic pressure that promotes proximal reabsorption in hypovolaemia.