The clearance of para-aminohippuric acid (PAH) at low plasma concentrations measures effective renal plasma flow (ERPF) because:
- A PAH is filtered only, not secreted or reabsorbed, and its clearance equals GFR
- B PAH is secreted by the loop of Henle and reabsorbed by the proximal tubule in equal amounts
- C PAH is filtered and completely secreted by proximal tubules, so none appears in renal venous blood ✓
- D PAH binds to plasma albumin and is extracted by the glomerulus in proportion to renal blood flow
Explanation
PAH at low plasma concentrations is approximately 90% extracted in one pass through the kidney via filtration (at the glomerulus) plus active secretion (organic anion transporters in proximal tubule S2 segment). Since essentially all PAH entering the kidney is excreted in urine, CPAH ≈ ERPF (the ~10% that reaches peritubular capillaries of juxtamedullary nephrons without being secreted explains the ~10% extraction imperfection). At higher plasma concentrations, tubular secretion saturates (Tm for PAH), and clearance falls. ERPF × (1/0.9) = total RBF if haematocrit and extraction ratio are known.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.