Filtration

Introduction

• Filtration fraction (FF) represents the ratio between glomerular filtration rate (GFR) and renal plasma flow (RPF) 
  • this in turn represents which fraction of RPF becomes filtered via the glomerular capillaries
    • the normal filtration fraction is ~20% and thus implies that 80% of the RPF is not filtered
      • the unfiltered RPF will then be part of the peritubular capillary blood flow after entering the efferent arteriole
  • Filtration fraction (FF) = GFR / RPF  
  • Filtered load = GFR x plasma concentration
• Renal plasma flow (RPF)
  • can be determined by using clearance of para-aminohippuric acid (PAH)
    • RPF = [U(PAH) x V] / [RA(PAH) – RV (PAH)]
      • U(PAH) = urine concentration of PAH (mg/mL)
      • V = urine flow rate (mL/min)
      • RA(PAH) = renal artery concentration of PAH (mg/mL)
      • RV(PAH) = renal vein concentration of PAH (mg/mL)
  • determining the RPF will allow you to determine the renal blood flow (RBF)
• RBF = RPF / (1 – hematocrit)