Renal Clearance

Introduction

• Renal clearance 
  • describes the amount (volume) of plasma that is completely cleared by the kidneys per unit time
    • in other words it reflects the excretion of a substance (e.g., medications)
    • this depends on
      • glomerular filtration
      • tubular reabsorption and secretion 
    • clearance decreases gradually with age but may have acute changes due to pathology    
  • equation
    • C(x) = (U_xV) / P_x 
      • C(x) = clearance of substance X (mL/min)
      • U_x = urine concentration of substance X (mg/mL)
      • V = urine flow rate (mL/min)
      • P_x = plasma concentration of substance X (mg/mL)
• Clearance ratios
  • C(inulin)
    • C(inulin) = GFR
    • inulin is freely filtered across glomerular capillaries
      • it is neither reabsorbed or secreted
        • amount of inulin filtered equals amount of inulin excreted
    • inulin is a reference substance called a glomerular marker
  • C(x) / C(inulin) > 1.0
    • C(x) > GFR
    • interpretation
      • there is a net secretion of substance X
        • e.g., para-aminohippuric acid 
          • can be used to determine renal plasma flow 
  • C(x) / C(inulin) < 1.0
    • C(x) < GFR
    • interpretation
      • there is a net reabsorption of subtance X
        • e .g., glucose