questions 2

Renal Clearance

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Topic updated on 06/12/17 4:28pm

Overview
  • Renal Clearance
    • volume of plasma from which a substance is completely cleared by kidneys per unit time
    • C(x) = (UxV) / Px
      • C(x) = clearance of substance X (mL/min)
        • ratio of urinary excretion of substance X to plasma concentration of substance X
      • Ux = urine concentration of substance X (mg/mL)
      • V = urine flow rate (mL/min)
      • Px = plasma concentration of substance X (mg/mL)
  • Clearance Ratios
    • C (inulin)
      • C (inulin) = GFR
      • inulin is freely filtered across glomerular capillaries, but is neither reabsorbed nor 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
        • substance X is freely filtered across glomerular capillaries, but is neither reabsorbed nor secreted
        • substance X is a glomerular marker
    • C (x) / C (inulin) > 1.0
      • C (x) > GFR
      • interpretation
        • substance X is freely filtered across glomerular capillaries and is secreted from peritubular capillaries into tubular lumen
          • net tubular secretion of substance X
          • e.g., para-aminohippuric acid (PAH)
    • C (x) / C (inulin) < 1.0
      • C (x) < GFR
      • interpretation
        • (1) substance X is not freely filtered across glomerular capillaries
          • e.g., albumin
        • (2) substance X is freely filtered across glomerular capillaries and is reabsorbed from tubular lumen into peritubular capillaries
          • net tubular reabsorption of substance X
          • e.g., glucose, amino acids, bicarbonate
Important substances
 
Substance
Filtered
Absorbed/secreted
Clearance
Notes
Glucose 1 -1 0 Except if transporters overwhelmed in diabetes
PAH   0.5 0.5 1 Used to estimate effective renal plasma flow and/or renal blood flow

≈ RPF (underest. by 10%)
RBF = RPF/(1-Hct)

Inulin 1 0 1 ≈ GFR
Creatinine 1 0.1 1.1 ≈ GFR (overest.)
Urea 1 -0.5 0.5 ≈ GFR (underest.)


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Qbank (1 Questions)

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(M1.RL.4695) A 45-year-old gentleman with a history of poorly controlled diabetes mellitus is referred to a nephrologist for evaluation of the possibility of early stage kidney failure. Upon evaluation, the nephrologist decides to assess the patient's renal plasma flow by performing a laboratory test. Which of the following substances would be the best for estimating this value? Topic Review Topic

1. Creatinine
2. Inulin
3. Glucose
4. Para-aminohippurate (PAH)
5. Urea

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