Answer to Challenge Question

Derive a formula for GFR based on the following information:

• Inulin is freely filtered by glomeruli and not reabsorbed or secreted by nephrons.

• The amount of injected inulin filtered by the glomeruli is equal to the amount of inulin subsequently found in urine.

Answer:

For inulin (or creatinine)

Amount filtered = Amount in urine

Amount = volume x concentration (VC)

V1C1 = V2C2

V1 = GFR

GFR = (V2C2)/C1

GFR = (U x INu)/INp

Using numbers for normal creatinine; e.g.,

V1C1 = V2C2
100ml/min x 1 mg/100ml = 1ml/min x 100mg/100ml

1 mg/min = 1 mg/min
Note that these numbers show the concentrating ability of the kidney; e.g.,
1 mg/dL of plasma creatinine become 100 mg/dL of urine creatinine
and
100 ml/min of filtrate becomes 1 ml/min of urine production