Understanding and Calculating Renal Blood Flow (RBF)

Renal blood flow, or RBF, is the volume of blood flowing into the kidneys. The kidneys need a large supply of blood so that they can filter the blood and maintain healthy electrolyte levels. The kidneys also need an adequate supply of oxygen-rich blood to survive. To test a patient's renal blood flow, you first need to inject a known amount of solute intravenously. Wait a few minutes, then draw blood samples from any artery and the renal vein; also obtain a urine sample. Once you have lab results with the concentrations of your solute in all the samples, you are ready to calculate your patient's RBF.

Things You'll Need

• Lab report including the following values:
• U: solute concentration in urine (mg/ml)
• V: urine excretion rate (ml/min)
• PA: solute concentration in arterial plasma (mg/100 ml)
• PV: solute concentration in renal vein plasma (mg/100 ml)
• hct: hematocrit

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Instructions

• 1

  Multiply U X V. For example, 0.1 mg/ml X 1.0 ml/min = 0.1.

• 2

  Divide PA by 100. For example, 1.0 mg/100 ml ÷ 100 = 0.01.

• 3

  Divide your value from step 1 by your value from step 2. For example, 0.1 ÷ 0.01 = 10. This is the clearance rate (C), which means that the 0.1 mg of solute that was excreted in the urine was dissolved in 10 ml of arterial plasma.

• 4

  Subtract PV from PA. For example, 1.0 mg/100 ml - 0.5 mg/100 ml = 0.5 mg/100 ml.

• 5

  Divide your result from step 4 by PA. For example, 0.5 mg/100 ml ÷ 1.0 mg/100 ml = 0.5. This is the extraction ratio (E).

• 6

  Divide C by E. For example, 10 ÷ 0.5 = 20. This is the renal plasma flow (RPF), so RPF=20 ml/min.

• 7

  Subtract hct from 1. For example, if the hematocrit is 0.5, then 1 - 0.5 = 0.5.

• 8

  Divide RPF by your result from step 7. For example, 20 ÷ 0.5 = 40 ml/min. This is the renal blood flow (RBF).