USMLE Step 1 Renal Physiology Review 59 02 Renal Clearance

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Play USMLE Audio MP3 59 02 Renal Clearance Below

Begin 59 02 Renal Clearance Transcription

Now let’s move on to renal clearance.

What is the key concept for renal clearance?

• Renal clearance can be calculated for any substance filtered by the kidneys.

What is actually going on in renal clearance? It’s the volume of plasma that is cleared of a substance per unit time.

• Clearance is calculated by multiplying two things, then dividing that product by something else.

First, what two things are multiplied together?

• The concentration of the substance in urine is multiplied by the volume of urine being passed over a unit time.

And what is this part of the calculation equal to? It starts with ER.

• The excretion rate of the substance.

What is that product then divided by?

• By the concentration of the substance in plasma.

Student doctor, please pause the tape briefly and summarize how the renal clearance of a substance is calculated.

Clearance equals the concentration of the substance in urine times the volume of urine being passed over a unit time. And then that product, divided by the concentration of the substance in plasma. The renal clearance of a given substance is actually the ratio of two things.

What two things?

• Clearance is the ratio of excretion rate of the substance to the concentration of the substance in plasma.

What are the units for clearance?

• mL/min.

When the clearance of a substance equals the glomerular filtration rate, what does this indicate about secretion and reabsorption of the substance?

• When clearance equals the glomerular filtration rate, this means the secretion and the reabsorption of the substance within the nephron are in balance. That is, they are equal.

What does it means when the clearance is less than the GFR?

• This means that the renal tubules are reabsorbing more of the substance than is being secreted.

And what does it mean when the clearance is greater than the GFR?

• When clearance is greater than the GFR the renal tubules are secreting more of the substance than they reabsorb.

What is the normal renal blood flow?

• Normal renal blood flow is 1.2 liters, or 1200 mL/min.

What is normal renal plasma flow?

• Renal plasma flow is normally 600 mL/min.

What is the normal GFR?

• 125 mL/min.

Student doctor, please pause the tape and summarize what we’ve discussed about renal clearance, how it is calculated, and what it tells us about secretion and reabsorption.

• The key concept for renal clearance is that it can be calculated for any substance filtered by the kidneys. Renal clearance is the volume of plasma that is cleared of a substance per unit time. It is calculated as follows. First the concentration of the substance in urine is multiplied by the volume of urine being passed over a unit time. This is then divided by the concentraion of the substance in plasma. The renal clearance of a given substance is actually the ratio of the excretion rate of the substance to the concentration of the substance in plasma. The units for clearance are mL/min. When the clearance of a substance equals the glomerular filtration rate, this indicates that secretion and reabsorption of the substance within the nephron are in balance. When clearance is less than the GFR, the renal tubules are reabsorbing more of the substance than is being secreted. When clearance is greater than the GFR the renal tubules are secreting more of the substance than they reabsorb. Normal renal blood flow is 1.2 liters/min and renal plasma flow is normally 600 mL/min. The normal glomerular filtration rate is 125 mL/min.

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