USMLE Step 1 Renal Physiology Review 59 10 Free Water Clearance & Reabsorption

USMLE Step 1 Renal Physiology Review 59 10 Free Water Clearance & Reabsorption

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Play USMLE Audio MP3 59 10 Free Water Clearance & Reabsorption Below

Begin 59 10 Free Water Clearance & Reabsorption Transcription

What is the term used to describe the amount of pure water that would have to be added to or removed from urine per unit time in order to make it have the same osmolality as plasma? It starts with FWC.

  • Free water clearance.

What kidney ability is free water clearance actually a measure of?

  • Free water clearance is a measure of the ability of the kidneys to excrete water.

What kind of urine generates a negative value for free water clearance?

  • Hyperosmotic urine.

What about with hyposmotic urine?

  • The value for free water clearance will be positive.

What kind of urine would generate a value of zero for free water clearance?

  • Isosmotic urine.

The first step in calculating free water clearance is to divide one osmolality by another, so what is this step?

  • Divide urine osmolality by plasma osmolality.

Then, what do you subtract that quotient from?

  • Step two, subtract that quotient from 1.

And finally, what do you multiply that difference by?

  • The volume of urine per unit time.

Okay, so put that all together now. To calculate free water clearance divide urine osmolality by plasma osmolality. Subtract that quotient from 1 and then multiply by the volume of urine per unit time. What are the two main things that will cause free water clearance to be negative?

  • The first one has to do with water intake. What is it? Water deprivation will cause free water clearance to be negative.

What is the second principal cause of antidiuresis or a negative value for free water clearance. It has to do with the blood?

  • A reduction in the volume of circulating blood.

What is this called?

  • Hypovolemia.

What is the opposite of free water clearance?

  • Free water reabsorption.

How is this calculated?

  • To calculate free water reabsorption, divide urine osmolality by plasma osmolality, subtract 1 from that quotient, and then multiply by the volume of urine per unit time.

During the production of hyperosmotic urine, will free water reabsorption be positive, negative, or zero?

  • Positive.

Student doctor, please pause the tape and summarize what we’ve discussed about free water clearance and reabsorption. Free water clearance is the term used to describe the amount of pure water that would have to be added to or removed from urine in order to make it have the same osmolality as plasma. It’s actually a measure of the kidney’s ability to excrete water. Hyperosmotic urine generates a negative value for free water clearance. Hyposmotic urine generates a positive value and isosmotic generates a value of zero. To calculate free water clearance, divide urine osmolality by plasma osmolality, subtract that quotient from 1, and then multiply by the volume of urine per unit time. The two main causes of antidiuresis or a negative value for free water clearance are water deprivation and a reduction in the volume of circulating blood or hypovolemia. The opposite of free water clearance is free water reabsorption. To calculate this, divide urine osmolality by plasma osmolality, subtract 1 from that quotient, and then multiply by the volume of urine per unit time. During the production of hyperosmotic urine, free water reabsorption will be positive.

 

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