# USMLE Step 1 Renal Physiology 1 Review 58 13 The Starling Equation

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### Play USMLE Audio MP3 58 13 The Starling Equation Below

### Begin 58 13 The Starling Equation Transcription

Okay, now let’s get back to the Starling equation. The first involves the subtraction of two hydrostatic pressures.

What is this step? Please pause the tape briefly.

**The first step in calculating the glomerular filtration rate using the Starling equation is to subtract the hydrostatic pressure in Bowman’s space from the hydrostatic pressure in the glomerular capillaries.**

Next, what do you subtract from this hydrostatic pressure difference?

**From that you subtract the oncotic pressure in the glomerular capillaries.**

Why are we leaving the oncotic pressure in Bowman’s space out of the calculation?

**You don’t have to worry about because it equals zero.**

Why does it equal zero?

**The oncotic pressure of Bowman’s space equals zero because the glomerular filtrate usually contains very little protein. So far we’ve subtracted the glomerular capillary oncotic pressure from the hydrostatic pressure difference between the glomerular capillaries and Bowman’s space.**

Now what do we do?

**We multiply that value by the filtration coefficient of glomerular capillaries.**

What is the symbol for the filtration coefficient?

**k**_{f}

Student doctor, please pause the tape briefly and summarize how the glomerular filtration rate is calculated using the Starling equation.

**Subtract the hydrostatic pressure in Bowman’s space from the hydrostatic pressure in the glomerular capillaries. From that subtract the oncotic pressure in the glomerular capillaries. Then multiply by the filtration coefficient for glomerular capillaries, k**_{f}.

What is a normal value for the hydrostatic pressure in the glomerular capillaries?

**About 45 mmHg.**

What about the hydrostatic pressure in Bowman’s space?

**10 mmHg.**

What is a typical value for glomerular capillary oncotic pressure?

**About 25 mmHg.**

What does the filtration coefficient usually equal?

**About 12.5 mL/minute per mmHg.**

What two things determine the filtration coefficient?

**The two things that determine the filtration coefficient are the glomerular capillary surface area and the permeability of the membrane.**

Contraction of what mononucleated stellate cells will reduced the filtration surface area? They start with M.

**Mesangial cells.**

How does contraction of these cells affect the filtration coefficient?

**When the mesangial cells contract the filtration coefficient is reduced.**

Alphabetically list the three things that will stimulate certain mesangial cells to contract. I’ll cue you with the first letter of each.

First, A.

**Angiotensin II.**

Second, N.

**Norepinephrine**

and third, V.

**Vasopressin or antidiuretic hormone.**

Student doctor, please pause the tape and summarize what we’ve discussed about how the glomerular filtration rate is calculated using the Starling equation.

To calculate the glomerular filtration rate using the Starling equation subtract the hydrostatic pressure in Bowman’s space from the hydrostatic pressure in the glomerular capillaries. From that subtract the oncotic pressure in the glomerular capillaries. Then multiply by the filtration coefficient for glomerular capillaries, k_{f}. The oncotic pressure in Bowman’s space is left out of the calculation because it equals zero. This is due to the fact that the glomerular filtrate usually contains very little protein. Normally, the hydrostatic pressure in the glomerular capillaries is about 45 mmHg. The hydrostatic pressure in Bowman’s space is about 10 mmHg and typical oncotic pressure in the glomerular capillaries is about 25 mmHg. The filtration coefficient usually equals about 12.5 mL/minute per mmHg. The filtration coefficient for glomerular capillaries is determined by the glomerular capillary surface area and the permeability of the membrane. Contraction of the mesangial cells will reduce the filtration surface area which reduces the filtration coefficient. Angiotensin II, norepinephrine, and ADH will cause certain mesangial cells to contract.

****END OF TRANSCRIPTION****

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