When capillary and interstitial hydrostatic pressures are 30 mm Hg and 10 mm Hg, respectively, and colloid oncotic pressure in capillaries and in interstitium are 22 mm Hg and 5 mm Hg, respectively, how much is the net filtration pressure?
**Core Concept**
The net filtration pressure is the balance between the forces driving fluid out of the capillaries (hydrostatic pressure) and the forces resisting fluid movement into the interstitium (colloid oncotic pressure). This concept is crucial in understanding the Starling forces that influence fluid movement across the capillary membrane.
**Why the Correct Answer is Right**
To calculate the net filtration pressure, we need to subtract the interstitial hydrostatic pressure from the capillary hydrostatic pressure and add the capillary colloid oncotic pressure to the interstitial colloid oncotic pressure. The formula is: Net filtration pressure = (Capillary hydrostatic pressure - Interstitial hydrostatic pressure) - (Interstitial colloid oncotic pressure - Capillary colloid oncotic pressure). Plugging in the given values: Net filtration pressure = (30 mm Hg - 10 mm Hg) - (5 mm Hg - 22 mm Hg) = 20 mm Hg + 17 mm Hg = 37 mm Hg.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not account for the interstitial hydrostatic pressure and colloid oncotic pressure in the calculation.
**Option B:** This option is incorrect because it incorrectly calculates the net filtration pressure by subtracting the capillary colloid oncotic pressure from the interstitial colloid oncotic pressure instead of adding the capillary colloid oncotic pressure to the interstitial colloid oncotic pressure.
**Option C:** This option is incorrect because it incorrectly calculates the net filtration pressure by adding the capillary hydrostatic pressure to the interstitial hydrostatic pressure instead of subtracting the interstitial hydrostatic pressure from the capillary hydrostatic pressure.
**Clinical Pearl / High-Yield Fact**
Remember the Starling forces formula: Net filtration pressure = (Capillary hydrostatic pressure - Interstitial hydrostatic pressure) - (Interstitial colloid oncotic pressure - Capillary colloid oncotic pressure). This formula is essential in understanding the forces driving fluid movement across the capillary membrane.
**Correct Answer:** . Net filtration pressure is 37 mm Hg.