Net capillary fluid filtration is enhanced by which of the following? Choose all that apply.
a. Decreased venous pressure
b. Decreased capillary plasma oncotic pressure
c. Increased precapillary resistance
d. Increased tissue hydrostatic pressure
Explain why you think that each of these are correct/incorrect.
The Correct Answer and Explanation is :
The correct answers to the question regarding factors that enhance net capillary fluid filtration are:
a. Decreased venous pressure
b. Decreased capillary plasma oncotic pressure
d. Increased tissue hydrostatic pressure
Explanation:
To understand which factors enhance capillary filtration, we need to review the Starling forces that govern the movement of fluids across capillary membranes. These forces include:
- Capillary hydrostatic pressure (Pc): Pressure exerted by the fluid within the capillary, pushing fluid out of the capillary.
- Plasma oncotic pressure (πp): The osmotic pressure exerted by proteins (mainly albumin) in the plasma, which pulls fluid into the capillary.
- Interstitial hydrostatic pressure (Pi): Pressure exerted by the fluid in the interstitial space, pushing fluid into the capillary.
- Interstitial oncotic pressure (πi): Osmotic pressure exerted by proteins in the interstitial fluid, pulling fluid out of the capillary.
The net filtration pressure (NFP) is the difference between the forces that push fluid out (capillary hydrostatic pressure and interstitial oncotic pressure) and the forces that pull fluid in (plasma oncotic pressure and interstitial hydrostatic pressure).
a. Decreased venous pressure
- Correct. Decreasing venous pressure reduces the back pressure on capillaries, leading to an increase in capillary hydrostatic pressure (Pc). This increase in capillary hydrostatic pressure enhances the filtration of fluid out of the capillaries into the surrounding tissue.
b. Decreased capillary plasma oncotic pressure
- Correct. Decreasing plasma oncotic pressure (πp) reduces the pulling force that retains fluid within the capillaries. As a result, less fluid is drawn into the capillary from the interstitial space, enhancing fluid filtration out of the capillary.
c. Increased precapillary resistance
- Incorrect. Increased resistance in the precapillary arterioles (like in the case of vasoconstriction) would reduce the blood flow to capillaries, which in turn reduces capillary hydrostatic pressure (Pc) and therefore decreases fluid filtration.
d. Increased tissue hydrostatic pressure
- Correct. Increased interstitial (tissue) hydrostatic pressure (Pi) pushes fluid into the capillaries. While this force may not be as significant as others in normal conditions, it can contribute to the filtration process.
In summary, decreased venous pressure, decreased capillary plasma oncotic pressure, and increased tissue hydrostatic pressure all contribute to the enhanced filtration of fluid from capillaries. However, increased precapillary resistance would have the opposite effect, reducing fluid filtration.