Questions: Why do plasma proteins typically remain in the bloodstream and not enter the tissue fluids? O Plasma proteins are too large to diffuse out of the capillaries. Plasma proteins coat the lining of the endothelium and block the membrane pores. Plasma proteins cannot pass through precapillary sphincters. Plasma proteins are hydrophobic and so remain in the bloodstream.

The answer is the first one: Plasma proteins are too large to diffuse out of the capillaries.

Explanation for each option:

1. Plasma proteins are too large to diffuse out of the capillaries.

   • This is correct. Plasma proteins, such as albumin, globulins, and fibrinogen, are large molecules that cannot easily pass through the small pores of the capillary walls. This size exclusion is the primary reason they remain in the bloodstream.

2. Plasma proteins coat the lining of the endothelium and block the membrane pores.

   • This is incorrect. Plasma proteins do not coat the endothelium in a way that blocks membrane pores. Their primary role is not to block pores but to maintain osmotic pressure and transport substances.

3. Plasma proteins cannot pass through precapillary sphincters.

   • This is incorrect. Precapillary sphincters regulate blood flow into capillaries but do not directly influence the movement of plasma proteins. The size of the proteins relative to the capillary pores is the limiting factor.

4. Plasma proteins are hydrophobic and so remain in the bloodstream.

   • This is incorrect. Plasma proteins are generally hydrophilic, not hydrophobic. Their solubility in blood and inability to pass through capillary walls due to size are the reasons they remain in the bloodstream.

In summary, plasma proteins remain in the bloodstream primarily because they are too large to diffuse out of the capillaries.