Why doesn't BCOP change along the length of a capillary?

• A. Plasma proteins readily cross capillary walls.
• B. Capillary hydrostatic pressure prevents any protein movement.
• C. Endothelial cells actively transport proteins back into the bloodstream.
• D. Plasma proteins are too large to leave the capillary.

Answer: (D)

The key idea is that the colloid osmotic pressure inside capillaries is driven by plasma proteins, especially albumin. These proteins stay in the blood because they’re too large to cross the capillary walls under normal conditions. As water and small solutes filter out along the capillary, the protein concentration in the blood remains essentially the same, so the osmotic pull back into the capillary (BCOP) stays constant along its length. If proteins could easily leave the capillary, BCOP would change as protein concentration in plasma changed from arterial to venous ends; the other options describe mechanisms that don’t primarily set BCOP.