Key differences in protein content between blood plasma and tissue fluid

Steps:

• Define blood plasma as the liquid component of blood, rich in proteins like albumin.
• Define tissue fluid as interstitial fluid formed by filtration from capillaries, excluding large proteins.
• Compare: Plasma retains proteins due to capillary permeability, while tissue fluid has lower protein levels.
• Evaluate options against these compositions to identify the accurate comparison.

Why A is correct:

• Capillary walls act as semi-permeable barriers; large protein molecules (e.g., globulins) are retained in plasma, resulting in higher protein concentration there than in tissue fluid.

Why the others are wrong:

• B: Blood plasma excludes all cells, including red blood cells, which are confined to whole blood.
• C: Blood plasma contains no white blood cells; both fluids lack them, as cells remain in blood vessels.
• D: Tissue fluid forms from plasma via ultrafiltration but returns to circulation via lymphatics, not directly to plasma.

Final answer: A