Which blood component primarily contributes to plasma osmotic (oncotic) pressure?

• A. Platelets (thrombocytes)
• B. Erythrocytes (red blood cells)
• C. Plasma proteins (especially albumin)
• D. Plasma electrolytes (e.g., $N^{+}a$ and $C^{-}l$)

Answer: (C)

Verified step by step guidance

1. Understand the concept of plasma osmotic (oncotic) pressure: it is the pressure exerted by solutes in the blood plasma that tends to pull water into the circulatory system.
2. Identify the main solutes in blood plasma that can influence osmotic pressure: these include plasma proteins, electrolytes, and cells like erythrocytes and platelets.
3. Recall that plasma proteins, especially albumin, are large molecules that cannot easily cross the capillary walls, thus they exert a significant osmotic pull to retain water within the blood vessels.
4. Compare the roles of other components: electrolytes are small and freely cross membranes, so they contribute less to oncotic pressure; erythrocytes and platelets are cells and do not directly contribute to plasma osmotic pressure.
5. Conclude that plasma proteins, particularly albumin, are the primary contributors to plasma osmotic (oncotic) pressure because of their size and concentration in plasma.