What is the most important source of oncotic pressure in plasma?

• A. Red blood cells
• B. Plasma proteins
• C. Electrolytes
• D. Water

Answer: (B)

Oncotic (colloid osmotic) pressure in plasma is produced mainly by large plasma proteins, especially albumin. Albumin is present at high concentration and is too large to cross the capillary wall easily, so it remains inside the vessels and draws water back into the bloodstream as fluid filters out at the capillary level. This protein-driven pulling force is the dominant contributor to plasma’s oncotic pressure, helping to keep fluid in the vasculature.

Red blood cells are cells suspended in plasma and do not generate oncotic pressure; they influence blood viscosity and oxygen transport but not the osmotic gradient. Electrolytes affect the overall osmolarity but do not produce the protein-based colloid pressure. Water is the solvent; on its own it does not generate oncotic pressure—it's the proteins that create the pressure gradient across the capillary wall.

So the most important source of oncotic pressure in plasma is plasma proteins, with albumin as the major contributor.