Which component of blood determines the majority of plasma oncotic pressure?

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

Answer: (B)

Plasma oncotic pressure is the pulling force that draws fluid back into the capillaries, and it is produced mainly by plasma proteins, especially albumin. These large proteins stay largely in the intravascular space and create a colloid osmotic force that keeps water inside the vessels. Red blood cells are cells, not dissolved solutes, so they don’t generate this osmotic pull. Electrolytes contribute to overall osmolarity but not to the specific oncotic (colloid) pressure produced by proteins. Water is the solvent; it doesn’t itself create the pulling force—it's the proteins that stay in the blood and draw fluid inward.