Which component makes up about 55% of blood's volume and is a slightly thick, straw-colored fluid mostly water with plasma proteins?

• A. Buffy coat
• B. Formed elements
• C. Serum
• D. Plasma

Answer: (D)

This is about identifying the liquid portion of blood that carries the cells and proteins. The component that makes up about 55% of blood and is a slightly thick, straw-colored fluid mostly water with plasma proteins is plasma. Plasma acts as the transport medium, carrying nutrients, hormones, wastes, and the numerous dissolved substances. Its proteins—albumin, fibrinogen, and globulins—serve important roles: albumin helps keep fluid in the vessels, fibrinogen is essential for clot formation, and globulins include antibodies involved in immunity. When blood is separated, the remaining solid parts are the formed elements (red cells, white cells, platelets); the thin layer between plasma and the formed elements is the buffy coat, which contains white blood cells and platelets. Serum, by contrast, is plasma minus the clotting factors, so it lacks fibrinogen. Thus, the described fluid is plasma.