What percentage of plasma is water?

• A. 92% water
• B. 50% water
• C. 75% water
• D. 100% water

Answer: (A)

Plasma is the watery, protein-containing component of blood, acting as the solvent in which cells and a variety of solutes ride along. The majority of plasma by weight is water—roughly 92%—which is why it serves as an effective medium for transport of nutrients, wastes, and biochemical signals. The remaining ~8% consists mainly of plasma proteins (like albumin, globulins, and fibrinogen) along with dissolved electrolytes, nutrients, gases, and waste products. Because there are these significant dissolved substances, plasma cannot be pure water, even though water is its dominant constituent. That’s why 92% water is the most accurate choice, while 50%, 75%, or 100% would either underrepresent or misrepresent the true composition.