83% of the blood consists of which substance?

• A. Water
• B. Plasma
• C. Oxygen
• D. Proteins

Answer: (A)

The main concept here is the overall composition of blood and why water dominates. Blood is a watery mixture: the fluid part, plasma, is mostly water and makes up about half of blood. Plasma itself is about 92% water, and the cells (red cells, white cells, platelets) contain water in their cytoplasm. When you combine the water from plasma with the water inside the cells, water accounts for most of the blood’s volume—roughly around 83%. The other components—plasma proteins, electrolytes, nutrients, gases—still fit into that mainly-water matrix but do not approach the same large share as water. Oxygen is present only dissolved in plasma in small amounts and is not a major constituent of blood. Proteins add up to a much smaller fraction of whole blood than water.