Which of the following forms about 55% of the volume of the blood in man?

Analyze the question: You're asked which blood component makes up about 55% of the total blood volume in humans.

Explain the concept: Blood has two main parts:

• Plasma (liquid part): about 55% of blood volume. It is mostly water (about 90%) and carries dissolved nutrients, hormones, salts, plasma proteins, and waste products such as urea and carbon dioxide.
• Cellular part (formed elements): about 45% of blood volume. It is made up of: • Erythrocytes (red blood cells): the vast majority of the cells, about 44% of the blood volume. They carry oxygen. • Leucocytes (white blood cells): fight infection. • Platelets (thrombocytes): help in blood clotting. • Leucocytes and platelets together make up less than 1% of the blood volume.

Solution:

• A. Leucocytes: less than 1%. ❌
• B. Platelets: less than 1%. ❌
• C. Plasma: about 55%. ✅
• D. Erythrocytes: about 45% (the packed cell volume, or haematocrit, is roughly 40-45%). ❌

Plasma is the largest single component of blood, so it is the one that makes up about 55%.

Answer: C. plasma

Exam tip: Remember "55 / 45": Plasma is 55, and the Cells are 45. Think of plasma as the "P" for the "Plenty" part, the bigger share. If a question asks about the cells' share, remember that red blood cells make up almost all of that 45%.