Which blood component is typically present in the highest numbers in healthy adults?

• A. Red blood cells
• B. Platelets
• C. White blood cells
• D. Plasma proteins

Answer: (A)

Red blood cells are the most numerous component of blood in healthy adults. Their main role is to carry oxygen from the lungs to tissues throughout the body using hemoglobin, so the body maintains a very large number to meet ongoing oxygen needs. This abundance is reflected in the hematocrit, the proportion of blood volume made up of red cells, which is typically around 40–45%. Platelets and white blood cells exist in much smaller numbers because they serve more specialized functions—clotting and immune defense, respectively. Plasma proteins are dissolved substances, not cells, so they aren’t counted the same way. Thus, red blood cells are present in the highest numbers.