Which formed element is the most abundant in blood?

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

Answer: (B)

Red blood cells are the most abundant formed element in blood. The formed elements—red blood cells, platelets, and white blood cells—make up the cellular portion, while plasma is the liquid component. In a healthy adult, red blood cells vastly outnumber the other formed elements, contributing roughly half of blood's volume as reflected by the hematocrit, whereas white blood cells and platelets are present in much smaller numbers. This large supply supports their primary role of gas transport: hemoglobin within each cell binds oxygen and carbon dioxide, and the biconcave shape increases surface area for efficient exchange. Platelets and white blood cells are important too, but their counts are comparatively small, and plasma is not a formed element.