Which of the following makes up the LARGEST component of whole blood?

Blood is composed of two main parts: plasma and hematocrit (i.e. red blood cells).

Plasma, the watery portion of blood, makes up roughly 55% of total blood volume with the remaining 45% coming mostly from red blood cells, or erythrocytes. White blood cells and platelets make up a very small portion of total blood.

It is important to maintain plasma volume so that blood consistency is maintained. Blood that contains less plasma and more erythrocytes would be thicker and more viscous, leading to thicker blood that is harder to transport through the blood vessels.