Whole blood is composed of plasma and formed elements.
Whole blood is composed of plasma (fluid portion of blood) and formed elements (blood cells and platelets). Plasma is the fluid portion of blood that contains proteins, ions, nutrients, hormones, antibodies, metabolites, enzymes, clotting factors, etc. Plasma is straw-colored and makes up approximately 55% of whole blood volume. The formed elements of blood are the red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). Blood cells and platelets make up approximately 45% of whole blood volume. Hematocrit is the percentage of total blood volume that is occupied by red blood cells. Hematocrit is also called packed red cell volume (PCV). To determine hematocrit, whole blood is centrifuged for a few minutes, which leads to the separation of the formed elements from plasma. After centrifugation, three distinct regions can be observed: (1) plasma, which is ~55% of total blood volume and is the top region, (2) red blood cells (hematocrit or PCV), which make up ~45% of total blood volume and are in the bottom later, and (3) a small, off-white region between plasma and red blood cells that is referred to as the buffy coat. The buffy coat contains the white blood cells and platelets and is less than 1% of total blood volume. Generally, venous blood is obtained for this purpose, which has a dark red color. Occasionally, arterial blood is obtained, which has a bright red color. The tube in which blood is collected has an anti-clotting agent (e.g., heparin or EDTA) in order to prevent blood clotting, which would interfere with proper separation of plasma and formed elements. If an anti-clotting agent is not included and blood is allowed to clot before centrifugation, the straw-colored region at the top is now referred to as serum. Serum is similar to plasma but does not contain any clotting factors (e.g., fibrin). Blood clots move to the bottom layer during centrifugation.