Human blood is called whole blood after anticoagulation treatment. After centrifugation, whole blood is mainly divided into three layers. From top to bottom, they are the plasma layer, the buffy coat layer and the red blood cell layer. The plasma layer mainly contains plasma, water, protein, salts and various ions; the buffy coat layer mainly includes platelet-rich area, lymphocyte-rich area, monocyte-rich area and granulocyte-rich area. These tangible cells The specific gravity is close, so they gather in the buffy coat layer; the red blood cell layer can be simply divided into young red blood cells and normal red blood cells. Since the two are in different growth stages of red blood cells, the specific gravity is slightly different.