The volume of red blood cells is also defined as hematocrit. After centrifugation of a blood tube, two layers can be seen: the upper phase consists of the plasma, while the red blood cells sink to the bottom of the tube, which is visible as a red layer (Figure 1). White blood cells and platelets settle in the interface between both layers. A low hematocrit value can be an indicator for anemia.

Image of two tubes. The tube on the left has not undergone any process of centrifugation, and the sample looks like one only red component. The tube on the right has undergone centrifugation, and the different blood components are separated in phases inside the tube. From top to bottom, the image shows the plasma, then white blood cells and platelets, and finally red blood cells.

Figure 1: Schematic representation of the determination of the hematocrit.