The diagram shows a portion of a blood vial, containing three distinct layers. The top layer, colored yellow, occupies the top half of the vial, and is Plasma, containing water, protein, nutrients, hormones, etc.. The second layer down is a narrow, white-colored layer occupying only a very small portion of the vial in the vertical center, is called the Buffy coat, and contains white blood cells and platelets. The third layer is colored red, occupies the bottom half the vial, and is Hematocrit, containing red blood cells.