A. Two Divisions: Humans have a total blood volume of about 5 L (depending on body size). Blood is divisible into 2 parts, the formed elements, which include the blood cells and platelets, and the plasma, or liquid phase, in which the formed elements are suspended and in which a variety of important proteins, hormones, and other substances are dissolved.
B. Basic Cell Types: There are 2 basic types of blood cells, the erythrocytes, or red blood cells, and the leukocytes, or white blood cells.
C. Clotting: Outside the blood vessels, blood undergoes a complex reaction called clot formation or coagulation, which plays an important role in repairing damaged vessels and preventing blood loss.
D, Hematocrit: When anticoagulants (heparin, citrate, etc) are added, blood samples can be separated in a centrifuge into 3 major fractions. The erythrocytes constitute the densest fraction and end up at the bottom of the tube. The hematocrit is the percentage of packed erythrocytes per unit volume of blood. In adults, normal hematocrit values vary from 35 to 50% and are sex dependent. Leukocytes are less dense and less numerous (about 1 % Of blood volume) and form a thin white or grayish layer over the erythrocytes. On top is a thin layer of platelets. The least dense is the clear layer of plasma, which constitutes 42 47% of the blood.
E. Differential Cell Count: Blood is also studied by spreading a drop on a slide to produce a single layer of cells (blood smear). The cells are stained, differentiated by type, and counted to reveal disease-related changes in their relative numbers. The smears are usually stained with Romanovsky-type dye mixtures containing eosin and methylene blue.
F. Staining Properties: All of the descriptions of the staining properties of blood cells refer to their appearance after staining with Romanovsky-type mixtures leg, Wright's or Giemsa). Blood cells and their components exhibit 3 major staining properties that allow the cell types to be distinguished:
1. Basophilia is an affinity for methylene blue. Basophilic structures stain purple to black. 3. Eosinophilia, or acidophilia, is an affinity for eosin. Eosinophilic structures stain salmon pink to orange.
4. Neutrophilia is an affinity for a complex of dyes (originally thought to be neutral) in the mixture. Neutrophilic structures stain salmon pink to lilac.

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