Blood cells and their derivatives include:  erythrocytes, also called red blood cells (RBCs), leukocytes, also known as white blood cells (WBCs), and thrombocytes, also termed platelets.
Plasma is the liquid extracellular material that imparts fluid properties to blood. The relative volume of cells and plasma in whole blood is approximately 45% and 55%,  respectively. The
TABLE Formed Elements of the Blood 10.1 Cells/L Formed Elements Male Female % Erythrocytes 4.3– 5.7 1012 3.9–5.0 1012 Leukocytes 3.5–10.5 109 3.5–10.5 109 100 Agranulocytes Lymphocytes 0.9–2.9 109 0.9–2.9 109 25.7–27.6a Monocytes 0.3–0.9 109 0.3–0.9 109 8.6a Granulocytes Neutrophils 1.7–7.0 109 1.7–7.0 109 48.6–66.7a Eosinophils 0.05–0.5 109 0.05–0.5 109 1.4–4.8a Basophils 0–0.03 109 0–0.03 109 0–0.3a Thrombocytes (platelets) 150–450 109 150–450 109 aPercentage of leukocytes.
volume of packed erythrocytes in a sample of blood is called the hematocrit. The hematocrit is measured by centrifuging a blood sample to which anticoagulants have been added, and then calculating the percentage of the centrifuge tube volume occupied by the erythrocytes compared with that of the whole blood. A normal hematocrit reading is about 39% to 50% in men and 35% to 45% in women; thus, 39% to 50% and 35% to 45% of the blood volume for men and women, respectively, consists of erythrocytes. Low hematocrit values often reﬂect reduced numbers of circulating erythrocytes (a condition called anemia) and may indicate significant blood loss caused by internal or external bleeding.
Leukocytes and platelets constitute only 1% of the blood volume. In a blood sample that has been centrifuged, the cell fraction (the part of the sample that contains the cells) consists mainly of packed erythrocytes (99%). The leukocytes and platelets are contained in a narrow layer at the upper part of the cell fraction called the buffy coat. As Table 10.1 indicates, there are nearly 1,000 times more erythrocytes (5  1012 cells/L of blood) than leukocytes (7  109/L of blood).
Although the blood cells are the major objects of interest in histology, a brief examination of plasma is also useful. The composition of plasma is summarized in Table 10.2. More than 90% of plasma by weight is water, which serves as the solvent for a variety of solutes, including proteins, dissolved gases, electrolytes, nutrients, regulatory substances, and waste materials. The solutes in the plasma help maintain homeostasis, a steady state that provides optimal pH and osmolarity for cellular metabolism.
Plasma proteins consist primarily of albumin, globulins, and fibrinogen.