INTRODUCTION Blood constitutes 6 to 8 percent of total body weight. In terms of
volume, women have 4.5 to 5.5 L of blood and men 5 to 6 L. In infants and children, blood
volume is 50 to 75 mL/kg in girls and 52 to 83 mL/kg in boys. The principal functions of blood
are the transport of oxygen, nutrients, and hormones to all tissues and the removal of metabolic
wastes to the organs of excretion. Additional functions of blood are (1) regulation of
temperature by transfer of heat to the skin for dissipation by radiation and convection, (2)
regulation of the pH of body fluids through the buffer systems and facilitation of excretion of
acids and bases, and (3) defense against infection by transportation of antibodies and other
substances as needed.
Blood consists of a fluid portion, called plasma, and a solid portion that includes red blood
cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). Plasma makes
up 45 to 60 percent of blood volume and is composed of water (90 percent), amino acids,
proteins, carbohydrates, lipids, vitamins, hormones, electrolytes, and cellular wastes.1 Of the
“solid” or cellular portion of the blood, more than 99 percent consists of red blood cells.
Leukocytes and thrombocytes, although functionally essential, occupy a relatively small portion
of the total blood cell mass.2
Erythrocytes remain within the blood throughout their normal life span of 120 days, transporting
oxygen in the hemoglobin component and carrying away carbon dioxide. Leukocytes,
while they are in the blood, are merely in transit, because they perform their functions in body
tissue. Platelets exert their effects at the walls of blood vessels, performing no known function
in the bloodstream itself.3
Hematology is traditionally limited to the study of the cellular elements of the blood, the
production of these elements, and the physiological derangements that affect their functions.
Hematologists also are concerned with blood volume, the flow properties of blood, and the
physical relationships of red cells and plasma. The numerous substances dissolved or suspended
in plasma fall within the province of other laboratory disciplines.4