The blood has three types of formed elements: erythrocytes (red blood cells), leukocytes (white blood cells),
and platelets or thrombocytes .
Erythrocytes: Also called red blood cells (RBCs), erythrocytes are the body’s most numerous blood cells.
Average normal values indicating numbers of RBCs in the blood may be recorded as 4.60 x 106/μL
(4.60 million/ μL) for women and 5.20 x 106/μL (5.20 million/μL) for men.
To transport oxygen and carbon dioxide through the circulation, each RBC contains approximately 280 million
hemoglobin molecules. Each hemoglobin molecule contains four iron atoms. As RBCs pass through the lungs,
iron atoms combine with oxygen molecules. RBCs travel the circulatory system until, at the tissues, iron atoms
release oxygen into interstitial fluid and hemoglobin molecules take up carbon dioxide. Back at the lungs, RBCs
release carbon dioxide and take up oxygen again.
Leukocytes: The body normally contains 4,500-11,000 white blood cells (WBCs) per μL of blood. This value
may be reported as 4.5-11.0 x 103/μL or k/μL (k = thousand). Unlike RBCs, WBCs occur in many different
types. Most WBCs are filled with tiny grains and are called granulocytes (gran = grain). The normal range for
granulocytes is 1.8-8.5 x 103/μL of blood. Granulocytes include:
• Neutrophils: 50%-70% of total WBCs or about 1.8-7.7 x 103/μL
• Eosinophils: up to 5% of total WBCs or about 0.0-0.450 x 103/μL
• Basophils: up to 2% of total WBCs or about 0.0-0.2 x 103/μL
Lymphocytes and monocytes are non-granular WBCs. The normal range for lymphocytes and monocytes are:
• Lymphocytes: 20%-47% of total WBCs or about 1.0-4.8 x 103/μL
• Monocytes: 3%-10% of total WBCs or about 0.0-0.8 x 103/μL
WBCs fight infection; some surround and destroy debris and foreign invaders while others produce antigen/
antibody reactions [an antigen is any substance (foreign or part of the body) which causes production of an
antibody; an antibody is a protein which neutralizes antigens].
The average adult circulation contains 5 liters of blood (roughly 5.28 quarts).
• Blood completes the entire systemic circuit – from left heart through the body to right heart – in 90 seconds.
• Every cubic millimeter of blood contains 5 million RBCs.
• RBCs survive about 4 months; neutrophils survive about 6 hours.
Platelets: Also called thrombocytes, platelets are cell fragments that travel in the bloodstream. The normal
range for platelets is 140-440 x 103/μL of blood.
Platelets help prevent blood and fluid loss by clumping together to begin the coagulation process. A blood clot is
formed when sticky platelets become covered with fibrin – a plasma protein that holds the blood clot together.