Boxes

box 9.1. Clinical Correlations: ABO and Rh Blood Group Systems 219

box 9.2. Clinical Correlations: Hemoglobin Disorders 220

box 9.3. Clinical Correlations: Hemoglobin Breakdown and Jaundice 237

S? overview of blood

Blood is a fluid connective tissue that circulates through the cardiovascular system

Like the other connective tissues, blood consists of cells and an extracellular component whose volume exceeds that of the cells. The total volume of blood in the normal adult is about 6 liters, which amounts to 7 to 8% of total body weight. Blood is propelled through the cardiovascular system by the pumping action of the heart to reach the body tissues. Its many functions include

• Transport of nutrients and oxygen directly or indirectly to cells

• Transport of wastes and carbon dioxide away from cells

• Delivery of hormones and other regulatory substances to and from cells and tissues

• Maintenance of homeostasis by acting as a buffer and by participating in coagulation and thermoregulation

• Transport of humoral agents and cells of the immune system that protect the body from pathogenic agents, foreign proteins, and transformed cells, i.e., cancer cells

Blood consists of cells and their derivatives and a protein-rich fluid called plasma

Blood cells and their derivatives include

• Eiythrocytes, also called red blood cells (RBCs)

• Leukocytes, also called white blood cells (WBCs)

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 volume of packed erythrocytes in a sample of blood is called the hematocrit. It is obtained by centrifuging a blood sample to which anticoagulants have been added. The hematocrit reading is then obtained by measuring the percentage of the centrifuge tube volume occupied by the erythrocytes, compared with the whole blood volume. A normal reading is about 39 to 50 in males and 35 to 45 in females; thus 39 to 50% or 35 to 45% of the blood volume, respectively, consists of erythrocytes.

Leukocytes and platelets constitute only 1% of the blood volume. In a blood sample that has been cen-trifuged, 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

TABLE 9.1. Formed Elements of the Blood

Cells/L

TABLE 9.1. Formed Elements of the Blood

Cells/L

Formed Elements

Male

Female

%

Eiythrocytes

4.3-5.7 x 10'2

3.9-5.0 x 1012

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