Porth's Essentials of Pathophysiology, 4e

lthough the lungs provide the means for gas exchange between the external and internal envi- ronments, it is the hemoglobin in the red blood cells that transports oxygen to the tissues. The red blood cells also function as carriers of carbon dioxide and participate in acid–base balance. The function of the red blood cells, in terms of oxygen transport, is discussed in Chapter 21, and acid–base balance is discussed in Chapter 8. This chapter focuses on the red blood cell, anemia, polycy- themia, and age-related changes in the red blood cells. The Red Blood Cell The erythrocytes or mature red blood cells are the most common type of blood cell, being 500 to 1000 times more numerous than other blood cells. The erythrocyte is a nonnucleated, thin, biconcave disk (Fig. 13-1). This unique shape contributes in two ways to the oxygen transport function of the erythrocyte. The biconcave shape provides a larger surface area for oxygen diffusion than would a spherical cell of the same volume, and the thinness of the cell mem- brane enables oxygen to diffuse rapidly between the exterior and the interior of the cell 1–3 (Fig. 13-2A). Another structural feature that facilitates the transport function of the red blood cell is the flexibility of its membrane. The biconcave shape and flexibility of the red cell membrane are maintained by a complex net- work of fibrous proteins, especially one called spectrin (Fig. 13-3). Spectrin forms an attachment with another protein, called ankyrin, that resides on the inner sur- face of the membrane and is anchored to an inte- gral protein that spans the membrane. 2 This unique arrangement of proteins imparts elasticity and stability to the red blood cell membrane and allows it to deform easily as it moves through narrow spaces in the vascu- lar network. The function of the red blood cell, facilitated by the hemoglobin molecule, is to transport oxygen to the tis- sues. Because oxygen is poorly soluble in plasma, about 95% to 98% is carried bound to hemoglobin. The hemo- globin molecule is composed of two pairs of structurally different alpha ( α ) and beta ( β ) polypeptide chains (see Fig. 13-2B). Each of the four polypeptide chains consists Disorders of Red Blood Cells 13 C h a p t e r A

The Red Blood Cell

Hemoglobin Synthesis Red Cell Production Red Cell Life Span and Destruction Laboratory Tests Anemia

Blood Loss Anemia Hemolytic Anemias Inherited Disorders of the Red Cell Membrane Sickle Cell Disease Thalassemias Inherited Enzyme Defects Acquired Hemolytic Anemias Anemias of Deficient Red Cell Production

Iron-Deficiency Anemia Megaloblastic Anemias Aplastic Anemia Anemia of Chronic Disease

Polycythemia

Primary Polycythemia Secondary Polycythemia Age-Related Changes in Red Blood Cells Red Cell Changes in the Neonate Hyperbilirubinemia in the Neonate Hemolytic Disease of the Newborn Red Cell Changes in the Elderly

277