Porth's Essentials of Pathophysiology, 4e

278

Hematopoietic Function

U N I T 3

Red cell antigens

Red cell membrane

Integral protein

Ankyrin

Actin

Spectrin

of a globin (protein) portion and a heme unit, which surrounds an atom of iron that binds oxygen. 1,3 Thus, each molecule of hemoglobin can carry four molecules of oxygen. Hemoglobin is a natural pigment; because of its iron content, it appears reddish when oxygen is attached and has a bluish cast when deoxygenated. The production of each type of globin chain is controlled by individual structural genes with five different gene loci. Mutations, which can occur anywhere in these five loci, have resulted in over 550 types of abnormal hemoglobin molecules. 1 There are two major types of normal hemoglobin— adult hemoglobin (HbA) and fetal hemoglobin (HbF). Adult hemoglobin consists of a pair of α chains and a pair of β chains. Fetal hemoglobin is the predominant hemo- globin in the fetus fromthe third through theninthmonths of gestation. It has a pair of gamma ( γ ) chains substituted for the α chains. Because of this chain substitution, HbF vascular catheter. Note the biconcave shape of each erythrocyte, which increases the surface area of these hemoglobin-filled cells, thus promoting more effective gas exchange. (From the Centers for Disease Control and Prevention Public Health Images Library. No. 7315. Courtesy of Janice Carr.) FIGURE 13-1. A highly magnified (× 11,397) electron micrograph of a number of red blood cells found enmeshed in a fibrinous matrix on the luminal surface of an indwelling

has a higher affinity for oxygen than adult hemoglobin. This affinity facilitates the transfer of oxygen across the placenta from the HbA in the mother’s blood to the HbF in the fetus’s blood. HbF is replaced within 6 months of birth with HbA. FIGURE 13-3. Cross-sectional side view of the biconcave structure of the red blood cell and diagram showing the cytoskeleton and flexible network of spectrin proteins that attach to the ankyrin protein, a transmembrane protein that resides on the inner surface of the membrane and is anchored to an integral protein that spans the membrane.

Plasma membrane

Hemoglobin

β 2

β 1

C

C

N NFe N N

C

C

FIGURE 13-2. (A) Biconcave structure of the red blood cell as shown in cross-section and in lateral surface view. (B) Hemoglobin molecule, showing the four iron (Fe)- containing heme subunits and their structure.

Heme

α 2

α 1

B

A

Hemoglobin

Red blood cell