Anemia

Definition of anemia

Anemia may be defined as any condition resulting from a significant decrease in the total body erythrocyte mass. Measurement of total body rbc mass requires special radiolabeling techniques that are not amenable to general medical diagnostic work. Measurements typically substituted for rbc mass determination take advantage of the body's tendency to maintain normal total blood volume by dilution of the depleted rbc component with plasma. This adjustment results in decrease of the total blood hemoglobin concentration, the rbc count, and the hematocrit. Therefore, a pragmatic definition of anemia is a state which exists when the hemoglobin is less than 12 g/dL or the hematocrit is less than 37 cL/L. Anemia may exist as a laboratory finding in a subjectively healthy individual, because the body can, within limits, compensate for the decreased red cell mass.

Physiologic compensation for decreased rbc mass

Each physiologic mechanism will be discussed below. It should be noted that, although there are many adjustments that can be made, one that cannot is decrease in the tissue requirement for oxygen. Actually, overall body oxidative metabolism increases in anemia because of the energy requirement of the compensatory activities.

Decreased hemoglobin oxygen affinity

Increased oxygen extraction of anemic blood by the tissues produces increased concentration of deoxyhemoglobin in the rbc, which stimulates the production of 2,3-diphosphoglycerate (2,3-DPG). 2,3-DPG shifts the hemoglobin-oxygen dissociation curve to the right, thus allowing the tissues to more easily strip the hemoglobin of its precious electron-accepting cargo:

1. Redistribution of blood flow

   In anemia selective vasoconstriction of blood vessels subserving certain nonvital areas allows more blood to flow into critical areas. The main donor sites who sacrifice their aerobic lifestyle are the skin and kidneys. Shunting of blood away from cutaneous sites is the mechanism behind the clinical finding of pallor, a cardinal sign of anemia. Although the kidney can hardly be thought of as a nonvital area, it receives (in the normal state) much more blood flow than is needed to meet its metabolic requirements. Although (by definition) total body red cell mass is decreased in anemia, in the chronically anemic patient the total blood volume paradoxically is increased, due to increased plasma volume. It is as if the body were trying to make up in blood quantity what it lacks in quality.

2. Increased cardiac output

   The heart can respond to tissue hypoxia by increased cardiac output. The increased output is matched by decreased peripheral vascular resistance and decreased blood viscosity (thinner blood flows more freely than thick blood), so that cardiac output can rise without an increase in blood pressure. Generally, anemia must be fairly severe (hemoglobin <>