Blood

The chief function of red blood cells is to deliver oxygen to the metabolizing tissues of the body. To ensure adequate performance of this essential function, the quantity of circulating red blood cells or red cell mass (as reflected by the blood hemoglobin level) is tightly regulated. If the red cell mass falls then the amount of oxygen delivered to body tissues is reduced. The peritubular cells of the kidney are particularly sensitive to reductions in oxygen delivery. Under such conditions, these cells are stimulated to release the hormone erythropoietin, which in turn acts upon the bone marrow to increase the production of red blood cells. This leads to improvement in oxygen delivery to body tissues, including the kidney, and thus secretion of erythropoietin is suppressed and the rate of red blood cell production reduced. In this way, red cell production may increase as much as tenfold in times of need, and the red cell mass is usually maintained in a very narrow range.

When these regulatory mechanisms fail, and the red cell mass falls to an abnormally low level, anemia results. Anemia may be caused by an increased rate of red cell loss or from a decreased rate of red cell production. Blood loss may be caused by bleeding or by a shortening of the red blood cell life span (known as hemolysis). Red cell production may be suppressed by exposure to various toxins (e.g., certain drugs or radiation), by nutritional deficiencies (e.g., of iron, folic acid, or vitamin B12), by bacterial or viral infections, or by involvement of the marrow by primary or metastatic cancer.

It has been shown in numerous population studies that older adults have, on average, lower hemoglobin levels than do younger adults. These observations have led some investigators to conclude that a decline in red cell mass is inherent to the aging process, and that age-specific normal ranges ought to be employed in diagnosing anemia in older people. This issue is complicated, however, by the fact that disorders associated with anemia occur more frequently with increasing age. When only healthy subjects are studied, however, the difference in mean hemoglobin between older and younger adults largely disappears. Therefore anemia in an elderly person cannot be ascribed simply to "old age"; an underlying cause should be sought.