Aerobic exercise can alter the number of red blood cells in several ways. Red blood cells carry oxygen and carbon dioxide through the bloodstream. In general, endurance training increases the number of red blood cells. However, in some cases, exercise can also lead to their destruction.
Red Blood Cells
Red blood cells, known as erythrocytes, contain a protein called hemoglobin; this protein binds both oxygen and carbon dioxide for transport through the body. The number of red blood cells may be expressed in terms of concentration or as hematocrit, the ratio of blood cells to blood volume. Concentrations average 4.7 million red blood cells per milliliter of blood for women and 5.2 million for men. Hematocrit averages 41 for women and 45 for men.
Changes with Exercise
When you begin an aerobic exercise program, blood volume increases rapidly during the first few weeks, then eventually levels off. This initial expansion is due mostly to an increase in blood plasma, resulting in a decline in hematocrit. This is sometimes called sports anemia, but it is a normal response to exercise, rather than true anemia. After about a month of training, red blood cell numbers also begin to increase to match the increase in plasma volume.
What Causes the Increase in Red Blood Cells
It is unclear what causes the increase in red blood cells with exercise. Erythrocytes are produced in bone marrow in response to erythropoietin, a hormone released by the kidney. Erythropoietin is secreted when low oxygen levels are low. However, exercise doesn't usually cause oxygen levels to drop low enough for that to occur. Moreover, most studies have not found elevated levels of erythropoietin with exercise. Another hypothesis is that growth hormones released during exercise may account for the change.
Destruction of Red Blood Cells with Exercise
Although aerobic training leads to increases in red blood cells, it can also lead to their accelerated destruction. Erythrocytes live for only about 70 days in endurance athletes, versus about 120 days in those who are sedentary. Several mechanisms may contribute to this, including increases in body temperature during exercise and oxidative stress. The main cause, however, appears to be trauma to the blood cells, most often due to the pounding of the feet in running.