The increase in red blood cells increases the ability of cells to pick up oxygen from the air. This occurs as a consequence of the body's adaptation to prolonged exposure to a given situation. Increasing the number of red blood cells means increasing the ability to carry oxygen through the blood, and thereby improving aerobic performance when the athlete.
If Cindy wanted to increase her resistance by taking supplements or drugs, she would try to increase oxygen uptake by red blood cells and help her metabolism break down the supplement or drug efficiently.
