Final answer:
High-altitude training stimulates the production of EPO and erythrocytes to compensate for lower oxygen levels, resulting in a temporary increase in red blood cells when an athlete returns to lower altitudes.
Step-by-step explanation:
When athletes train at high altitudes, the lower partial pressure of oxygen stimulates their kidneys to produce more erythropoietin (EPO), which in turn boosts the production of erythrocytes (red blood cells). This process is known as acclimatization and helps the body compensate for lower oxygen levels by having more red blood cells to transport oxygen. Upon returning to lower altitudes, the enhanced erythrocyte count provides a temporary advantage as tissues receive more oxygen, potentially improving athletic performance. However, this increased count is temporary as the body will eventually reduce EPO production to return the erythrocyte count to normal levels, maintaining homeostasis.