In the above description (2.1 Air pressure, on the previous page), together with the physiological knowledge of hematology, we can explain why some top athletes travel to altitude before a competition.
What these athletes do is train the body at high altitude where there is lower oxygen gas pressure than at sea level or the lowlands where most people live. Receptors in the body will detect that the oxygen supply to the body is lower than demand and will compensate for this. It does this by producing more red blood cells whose role is to transport oxygen in the body. The higher the number of red blood cells, the more oxygen gas can be transported. After some weeks at altitude, the athletes travel back to lower altitudes to compete, their body has adapted to transport oxygen gas at low air pressures at altitude. At low altitude the athletes therefore have a surplus of red blood cells, meaning that they have more red blood cells that can bind and transport oxygen, enabling their muscles to do more work before fatigue sets in with the production of lactic acid. Lactic acid is produced when tissues metabolise anaerobically that is without using oxygen.