Whenever conditions arise under which unusual demands are made within the tissues for a supply of oxygen, the organism responds in ways that make for better arterialization of the blood. Under ordinary circumstances we breathe more rapidly in order to increase the pulmonary ventilation, and the circulation speeds up so as to carry the respiratory gases back and forth more promptly between the lungs and the tissues. When a more chronic stage of oxygen deprivation occurs, such as is the case at higher altitudes where the atmosphere is rarefied, an additional interesting adjustment takes place in the form of an increase in the erythrocytes and hemoglobin in the blood. In other words, as oxygen becomes scarcer in the environment, the physiologic carriers of the indispensable gas in the body become more abundant.
The mechanism by which such beneficent compensatory adjustments are brought about has been the subject of considerable dispute.
THE BLOOD DURING EXERCISE. JAMA. 1922;79(24):2004. doi:10.1001/jama.1922.02640240038017