1 ) 

After a few weeks at high altitude, the number of red blood cells increases to compensate for the lower availability of oxygen in the air.

2 ) 

At high altitude, the bone marrow produces more red blood cells to provide binding sites for the increased number of oxygen molecules available.

3 ) 

At high concentrations of oxygen in the blood, a cytokine stimulates the production of red blood cells to increase the oxygen-carrying capacity of the blood.

4 ) 

After only a few days at high altitude the number of red blood cells increases considerably to compensate for the lower availability of oxygen in the air.