People living at sea level have around 5 million RBC per cubic millimeter of their blood whereas those…
People living at sea level have around 5 million RBC per cubic millimeter of their blood whereas those living at an altitude to 5400 metres have around 8 million. This is because at high altitude:
people eat more nutritive food. therefore more RBCs are formed
people get pollution-free air to breathe and more oxygen is available
atmospheric $\mathrm{O}_2$ level is less and hence more RBCs are needed to absorb the required amount of $\mathrm{O}_2$ to survive.
there is more UV radiation which enhances RBC production
Solution
As we move from low altitude to high altitude, the air starts getting thinner and thinner. The partial pressure of oxygen starts decreasing with increasing altitude. Now, since the partial pressure of oxygen is less in our lungs, the binding of oxygen to haemoglobin in RBCs also decreases. As a result, there is decreased oxygen supply to the tissues thereby starving them of oxygen. This situation leads to the release of the hormone erythropoietin from the kidneys, which stimulates the bone marrow to increase the production of RBCs.
Related Theory
Oxygen is carried in the blood in two forms: (i) dissolved in plasma and RBC water (about $2 \%$ of the total) and (ii) reversibly bound to haemoglobin (about $98 \%$ of the total). The haemoglobin molecule has four binding sites for oxygen molecules: the iron atoms in the four heme groups. Thus, each $\mathrm{Hb}$ tetramer can bind four oxygen molecules.