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People living at sea level have around 5 million RBC per cubic millimeter of their blood whereas those living at an altitude of 5400 metres have around 8 million. This is because at high altitude

1. atmospheric O₂ level is less and hence more RBCs are needed to absorb the required amount of O₂ to survive
2. there is more UV radiation which enhances RBC production
3. people eat more nutritive food, therefore more RBCs are formed
4. people get pollution-free air to breathe and more oxygen is available

Correct answer: atmospheric O₂ level is less and hence more RBCs are needed to absorb the required amount of O₂ to survive

Solution

At high altitude, atmospheric oxygen pressure is lower, so less oxygen enters the blood with each breath. The body compensates by producing more RBCs to increase oxygen-carrying capacity.

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