why people live on mountainous have high amount of red blood cells than those in low lands areas?

Question: why people live on mountainous have high amount of red blood cells than those in low lands areas?
- Keywords:
  people,
  red blood cells
Asked by charo to Joseph on 6 Jul 2017.
- Joseph Olwendo answered on 6 Jul 2017:
  
  This is because blood contains haemoglobin – a specialised protein that binds to oxygen in the lungs so that the oxygen can be transported to the rest of the body. The amount of haemoglobin in blood increases at high altitude.

Question: why people live on mountainous have high amount of red blood cells than those in low lands areas?