As a mountain climber is hikes up mount rainier, it becomes harder for him to breath the higher he goes.

why does this happen? how does low air pressure affect your breathing?

There is less oxygen than what your used to. Your body has to breathe more this leads to shortness of breath, tiredness and dizziness. The oxygen drop and much less air pressure can be really hard on your body. Your blood pressure and heart rate will increase dramatically and your body works harder for your veins to pump oxygen throughout the body.