well, as you go higher,the oxygen molecules are less up there so you need to breath in more to get the same amount of oxygen you got when you were lower
example, lets say at sea level you get x cups of oxygen per breath
but at 1000 meters above sea level, you get 0.5 cups of oxygen per breath
you would need to breath in twice as much to get the same as much oxygen
and you need oxygen to move and live and be alive
