Why is it hard to breathe at higher elevations

It is hard to breathe at higher elevations because of lower air pressure. Due to lower air pressure it becomes hard for the oxygen to enter our vascular system. At sea level the atmospheric pressure is about 14.7 pounds per square inch. This atmospheric pressure allows us the oxygen to reach our lungs and to the vascular system with ease. The higher we go, the atmospheric pressure starts decreasing and harder it becomes for us to breath. The lungs gets affected by the lower atmospheric pressure and so it cannot function normally as it can do at sea level.

Ithink 2 is the correct