Each time you inhale, gases are drawn into your lungs, and oxygen is absorbed into the bloodstream, carried to the cells and used to fuel the myriad of actions that your body performs every second. Meanwhile, carbon dioxide is being expelled from your system each time you exhale. This breathing cycle continues through the lifetime of all human beings. We depend on the correct mixture of gases in the atmosphere to sustain our lives. The human being has evolved to be dependent upon the pressure of the atmosphere on Earth for breathing as well.
Atmospheric pressure is a measure of the density and temperature of the gas molecules in the air.
The atmospheric pressure allows us to inhale and exhale while breathing and also makes it possible to keep certain gases dissolved within the bloodstream. Breathing is an automatic body function and is essential to human survival. Thus, the correct mixture of gases in the atmosphere and the pressure of the atmosphere must both be maintained in order for us to breathe. There is no atmosphere is space. There are gas molecules, but they are so few and far between that they do not produce a measurable pressure.