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. The vacuum of space is more complete than any vacuum that can be produced in a laboratory on Earth. Therefore, space travelers must carry their own pressurized atmosphere with the correct mixture of gases in a leak-proof spacecraft cabin, or, when they venture outside the spacecraft, in suits pressurized by oxygen tanks.