Question

How does human body correctly inhale oxygen from a mixture of gases?

Answer 1

by partial pressure of gases.

Answer 2

 When we breathe in, we inhale the same mixture of gases contained in the atmosphere as our nose cannot filter out the oxygen from other gases.
During the respiration process, the total composition of the air that we breathe in also depends on our surrounding environment. Let say, the underwater divers inhale air containing more oxygen or helium. Similarly, the space suits worn by astronaut supplies pure oxygen for them to breathe in space.