Air composition

Dry air is a gas mixture: about 78% nitrogen and 21% oxygen by volume. The remaining ~1% is argon, carbon dioxide and other trace gases, plus a variable amount of water vapour.

Nitrogen is the inert bulk; oxygen is the part your body and the engine need. The proportions stay the same as you climb, but the total pressure falls, so each breath holds less oxygen with height.

Note: the exact 78/21 split is grounded in the FAA Aviation Weather Handbook (FAA-H-8083-28) Table 4-1, not the local TC AIM (which mentions nitrogen "80 percent" only in a decompression-sickness context).

Trap: it is nitrogen at ~78% and oxygen at ~21%, not the other way round.

Practice