Federal Aviation Administration regulations require the use of supplemental oxygen, either through an oxygen mask or cabin pressurization, for flights above 12,500 feet. The oxygen content of a pressurized cabin must be at least equal to that of outside air at about 8,000 feet. To provide air that is dense enough to breathe safely, the airplane must first be sealed tightly. Then air is pumped in off of the plane's engines. In the course of normal operation, a jet engine sucks in air through intake ducts.
Fans compress the air and injectors add fuel.
The mixture is ignited and the hot gasses that result stream out the back, pushing the plane forward. Before fuel is added, some of the air is siphoned off from the turbocharger and funneled into the plane. Because it has been compressed, the air is quite hot, so it is sent through air conditioning packs before it is pumped into the cabin. To maintain proper internal pressure inside the cabin, air is released through an outflow valve as new air is pumped in to the cabin, or as the plane descends. On older airplanes, all of the air that circulated in the cabin was sucked in from outside.