Air planes have pressurised cabins for passenger safety while flying high in the atmosphere.
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Answer:
At the higher altitudes, the air is less dense. This means two things: 1. an airplane needs less fuel, and 2: humans need help to breathe.
Our bodies depend on the partial pressure of oxygen in the atmosphere to help the oxygen cross through our blood vessels in our lungs. On the ground is the optimal partial pressure. When we increase our altitude, the ability of oxygen to penetrate the blood vessels in our lungs at the lower partial pressure decreases. So, in an airplane, the air is compressed so that our bodies think that they are on the ground, and the compressed oxygen is more easily able to pass thru our lungs. Typically, this compression results in a cabin altitude of about 8,000 feet when the airplane is about 35,000 feet up.
In an emergency, if the airplane loses pressurization, there is supplemental oxygen on board, and its use is demonstrated before every flight. This supplemental oxygen will increase the amount of oxygen available to compensate for the lesser amount of oxygen available at the higher altitudes. The captain will then perform a maneuver called the “high dive”… a rapid descent from where you are to about 10,000 feet, where it is no longer necessary for the supplemental oxygen because the partial pressure of oxygen allows it to cross into our lungs again.
BTW, did you know that when you breathe, you actually create a vacuum in your lungs and the air rushes in to fill the vacuum