Breathless …

So you’re sitting comfortably in your seat when you feel a tell-tale tickle or pressure in your ears. Maybe you try to pinch your nose and blow gently or swallow hard to clear the problem, wondering what’s going on. You haven’t noticed the plane descending, which is the time when ear pressurization-equalization issues usually happen. Maybe the aircraft is experiencing some sort of problem (or routine adjustment?) in the pressurization system.
PressValve
Since the advent of jet liners (and high altitude flight) the cabin air has to be kept at a greater pressure than the outside air or we’d pass out from lack of oxygen. This is accomplished by making the fuselage (more-or-less) airtight, then forcing in air. A special outflow valve gives us  precise control over pressure changes inside the plane. These valves are usually located on the lower fuselage of the aircraft, near the rear. The next time you’re sitting at the departure lounge or looking out the window while parked at the gate, look for the valves on the planes around you. They look like a significant-sized hole (circular or rectangular) equipped with a moveable valve (see the photo for a typical example). On the ground they are normally fully opened. This valve is gradually closed and regulated by an automatic sensing system, once the flight begins.
Aircraft cabin pressure problems are extremely rare thanks to reliable and redundant designs of the systems and constant vigilance by the pilots, but when they do happen they are considered, potentially, very serious. This is because when our brains go without oxygen too long we suffer brain damage and even death. So, maintaining adequate cabin pressurization is critical to our survival.
The degree of danger from pressurization problems depends upon a few factors such as our altitude and and how suddenly pressure changes occur.

NEXT — The problems we face when control of pressurization is lost…

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