Commercial aircraft cabins are usually pressurized to the pressure of the atmosphere at about \(2 \mathrm{km}\) above sea level. Why don't you feel the lower pressure on your entire body?

Atmospheric pressure is a fundamental concept that affects everything from weather patterns to how we breathe. It is defined as the weight of the air above us, exerted on every surface it contacts. At sea level, the standard atmospheric pressure is 101.325 kilopascals (kPa). As we increase in altitude, atmospheric pressure decreases because the density of the air becomes lower and there's less air pushing down from above. This is why, at high altitudes, humans can experience altitude sickness due to the lack of oxygen and lower pressure.

Commercial airliners counteract this effect by pressurizing cabins to mimic lower altitude conditions. It's like creating a bubble of sea-level atmosphere that travels with passengers high above the actual sea level. If this wasn't done, the drastic difference in pressure as the plane ascends would lead to discomfort and potentially dangerous health effects.

The human body is adept at handling changes in pressure, up to a certain point. This capability is due to our body's composition – primarily water, a virtually incompressible fluid, and solid tissues. However, air-filled cavities such as ears and sinuses can experience pressure differences that need to balance out, which is why you might experience ear popping during ascend and descend of a flight.

The equilibrium in our inner ear adjusts to changes in external pressure, thanks to the Eustachian tube, which balances pressure on both sides of the eardrum. In an airplane, the cabin pressurization system is designed to minimize the pressure gradient and allow our bodies to equalize internal pressure more comfortably with that of the cabin air. The effective management of cabin pressure is crucial for passenger comfort and health, preventing potential pressure-related issues such as barotrauma or decompression sickness.

Pressure gradient equilibrium is the balance that must be achieved between two pressure systems. For humans, this is especially relevant when there are sudden changes in altitude, such as during ascent and descent in a flight. The cabin pressurization system works to maintain a steady pressure gradient between the inside of the aircraft and the atmospheric pressure outside as it climbs or descends.

The goal is to reproduce a pressure environment similar to what we experience up to about 2 kilometers above sea level. Maintaining this equilibrium minimizes the danger of hypoxia, where the body is deprived of adequate oxygen supply, and other potential ailments due to pressure differences. The carefully controlled pressurization takes into account the pressure gradient force, which is the driving force for the movement of air and facilitates the comfortable transition for passengers as the aircraft changes altitude.