A Little Pressure
In addition to its higher pressure, the lower atmosphere has a higher partial pressure, which is the amount of a gas that will dissolve into a liquid. In essence, the total pressure of the air is the sum of the partial pressures of oxygen and nitrogen. The greater the partial pressure of a gas, the more of it will be pushed into a liquid. In the case of the human body, we’re most concerned with the amount of oxygen that dissolves into our blood, which transports the oxygen to the body’s cells.
Remember, oxygen, which the body uses to fuel most of its key physiological functions, represents only about 21 percent of the make-up of the air, while nitrogen, which the body hardly uses, comprises most of the rest of the atmosphere. So oxygen, which amounts to about a fifth of the gas in the air, exerts only about a quarter of the 14.7 pounds per square inch of the total pressure. (The reason a fifth of the gas exerts a quarter of the partial pressure is that oxygen weighs more than nitrogen.)
As air pressure gets lower, the partial pressure of the gases that make it up also decreases. In human terms, that means the oxygen that is so important to running the body has a harder time getting “pressed” into the bloodstream because of the lower partial pressure.
In addition to providing the lifting force that permits us to fly, the atmosphere also acts as an effective radiation filter and a shield from falling objects. It helps destroy small stones and ice particles from space before they thump us on the head, and it filters out some of the sun’s damaging rays that can cause cancer. Airline pilots and flight attendants are thought to have a higher rate of some types of cancer because they spend so much fame in the high atmosphere, where the sun’s rays are not fully filtered. Most passengers, though, don’t need to worry because they don’t spend enough time in the air for the radiation to have Tasting effects.