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In school, we learn that during inhalation, the diaphragm expands, causing air to get sucked into our lungs. You can feel this suction by putting your hand over your mouth while inhaling.

Why is that? Does the expanded capacity of the lungs cause the air outside my body to rush into my body to, shall we say, keep the lungs full?

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You are correct that your chest muscles are in fact pulling the lungs "open," which creates a pressure differential and draws air into the lungs. When the muscles relax, the chest cavity collapses to its original state, expelling the air (not 100% of it!). You may have heard of a "collapsed lung" injury. What happens there is that the lung is ripped loose from the surrounding muscles, and thus cannot be reinflated.

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  • $\begingroup$ Put simply: Air abhors a pressure difference. $\endgroup$ – moonman239 Nov 6 '15 at 16:51
  • $\begingroup$ @moonman239 Actually, all matter abhors a pressure difference. Solids just have a hard time reacting in a quick and dramatic way; fluids cannot fill the volume, just move in the direction of the pressure to the extent possible. $\endgroup$ – Carl Witthoft Nov 6 '15 at 17:19
  • $\begingroup$ I still have my favorite physical law: Nature abhors a vacuum. It especially seems to apply to computers and software, and even to wallets :) $\endgroup$ – Mike Dunlavey Nov 7 '15 at 0:43
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From what I've gathered, I think my initial guess is correct. Air tries to maintain a constant pressure. According to the ideal gas law, there are two ways to maintain the same amount of pressure with an increasing volume: 1) increase the amount of gas, and 2) increase the temperature of the gas.

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Just like when you "enlarge" an accordion the air rush inside. In both cases, muscles pull the air bag, in order to suck air.

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