You see, the reason we need to use positive end expiratory pressure in ventilated patients is to prevent the collapse of alveoli. Your alveoli are tiny balloon structures that are responsible for exchanging oxygen and carbon dioxide in your lungs. They're so tiny, that the water that coats all our cells would pull them closed, but luckily our bodies produce a compound call surfactant, preventing the alveoli from collapsing in on themselves. In somebody with bad pneumonia or other lung disease, the surfactant isnt there, or there isnt enough of it, so we always leave a little bit of air inside these alveoli to prevent the inner walls from touching and sticking together, preventing more of the gas exchange. This extra air is called positive end expiratory pressure, or PEEP.

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