you need to know diff bt negative pressure ventilation (which is normal ventilation) and positive presssure ventilation (which is mechanical ventilation). In negative pressrue ventilation the diaphragm contracts making a - intrapleural pressure which allows alveoli to expand. in positive ventilation (the pt diaphragm is not contracting thus not expanding chest cavitiy and not creating the - pressure) the machine is creating positive pressure inside alveoli so so alveoli expands. :)

During normal inspiration, alveoli starts from FRC(0mmH2O) to -1 mmh2O; this NEGATIVE pressure sucks air in. In PPV, the machine is pushing air making person do "inspiration" but now alveolar pressure is POSITIVE. & then expiration occurs passively (by elastic recoil). PEEP is an additional VERSION of PPV. Here, we do PPV but, in expiration, after some passive recoil (END EXPIRATORY PRESSURE), the machine pushes some air. So the lung does not fully deflate and return back to FRC. That's how we stabilize the small alveoli from getting collapsed after every PPV. REMEMBER, in PEEP, lung NEVER GOES BACK TO FRC. So, Alveolar pressure will ALWAYS be POSITIVE. Normally IPP is -5 after EXPIRATION (END-TIDAL) and -8 after INSPIRATION. Here we are giving +10 with Ventilation. That will overcome normal END-TIDAL (-5) making END-TIDAL IPP to +5. And of course, this +5 has no chance to become even +4 because the machine will push the air during inspiration which will make IPP even more +ve.PEEP is used to Rx Hypoxemia of ARDS. The downside is, this +ve IPP decrease VR to Rt heart and also Lt heart> reducing Cardiac Output.

Here is an Awesome video that will help: https://www.youtube.com/watch?v=24gsHagkL_8

imagine the patient is a balloon and were inflating him, everything will be up, there won't be any negative pressure ie suction