they cannot breathe for themselves
they need more oxygen than is present in the air.
The air is pumped into the lungs, moved around the body by the blood cells and then the waste gasses are released when the patient exhales. This exhale is a reaction to air being pumped into the lungs. The natural elasticity of the lungs pushes the gasses out and this is usually removed from the body by a tube containing a one way valve.
There is a chamber of air in the ventilator that is compressed and the air is pushed into the lungs of the patient. In the past, this has just been room air, but with the ventilators that we now have in hospitals throughout the world, doctors and nurses choose what goes in them. This can be anything from 21% oxygen to pure oxygen.
The air is pushed into the lungs of the patient via a tube. This tube can either be a nasal tube that goes into the nostrils of the patient, in the form of a mask or it can be given to the patient by a tracheotomy. A tracheotomy is where a tube is inserted into the windpipe of the patient, in their chest or lower neck. This is said to be invasive but it has been proven to be the best way to use a ventilator on a patient who is unconscious and will remain in an unconscious state, on a ventilator for some time.