system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens to the heart during CPR?
gpt
The compression on the chest squeezes the heart, pushing some blood out of it. When the compression stops the change in pressure allows the blood to fill the heart again, much as it does between heartbeats. The valves in the heart keep everything going in the right direction. 

The compressions also ventilate some air in and out of the lungs as the chest size expands and contracts. It is not as good as regular breathing though or having a bag mask, but it should be enough to keep the bare minimum of oxygen circulating.