How does it work
Doing CPR in a person whose heart has stopped beating can restore the flow of blood. Therefore, oxygen (and other nutrients) can be circulated to the organs while the metabolic wastes can be transported away from them. There are two theories that explain how chest compression can achieve this purpose: the cardiac pump theory and the thoracic pump theory.
First, the cardiac pump theory postulates that blood flows because the heart is squeezed between the sternum and the spine during chest compression.
Second, the thoracic pump theory postulates that blood flows from the thoracic cavity because the pressure inside the thoracic cavity is greater than the pressure inside the blood vessels which are outside the thoracic cavity. Blood can then flow forward from the thoracic cavity (the heart) to the periphery because of the pressure difference. Backward flow is restricted by the valves inside the blood vessels (the veins).
In real life, both theories play a role in chest compression CPR and they are not mutually exclusive.