Eli5: When doing CPR, why does it help to breathe into the person’s mouth, when we exhale co2?

Question

Eli5: When doing CPR, why does it help to breathe into the person’s mouth, when we exhale co2?

In: Biology

Answer 1

~~When you breathe normally, your lungs only absorb about 5% of the oxygen you inhale.~~ (edit: see response below) That oxygen is then replaced by c02. The air you exhale still has plenty of oxygen left in it.