Final answer:
During chest recoil in CPR, a vacuum is created, decreasing thoracic pressure and aiding the return of blood to the heart, while also allowing air to passively leave the lungs as lung tissues recoil.
Step-by-step explanation:
During the recoil of the chest in between compressions, a vacuum is created as the chest wall expands and the diaphragm relaxes. This vacuum leads to a decrease in thoracic pressure, which, according to Boyle's Law, allows blood to flow back into the relaxed thoracic cavity. This return of blood to the heart is crucial as it primes the heart with blood to be pumped out during the next compression, ensuring continuous circulation. Furthermore, this negative pressure also helps in the passive movement of air out of the lungs as the lung tissues recoil and the volume within the thoracic cavity decreases