Final answer:
After birth, air in the alveoli causes blood vessels to relax or dilate, which improves blood flow and facilitates the exchange of gases.
Step-by-step explanation:
After birth, air in the alveoli causes vessels to relax. This occurs because the partial pressure of oxygen in the alveoli increases, leading to the dilation of the pulmonary arterioles. This physiological response enhances blood flow in the area, facilitating the efficient exchange of carbon dioxide for oxygen. During this process, amniotic fluid that remains in the lungs after birth is absorbed, and the lungs begin to function independently, handling the task of gas exchange previously managed by the placenta.