Blood flows as the heart pumps, it's like the blood won't circulate unless the heart pumps, something like that Gas exchange happens in alveoli, the tiny like structure present in the lungs, but first the inhaled air must pass through trachea, then the bronchi, then the bronchioles, then to the alveoli, then vice versa, That's all I know, I hope I helped^_^, To God be the glory