So how does gas exchange actually take place within each individual alveolis? Within our alveolar sac? So recall that the rich ventricle of the heart pumps deoxygenated blood into the pulmonary trunk, which extends into the pulmonary arteries. And these arteries bring deoxynated blood into the lungs. Now, eventually, the pulmonary arteries divide into smaller arteries, and they ultimately divide into these pulmonary arterioles that is shown by number five. And these pulmonary arterioles essentially circle around the alveoli until they connect with the pulmonary capillary.