So these cells are the endothelial cells of our pulmonary arterial, and these cells are the endothelial cells of our pulmonary Venuel. Okay, so now that we know what the structure of our alveolus actually looks like, let's discuss how gas exchange actually takes place and why. Oxygen is taken up by the capillaries, but carbon dioxide is released by the capillaries. 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.