Each alveolus is surrounded by a web of blood capillaries supplied by the
pulmonary artery. The pulmonary artery carries deoxygenated blood from the heart to the lungs, where it branches off into smaller vessels until it reaches the capillaries surrounding the alveoli. Here, oxygen from the breathed air diffuses through the thin walls of the alveoli and into the blood, while carbon dioxide from the blood diffuses into the alveoli to be exhaled. The oxygen-rich blood then travels back to the heart through the pulmonary veins, where it is pumped to the rest of the body.
pulmonary artery and the pulmonary veins. The pulmonary artery carries deoxygenated blood from the heart to the lungs, where it divides into smaller arteries and eventually into a network of capillaries that surround the alveoli. In the alveolar capillaries, the exchange of gases occurs - oxygen from the inhaled air diffuses into the bloodstream, while carbon dioxide from the bloodstream diffuses into the alveoli.
After oxygen and carbon dioxide exchange takes place in the alveoli, the oxygen-rich blood is carried away from the capillaries by the pulmonary veins. The pulmonary veins transport the oxygenated blood back to the heart, specifically to the left atrium. From the left atrium, the blood is then pumped into the left ventricle and subsequently distributed to the rest of the body through the systemic circulation.
This continuous flow of deoxygenated blood to the lungs through the pulmonary artery, exchange of gases in the alveolar capillaries, and transportation of oxygenated blood back to the heart via the pulmonary veins allows for the exchange of oxygen and carbon dioxide during respiration.