Where does oxygen enter the blood and carbon dioxide leave blood?

Oxygen enters the blood in the lungs. The lungs are made up of millions of tiny air sacs called alveoli. The alveoli are lined with capillaries, which are tiny blood vessels. Oxygen from the air diffuses across the alveoli and into the capillaries. The capillaries then carry the oxygen-rich blood to the heart, which pumps it out to the rest of the body.

Carbon dioxide leaves the blood in the lungs. Carbon dioxide is a waste product that is produced by the cells of the body. The carbon dioxide diffuses from the capillaries into the alveoli. The alveoli then expel the carbon dioxide through the nose and mouth.