Gas exchange occurs between the air in the alveoli and the red blood cells, located in the blood in the capillaries. During gas exchange, oxygen from the air in the alveoli passes into the red blood cells in the capillaries. The blood becomes oxygen-rich. At the same time, carbon dioxide, a waste product carried in the blood and red blood cells, moves into the alveoli. The air rich in carbon dioxide flows out of the alveoli and bronchial tubes toward the nose and mouth. This is known as exhalation.

Meanwhile, the oxygen-rich blood flows back to the heart through the pulmonary veins. The heart then pumps the blood to the rest of the body. Exhalation occurs when your diaphragm and rib cage muscles relax to make the chest cavity smaller. Air rich in carbon dioxide is forced out of the lungs and through the nose and mouth. The lungs are now prepared for another breathing cycle.