Blood that reaches the lungs travels throughout a network of small blood vessels, where oxygen moves into the blood and carbon dioxide moves out of the blood, according to the National Heart, Lung and Blood Institute. This oxygen-rich blood is transported through the pulmonary veins and back to the heart, where it is pumped out to the rest of the body.
The air exchange in the lungs is important to the body's overall functioning. Oxygen that enters the blood in the lungs is obtained through air that is breathed in, or inhaled. The National Heart, Lung and Blood Institute explains that inhaled air travels down the windpipe, through the bronchial tubes and into small air sacs called alveoli. Small blood vessels called capillaries surround the alveoli. Oxygen passes through the thin walls of the alveoli and into the blood in the capillaries. A protein in the red blood cells called hemoglobin helps to transport the oxygen. At the same time, carbon dioxide passes from the blood into the alveoli. When air is breathed out, or exhaled, the air contained in the alveoli, which is rich in carbon dioxide, is forced out of the lungs and exits the body through the nose and mouth.