Oxygen Enters the Body
When you take a breath, oxygen is moved in through the nose and into the trachea, or windpipe. The trachea splits into two tubes, or bronchi, which connect to the lungs, then branches out into smaller bronchioles. As the lungs expand and take in air, the diaphragm -- a muscle beneath the ribs -- contracts, reducing pressure in the chest cavity and creating suction.
Air Moves Through the Lungs
Once inside the lungs, oxygen travels from the bronchioles to millions of tiny capsules, or sacs, called alveoli. Inside the alveoli, oxygen is exchanged for carbon dioxide. The new oxygen is transported by red blood cells in the arteries throughout the body, where tissues use the oxygen for energy.
Carbon Dioxide is Expelled
In the process of using the oxygen, the body's cells create carbon dioxide, which the body cannot use. The carbon dioxide is moved by the blood inside the veins back to the lungs, where it is exchanged in the alveoli. When you exhale, the carbon dioxide is moved from the lungs, up through the trachea and out of the nose and mouth.