Breathing is vital to staying alive. Us humans need oxygen so that our bodies can perform all kinds of functions that keep us from pushing up daisies.
Starting with a broad view of breathing, inside the thoracic cavity, the diaphragm is a muscle attached to the bottom of our lungs. Its function is to pull downwards, expanding our lungs, creating a vacuum that forces air through our mouth or nose and into the lungs. The diaphragm then pushes back up, in turn moving air back out again when we exhale.
As we inhale, air is sucked through the mouth or nose, down the pharynx, also called the throat. Air further travels down through the larynx and deeper through the trachea. The trachea splits off into the bronchial tubes, each tube connecting to a lung. The bronchial tubes, also called bronchi, branch into more bronchi, more and more until the smaller branches are called bronchioles. The bronchioles split off into tiny air sacs called alveoli. Deoxygenated blood goes to the alveoli and makes an exchange of carbon dioxide, or CO2, for oxygen, O2.
Replenished and oxygenated blood is then transported to the heart through the pulmonary vein, one of the very few veins that carry oxygenated blood. The heart pumps the oxygen-rich blood through large arteries that carry blood all throughout the body, branching off into arterioles and finally down to capillaries. The capillaries are the blood vessel that makes direct exchanges with cells. The cells receive oxygen and the capillaries receive CO2, cell waste and other products.
The capillaries join into venules, which connect into veins. The veins then bring the deoxygenated blood back to the lungs, and the process repeats, with the diaphragm sucking air into the lungs, the alveoli and capillaries exchange CO2 for O2, and the pulmonary vein transporting the fresh blood back to the heart.