All body cells need air, or oxygen, to perform their different functions. Without oxygen, these functions would stop completely. Throughout your life you breathe in oxygen continuously and automatically without saying, “I will breathe,” or “I won’t breathe.”
Once this oxygen reaches your lungs, it enters small air sacs called alveoli through the alveoli’s very thin walls. These walls are filled with tiny blood vessels, or capillaries. The oxygen enters the capillaries, which carry it to your heart. Your heart then pumps the oxygen-filled blood to all the cells in your body.
With each breath, you inhale about 1 pint of air. This new air then mixes with the air already in your lungs. However, when you are running or exercising, you can inhale up to 4 quarts of air with each breath!