In our discussion about how the lungs work, we said that you need oxygen as fuel for everything you do. To help you better understand how the heart and lungs interact with each other, let's follow the journey of an oxygen molecule in the air you inhale. Starting from your mouth or nose, the oxygen molecule moves through smaller and smaller airways until it reaches tiny air sacs known as alveoli. There, the oxygen molecule moves across the thin walls of the alveoli and blood vessels into the blood where it is carried inside a red blood cell. As you may remember from our discussion of how the heart works, this blood was pumped from the right side of the heart and is now passing through the lungs on its way to the left side of the heart. After being carried in the blood to the left side of the heart, the red blood carrying the oxygen molecule is then pumped out to the rest of the body.