Everything starts when you breathe in through your nose and mouth. Air flows down through the windpipe, past the voice box or vocal cords, to where the lower–most ribs meet the center of your chest. Your windpipe divides into two tubes, which lead to the two lungs, which fill most of your ribcage. This is known as the Pulmonary Circulation System, or the Respiratory System.

Lungs are sponge–like. Inside each lung are tubes, called bronchi, which branch into even smaller tubes, much like the branches of a tree. At the end of these tubes are millions of tiny bubbles or sacs called alveoli.