Up and down. Up and down. This is what your chest does several times every minute of every day. It seems pretty simple, doesn't it? But hidden under your skin and beneath your rib cage is a very complex organ that causes the constant rise and fall of your chest: your lungs. Your lungs are one of the largest organs in your body. The outside of your lungs look pink and rubbery. The inside of your lungs look a lot like sponges.
Every time you inhale, your chest rises. A breath of air travels in through your nose or mouth. Then it makes its way down your trachea or windpipe. Finally it arrives in your lungs. The air then fills up millions of air sacs. Six hundred million, to be exact! These sacs, or alveoli, let the oxygen from the air pass into your bloodstream. The newly oxygenated blood travels all the way to your heart. Your heart then pumps that blood all through your body!