Our lungs are like two spongy bags. Just as sponge can be filled with water and then squeezed dry again, so our lungs fill with air when we breathe in and then release most of the air again when we breathe out. As air enters the lungs it passes down a series of smaller and smaller tubes, which are known as the bronchial tubes, until it ends up in thousands of tiny air sacs. At this point oxygen from the air passes through the lining of the lung and into the blood. This oxygen is vital to the blood stream, as it helps to carry food energy to every other part of the body. Oxygen passes into the blood stream through the lungs, and so, in a similar way, waste products, such as carbon dioxide, pass out of the blood and are removed from the body as we breathe out again.