When you breathe in via your nose or mouth, air goes down the pharynx (back of the throat), through the larynx (voice box), and into the trachea (windpipe). The trachea is separated into two air channels known as bronchial tubes. One bronchial tube connects to the left lung, while the other connects to the right lung. The bronchi divide into smaller and smaller branches called bronchioles. Each bronchiole ends in a tiny capillary that leads to a small alveolus (air sac). Alveoli fill with oxygen and release it into the blood stream through microscopic holes called pulmonary veins. The heart lies in the middle of this network of blood vessels and airways called the pulmonary system. It is connected to it by the pulmonary artery and vein which carry deoxygenated blood and oxygen-rich blood, respectively, into and out of the heart. Air flows from the top of the lungs down to the bottom. This movement creates a vacuum that draws in more air. When you breathe in, you expand your chest muscles to allow more air into your lungs. As you breathe out, your chest and belly button will go up and your back will come down. This moves more air out of your lungs. The pulmonary system is very important for survival because it allows us to breath air and remove carbon dioxide from our bodies. If it were not able to do this, we would die after a few minutes without oxygen.