The lungs are made of many tubes or branches. As air enters the lungs, it first goes through branches called the bronchi, then through smaller branches called bronchioles, and finally into the air sacs. Gas exchange occurs in the air sacs where oxygen is exchanged with carbon dioxide. The carbon dioxide in the air sacs now need to be exhaled, or breathed out. In the reverse process to inhaling, the diaphragm and the rib muscles relax, causing the lungs to be smaller. As the air pressure in the lungs is greater when the lungs are smaller, air is forced out. The air sacs are able to contract and expand because they are coated with a lubricant, surfactant, this lubricant helps the air sacs inflate while also preventing the collapse of the lungs during deflation.The exhaled air has a high concentration of carbon dioxide and a low concentration of oxygen. The maximum volume of air that can be breathed in and breathed out is called the vital capacity of the lungs and is up to five liters.