Respiratory system
The carbon dioxide in the air sacs now needs to be exhaled or breathed out. The diaphragm and the rib muscles relax, making the lungs smaller.  As the air pressure in the lungs is greater when the lungs are smaller, the 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.