Respiratory system Ventilation
To inhale, the lungs expand, decreasing the air pressure in the lungs. As the air pressure inside the lungs are lower when it has expanded, air from outside at higher pressure comes rushing in to the area of low pressure in the lungs. Biology Made Simple'. The lungs are made up of many tubes or branches. As air enters the lungs, it first travels through branches called the bronchus, 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 exhaled air has a high concentration of carbon dioxide and a low concentration of oxygen. The maximum volume of air that can be inhaled and exhaled is called the vital capacity of the lungs and is up to five liters.