The rib cage, lungs, and diaphragm all work together to help us breathe. Well show you how this works. Well focus on what happens inside the chest cavity. the chest cavity Here is a drawing of the rib cage which surrounds the chest cavity. rib cage Here well show a simplified diagram of the rib cage. rib cage We show the ribs as if they are cut through here. This is called cross section. rib cage ribs At the bottom of the chest cavity, there is a large band of muscle, called the diaphragm. The diaphragm is connected to the rib cage to form a totally sealed cavity. Now well have a look at whats inside the chest cavity. rib cage ribs diaphragm The lungs take up most of the space in the chest cavity. We are showing them as pink here. right lung rib cage left lung diaphragm There are two strong, thin membranes around each lung. We show them here as black lines. (click) The outer membrane is stuck to the inside of the wall of the chest cavity. right lung membrane stuck to wall of chest cavity rib cage left lung membrane stuck to wall of chest cavity diaphragm And the inner membrane is stuck to the outside surface of the lung. These strong membranes protect the lungs and the chest cavity wall as the lungs move. right lung membrane stuck to wall of chest cavity membrane stuck to lung rib cage left lung membrane stuck to wall of chest cavity membrane stuck to lung diaphragm Between these two membranes is a cavity filled with watery fluid. This layer of fluid provides lubrication so movements of the lungs dont irritate tissues. It also helps to stick the outside surface of the lung to the wall of the chest cavity. right lung membrane stuck to wall of chest cavity membrane stuck to lung rib cage fluid filled cavity rib cage left lung membrane stuck to wall of chest cavity membrane stuck to lung fluid filled cavity diaphragm Lets look at what happens during inhalation. (click) The muscles between the ribs contract and pull the rib cage upward and outward. rib cage pulled upward and outward At the same time, diaphragm muscles contract and pull the diaphragm downward. Lets watch as this happens. rib cage pulled upward and outward diaphragm pulled downward rib cage pulled upward and outward diaphragm pulled downward If we watch this again, we see that as this occurs, the volume inside the chest cavity increases. Now when the volume inside a sealed container increases, the pressure inside will decrease. So as we pull the rib cage out and up and pull the diaphragm down, we are DECREASING the pressure inside the chest cavity. When the pressure inside the chest cavity is decreased, it becomes lower than the pressure of the surrounding atmosphere. Air will always move from an area of higher pressure to an area of lower pressure. So which way do you think air will move? atmospheric pressure We see that air moves in through the air passages in order to equalize the decreasing pressure inside the chest cavity. And that is the way we draw air into our lungs. AIR So lungs do not have muscles of their own! We are able to inhale because (click) muscles in the rib cage pull it up and out, while muscles in the the diaphragm contract to pull it down. (click) And as this happens air moves in to equalize the decreasing pressure in the chest cavity. rib cage pulled upward and outward diaphragm pulled downward AIR