Expanding your chest draws air into your lungs - that may seem obvious. To understand how breathing really works, we need to learn a little bit about gas pressures. Pressure is caused by gas molecules striking the wall of their container. The pressure exerted by the gas molecules depends on the size of the container. When we increase the volume of the container without adding molecules, the molecules strike the container walls less often, thus exerting less pressure against the walls. When we reduce the volume of the container, the gas molecules strike the wall more often, thus exerting more pressure. These observations illustrate Boyle's Law, which states that the pressure of a gas is inversely proportional to the volume of its container. Thus, if you increase the volume of a container, the pressure will decrease, and if you decrease the volume of a container, the pressure will increase. To understand how Boyle's Law affects breathing, we need to define pressures related to ventilation. Atmospheric pressure is the pressure of the air around us. It is due to the weight of the overlying atmosphere. At sea level, it is about 760 millimeters of Mercury. Intrapulmonary pressure is the air pressure within the alveoli of the lungs. Intrapleural pressure is the pressure found within the pleural cavity. The intrapleural pressure is normally less than intrapulmonary pressure. This occurs because of the lungs' elastic tendency to collapse, which is opposed by the chest's elastic tendency to expand. The intrapleural pressure creates a vacuum or suction effect, pulling the lungs outwards towards the walls of the thoracic cavity. This is similar to the wet suction cups of a shower caddy adhering to the shower wall. When the suction cups expand outwards, this decreases the pressure between the wall and the cup, helping the suction cup to adhere to the wall. The cohesive properties of the water also help it to adhere, holding the caddy in place. Therefore, when contraction of the inspiratory muscles expands the thoracic cavity, the suction will pull the lungs outwards with the chest wall, expanding the lungs. Now that you have learned how pressure and volume are related, you need to understand that pressure differences drive the movement of gases. Gas molecules will always move from areas of greater pressure to areas of lower pressure... until the pressures in the two areas become equal. Let's see how this works during ventilation. When the lungs are at rest, air is not moving into or out of the lungs. This is because intrapulmonary pressure and atmospheric pressure are equal. To breathe in, your muscles expand the thoracic cavity and therefore the lungs. This lowers intrapulmonary pressure below atmospheric pressure. Air flows down its pressure gradient into the lungs - that is, you inspire. The flow of air ceases when intrapulmonary pressure again becomes equal to atmospheric pressure. During expiration, the volume of the lungs decreases, increasing intrapulmonary pressure beyond atmospheric pressure. Air flows out of the lungs down its pressure gradient until the intrapulmonary pressure again equals atmospheric pressure. Now let's look at what happens if the chest wall is punctured, creating an opening between the atmosphere and the pleural space. Remember that the intrapleural pressure is normally less than intrapulmonary and atmospheric pressure. When the chest wall is punctured, air moves down its pressure gradient from the atmosphere into the pleural cavity, a condition known as pneumothorax. This influx of air increases the intrapleural pressure, reducing the suction, that holds the lung open. Without this suction, the lung on that side begins to deflate. Notice how the chest wall expands outward while the lung collapses inward, each structure assuming a natural position, as their elastic tissues recoil. If air continues to enter, the lung may collapse fully. At that point, the pressure in the pleural space will equal atmospheric pressure.