![]() However, due to certain characteristics of the lungs, the intrapleural pressure is always lower than, or negative to, the intra-alveolar pressure (and therefore also to atmospheric pressure). Similar to intra-alveolar pressure, intrapleural pressure also changes during the different phases of breathing. Intrapleural pressure is the pressure of the air within the pleural cavity, between the visceral and parietal pleurae. It equalizes at 760 mm Hg but does not remain at 760 mm Hg. Alveolar pressure changes during the different phases of the cycle. Because the alveoli are connected to the atmosphere via the tubing of the airways (similar to the two- and one-liter containers in the example above), the interpulmonary pressure of the alveoli always equalizes with the atmospheric pressure. Intra-alveolar pressure is the pressure of the air within the alveoli, which changes during the different phases of breathing (Figure 2). A pressure that is equal to the atmospheric pressure is expressed as zero. Therefore, negative pressure is pressure lower than the atmospheric pressure, whereas positive pressure is pressure that it is greater than the atmospheric pressure. Typically, for respiration, other pressure values are discussed in relation to atmospheric pressure. One atm is equal to 760 mm Hg, which is the atmospheric pressure at sea level. Atmospheric pressure can be expressed in terms of the unit atmosphere, abbreviated atm, or in millimeters of mercury (mm Hg). ![]() Atmospheric pressure is the amount of force that is exerted by gases in the air surrounding any given surface, such as the body. Pulmonary ventilation is dependent on three types of pressure: atmospheric, intra-alveolar, and interpleural. If the two- and one-liter containers were connected by a tube and the volume of one of the containers were changed, then the gases would move from higher pressure (lower volume) to lower pressure (higher volume). In this formula, P 1 represents the initial pressure and V 1 represents the initial volume, whereas the final pressure and volume are represented by P 2 and V 2, respectively. Boyle’s law is expressed by the following formula: Therefore, the pressure in the one-liter container (one-half the volume of the two-liter container) would be twice the pressure in the two-liter container. Pressure and volume are inversely related. Likewise, if volume decreases, pressure increases. Boyle discovered that the pressure of a gas is inversely proportional to its volume: If volume increases, pressure decreases. Boyle’s law describes the relationship between volume and pressure in a gas at a constant temperature. At a constant temperature, changing the volume occupied by the gas changes the pressure, as does changing the number of gas molecules. Therefore, the pressure is lower in the two-liter container and higher in the one-liter container. In this case, the force exerted by the movement of the gas molecules against the walls of the two-liter container is lower than the force exerted by the gas molecules in the one-liter container. For example, a certain number of gas molecules in a two-liter container has more room than the same number of gas molecules in a one-liter container. In a gas, pressure is a force created by the movement of gas molecules that are confined. ![]() Inspiration (or inhalation) and expiration (or exhalation) are dependent on the differences in pressure between the atmosphere and the lungs.
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