Inspiration and expiration follow Boyle’s Law: as volume increases, pressure decreases. Thus, expansion of the lungs on inspiration increases lung volume, pressure becomes lower inside the lungs than outside and air flows into the lungs. Expiration decreases the volume of the lungs. Pressure increases slightly within the lungs and the air flows back out.
The ability of the lungs to stretch is compliance. Stiff lungs have low compliance. They require more work to stretch them. Expanded lungs are constantly pulling against the thoracic wall, because they are elastic. Expanding the lungs is somewhat like stretching a rubber band. This is why it takes energy for inspiration and quiet expiration can be passive. High compliance lungs stretch readily, but are not elastic. They require forced expiration to empty.
The lungs are tightly attached to the thoracic wall, so the pulling of lungs due to elasticity causes a negative intrapleural pressure. Regardless of the stage of inspiration or expiration, the intrapleural pressure remains negative, keeping the lungs open. Disruption of the pleural sac (say, by a knife wound) disrupts the connection between the lungs and thoracic wall and the elasticity of the lungs causes them to collapse.
Two measures are commonly used to describe inspiration and expiration. Minute volume is the volume breathed per minute, so it equals the tidal volume x respiratory rate. Peak expiratory flow measures the maximum speed of expiration (volume / time).
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