ABSTRACT
FORCES INVOLVED IN AIR MOVEMENT Boyle's Law Air, being a mixture of gases, moves about simply: it passively follows gradients of pres sure established around it. To move air from the environment into the lung, the respiratory system must generate a reduced pressure in the lung relative to atmosphere, and to move the air out, the opposite pressure gradient must be created. These pressure gradients are generated by changing the volume of the respiratory system, as pressure (P) and volume (V) must change in opposite directions according to Boyle’s law: PV = constant (Fig. 1). Thus, during inspiration, the chest wall and muscles expend energy to increase the volume of the respiratory system, which abruptly drops alveolar pressure relative to atmo spheric pressure and causes air to flow into the lung. During expiration, the respiratory muscles relax, allowing the elastic recoil of the lung to increase alveolar pressure and therefore expel air back to the environment, which returns the chest wall and lung to their resting position (functional residual capacity, or FRC).
