Final answer:
The direction and magnitude of air flow during breathing is determined by changes in intrapleural pressure, which is the force exerted by gases in the pleural cavity. Intrapleural pressure is typically lower than intra-alveolar pressure, resulting in a pressure gradient that allows air to flow during breathing.
Step-by-step explanation:
The direction and magnitude of air flow during breathing is determined by changes in intrapleural pressure. Intrapleural pressure is the force exerted by gases in the pleural cavity. Typically, intrapleural pressure is lower, or negative to, intra-alveolar pressure. The difference in pressure between intrapleural and intra-alveolar pressures is called transpulmonary pressure.