Final answer:
Transpulmonary pressure, which is the difference between intrapleural pressure and intra-alveolar pressure, is what keeps the lungs from collapsing. This pressure allows for lung inflation and deflation during breathing and maintains lung volume.
Step-by-step explanation:
The pressure that keeps the lungs from collapsing is the transpulmonary pressure, which is the difference between the intrapleural pressure and the intra-alveolar pressure (intrapulmonary pressure). Intrapleural pressure is always negative relative to intra-alveolar pressure, which means it is lower than both the intra-alveolar pressure and atmospheric pressure. During breathing, the natural elasticity of the lungs and the chest wall creates these pressure gradients that keep the lungs expanded.
Transpulmonary pressure determines the size of the lungs. A higher transpulmonary pressure corresponds to larger lungs. This pressure develops because of the competing forces of lung elasticity and surface tension pulling inwards, and the outward pull of the pleural fluid and thoracic wall. The careful balance of these pressures allows the lungs to inflate and deflate without collapsing.