Normal, spontaneous breathing begins with contraction of the muscles of inspiration, which effect expansion of the thoracic cage and create a pressure in the pleural space that is negative with respect to that of the atmosphere. Since the airway pressure in the lung at end-expiration is in equilibrium with the atmosphere, the transthoracic negative pressure gradient generated by inspiration is transmitted across the lung parenchyma, and the lung expands.
Mechanical ventilation, in contrast, usually refers to a machine blowing gas into the airway during inspiration, with lung inflation resulting from the pressure generated, which is positive relative to the atmosphere. During either spontaneous breathing or mechanical ventilation, the relationship of inflating pressure (negative or positive) to volume is defined as "compliance" (Figure). Expansion is limited by the amount of pressure generated or applied, by the volume of the lungs, and also by the inherent property of elastic recoil in both
Shapiro MB, Bartlett RH. Pulmonary Compliance and Mechanical Ventilation. Arch Surg. 1992;127(4):485-486. doi:10.1001/archsurg.1992.01420040131024