The transpulmonary pressure (PL) is calculated as:

Transpulmonary pressure (PL) is defined as the difference between alveolar pressure and pleural pressure. It reflects the pressure gradient that determines the lung's ability to expand; essentially, it is the force that keeps the lungs inflated against the elastic recoil of lung tissues. A higher transpulmonary pressure indicates a greater potential for lung expansion, which is critical during mechanical ventilation and when assessing a patient's respiratory mechanics.

In normal physiology, the alveolar pressure is the pressure within the lung's alveoli, while the pleural pressure is the pressure in the pleural cavity surrounding the lungs. Since pleural pressure is typically negative relative to atmospheric pressure, subtracting pleural pressure from alveolar pressure gives a true indication of the distending force acting on the lungs. This calculation is crucial in understanding lung mechanics and managing ventilatory support.

The other options do not accurately represent the relationship required to calculate transpulmonary pressure. Thus, the definition and calculation of transpulmonary pressure as alveolar pressure minus pleural pressure is a fundamental concept in mechanical ventilation and respiratory physiology.