The thing being tested is the pressure gradient that actually drives air through the airways. The difference between airway opening pressure (Paw) and alveolar pressure (Palv) is the transairway pressure—the driving force for airflow across the conducting airways. When Paw is higher than Palv, air moves into the lungs; when Palv rises above Paw, air flows out. This gradient is central to how ventilators push air into the alveoli and how resistance in the airways shapes flow, since Flow roughly equals (Paw minus Palv) divided by airway resistance. Other terms describe different interfaces: transpulmonary pressure is Palv minus pleural pressure, reflecting the distending pressure on the lung itself; transthoracic pressure involves pressures across the chest wall (often related to airway or pleural pressures but not the direct Paw-to-Palv gradient); transrespiratory pressure refers to the pressure difference across the entire respiratory system (such as from the airway opening to the body surface) and does not specifically isolate the airway-to-alveoli gradient.