Final answer:
The intrapleural pressure is the lowest pressure at rest during respiration, typically around –4 mm Hg and always lower than atmospheric and intra-alveolar pressures.
Step-by-step explanation:
The question asks which is the lowest pressure at rest during respiration between transpulmonary, atmospheric, intra-alveolar, and intrapleural pressures. During the respiration cycle, the intrapleural pressure is always lower than atmospheric pressure and intra-alveolar pressure, typically remaining around –4 mm Hg. This negative pressure is essential for keeping the lungs expanded. Atmospheric pressure is equal to the pressure exerted by the environment and is equivalent to one atmosphere or 760 mm Hg at sea level. The intra-alveolar pressure, or intrapulmonary pressure, changes with the phases of breathing but always equalizes with the atmospheric pressure. Therefore, it is not lower than atmospheric pressure. The pressure difference between intrapleural and intra-alveolar pressures is termed transpulmonary pressure, which reflects the distending pressure across the lung itself. Given these points, the correct answer is that intrapleural pressure is the lowest pressure at rest during respiration.