Final answer:
Intrapulmonary pressure, or intra-alveolar pressure, always eventually equalizes with the atmospheric pressure as air moves to balance the pressures during the breathing cycle.
Step-by-step explanation:
Intrapulmonary pressure, also known as intra-alveolar pressure, fluctuates during the different phases of breathing. It is the pressure inside the alveoli of the lungs, which changes as we inhale and exhale. Importantly, the intrapulmonary pressure is always eventually equalized with the atmospheric pressure. Whenever there is a difference in pressure between the inside of the lungs and the surrounding atmosphere, air moves to equalize the pressures. Essentially, if the intrapulmonary pressure is greater than the atmospheric pressure, air flows out of the lungs, and if it is lower, air flows into the lungs.
This pressure equilibrium is critical for effective breathing, and is maintained despite various forces acting within the thorax, such as the elasticity of the lung tissues and the surface tension of the alveolar fluid. The difference in pressure between the intrapleural pressure and the intra-alveolar pressure is termed the transpulmonary pressure, which influences the size of the lungs during the respiratory cycle. While the intrapleural pressure remains negative relative to both the atmospheric pressure and the intra-alveolar pressure, due to the outward pull of the thoracic wall being slightly greater than the inward pull of the lungs, it does not equalize with the atmospheric pressure like the intrapulmonary pressure does.