Final answer:
Intrapulmonary or intra-alveolar pressure is the pressure within the alveoli that equalizes with atmospheric pressure when there is no air flow, such as at the end of expiration.
Step-by-step explanation:
Intrapulmonary pressure, also known as intra-alveolar pressure, is the pressure of the air within the alveoli. This pressure changes during the different phases of breathing. Since the alveoli are connected to the atmosphere through the airways, the intrapulmonary pressure always equalizes with the atmospheric pressure. Therefore, at rest, when there is no airflow in or out of the lungs, intra-alveolar pressure is equal to atmospheric pressure and expressed as zero. During inspiration, the volume of the alveoli increases, leading to a drop in pressure according to Boyle's law which states that in a gas, pressure decreases as volume increases. Conversely, during expiration, the volume of the alveoli decreases, resulting in increased intra-alveolar pressure. However, the important point to note is that between breaths, at the end of expiration, when no air is flowing, the pressure within the alveoli equalizes to atmospheric pressure.