Final answer:
During inspiration, intrapulmonary pressure drops to less than atmospheric pressure, allowing air to flow into the lungs due to the created negative pressure gradient.
Step-by-step explanation:
During inspiration, or inhalation, the volume of the thoracic cavity increases, leading to a decrease in intrapulmonary pressure (also known as intra-alveolar pressure) according to Boyle's Law, which states that in a gas, pressure increases as volume decreases, and vice versa. The intrapulmonary pressure drops below atmospheric pressure, creating a pressure gradient that allows air to flow into the lungs. It's this gradient that is crucial for the process of inspiration, wherein air is drawn into the lungs due to the negative pressure relative to the atmosphere.
As stated, intrapleural pressure, the pressure within the pleural cavity, is always negative compared to intrapulmonary pressure, even during the changes that occur throughout the breathing cycle. Thus, during inspiration, when the intrapulmonary pressure decreases further, it remains less than atmospheric pressure, allowing the entry of air into the lungs.