Final answer:
During inhalation, the pleural cavity's pressure decreases as the volume of the thoracic cavity increases, creating a more negative intrapleural pressure relative to the usual -4 mm Hg. This increase in volume and corresponding negative pressure allows lungs to expand and air to flow in, upheld by the concept of Boyle's Law.
Step-by-step explanation:
The pressure of the pleural cavity (Ppl) changes during inhalation as part of the respiratory cycle. When inhalation begins, the muscles of the thoracic cavity, particularly the diaphragm and the intercostal muscles, contract, expanding the volume of the thorax. According to Boyle's Law, an increase in volume leads to a decrease in pressure. Hence, as the volume of the pleural cavity increases, the intrapleural pressure becomes more negative compared to the usual -4 mm Hg. This further negative intrapleural pressure is essential to allow the lungs to expand and air to flow in, a process driven by the pressure gradient where air moves from an area of higher pressure to an area with lower pressure.
During the transition from the beginning of inhalation to the end, the negative pressure is crucial in keeping the airways open and facilitating the inflow of air. This is balanced by the transpulmonary pressure, which is the difference between the intrapleural and intra-alveolar pressures and determines the lung size. An increase in transpulmonary pressure signifies a larger lung expansion during inhalation. It's during inhalation that the negative pressure is most critical because it assists in expanding the lung volume to draw air into the alveoli.