Final answer:
The intrapleural pressure becomes negative just before an inspiration begins.
Step-by-step explanation:
The intrapleural pressure becomes negative just before an inspiration begins. Intrapleural pressure is the pressure of the air within the pleural cavity, and it is always lower than the intra-alveolar pressure and atmospheric pressure. This negative pressure helps keep the airways of the lungs open and allows air to rush into the respiratory passages during inhalation.