Final answer:
During inspiration, the intraoral pressure is negative in comparison to the atmospheric pressure due to an increase in thoracic cavity volume which causes a decrease in intra-alveolar pressure.
Step-by-step explanation:
During inspiration, intraoral pressure is negative relative to atmospheric pressure. When we inhale, the volume of the thoracic cavity increases due to the contraction of the diaphragm and intercostal muscles, leading to a decrease in intra-alveolar pressure (intrapulmonary pressure). This decrease in pressure inside the alveoli compared to the atmospheric pressure creates a negative pressure, which is essential for air to flow into the lungs.
According to Boyle's Law, in a gas, pressure increases as volume decreases. Therefore, the larger volume inside the thoracic cavity during inspiration results in a lower pressure than the atmospheric pressure. The intrapleural pressure, although also negative, must be differentiated from intraoral pressure, as it is the pressure within the pleural cavity and not within the airways.