Final answer:
During inhalation, contraction of the diaphragm and external intercostals decreases intrapleural pressure, drawing air into the lungs. In contrast, relaxation during expiration increases intrapulmonary pressure, expelling air. Intrapleural pressure remains negative compared to intra-alveolar pressure throughout the breathing cycle.
Step-by-step explanation:
When the diaphragm contracts and flattens, and the external intercostals contract to expand the chest wall outward, there is an increase in the volume inside the pleural cavities. This results in a decrease in the intrapleural pressure, facilitating the flow of air into the lungs. During inhalation, the intrapleural pressure is lower than the atmospheric pressure, drawing air into the airways. Conversely, during expiration, as the diaphragm and intercostal muscles relax, the thoracic cavity and lungs decrease in volume, causing an increase in intrapulmonary pressure, pushing air out of the lungs. It is important to note that intrapleural pressure remains negative relative to intra-alveolar pressure due to the elasticity of the lungs and the chest wall dynamics.