Final answer:
Contraction of the diaphragm and external intercostal muscles reduces intrapleural pressure, aiding in inhalation. Intrapleural pressure is always kept negative relative to intra-alveolar pressure to maintain lung expansion. Inspiration increases thoracic volume, lowers pressure, and draws in air, while expiration is passive and increases pressure to expel air.
Step-by-step explanation:
When the diaphragm and external intercostal muscles contract, the chest wall expands outward. This action increases the volume inside the pleural cavities, which in turn decreases the pressure within them, known as the intrapleural pressure. The intrapleural pressure becomes more negative relative to the atmospheric pressure, creating a pressure differential that draws air into the lungs.
The intrapleural pressure is always negative compared to intra-alveolar pressure; a negative pressure is essential for keeping the lungs expanded against the elastic recoil of the lung tissue and surface tension of alveolar fluid. This delicate balance of forces, facilitated by mesothelial cells and the lymphatic system, maintains the normal respiratory function, ensuring effective ventilation.
During inspiration, as the volume of the thoracic cavity increases due to the contraction of these muscles, the intrapleural pressure decreases, leading to airflow into the lungs. Conversely, during expiration, which is usually a passive process, the diaphragm and intercostal muscles relax, the thoracic cavity and lungs decrease in volume, leading to an increase in intrapleural pressure which facilitates the expulsion of air from the lungs.