Final answer:
During inspiration, intrapleural pressure becomes more negative compared to expiration due to the expansion of the thoracic cavity. This is necessary to allow air to flow into the lungs. The competing forces of lung and thoracic wall elasticity, along with pleural fluid surface tension, maintain this crucial negative pressure.
Step-by-step explanation:
During inspiration, or inhalation, the intrapleural pressure becomes more negative relative to its pressure during expiration. The process is driven by the expansion of the thoracic cavity, which is a result of the contraction of the diaphragm and the intercostal muscles.
The expansion increases the volume of the pleural cavity and due to Boyle's Law, as the volume increases, the pressure decreases. This increased negative pressure is necessary to lower the pressure within the lungs to below atmospheric pressure, allowing air to flow into the lungs.
The negative intrapleural pressure is an essential aspect in the mechanics of breathing and is maintained by the balance of forces exerted by the elasticity of the lungs and thoracic wall. The elasticity of the lungs tends to pull them inward, whereas the thoracic wall's natural elasticity, along with the surface tension of the pleural fluid, exerts an outward pull.
This delicate balance ensures that the lungs do not collapse and can expand during inhalation. Ultimately, these competing forces lead to an intrapleural pressure of approximately -4 mm Hg during the breathing cycle, which becomes more negative during inspiration compared to expiration.