The intrapleural pressure is negative both during inspiration and expiration because
-
Intrapulmonary pressure is always negative
-
Thoracic cage and lungs are elastic structure
-
Transpulmonary pressure determines the negativity
-
Surfactant prevents the lungs to collapse
Explanation:
The intrapleural pressure becomes more negative at the end of inspiration and less negative at expiration. It ranges between -3 and -6 mmHg. The elasticity of lung and rib cage causes it to recoil and thus creating the negative pressure. The lymphatic drainage of the pleural fluid also creates a negative pressure.