The intrapleural pressure is negative both during inspiration and expiration because
Intrapulmonary pressure is always negative
Thoracic cage and lungs are elastic are elastic structure
Transpulmonary pressure determines the negativity
Surfactant prevents the lungs to collapse
Intrapleural pressure is the pressure within the pleural cavity. Intrapleural pressure is always negative, which acts like a suction to keep the lungs inflated. The negative intrapleural pressure is due to three main factors: 1. The surface tension of the alveolar fluid Ref: guyton and hall textbook of medical physiology 12 edition page number:317,318,319
Respiratory system