<prompt> The intrapleural pressure is negative both during inspiration and expiration becauseIntrapulmonary pressure is always negativeThoracic cage and lungs are elastic are elastic structureTranspulmonary pressure determines the negativitySurfactant prevents the lungs to collapse  <response>: 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 