This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory system, Subject name: Physiology
[{'content': 'The intrapleural pressure is negative both during inspiration and expiration because \nSelect and explain. \nA. Intrapulmonary pressure is always negative, B. Thoracic cage and lungs are elastic are elastic structure, C. Transpulmonary pressure determines the negativity, D. Surfactant prevents the lungs to collapse', 'role': 'user'}, {'content': 'Answer: B \nExplanation: 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', 'role': 'assistant'}]