This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
[{'content': 'Normal intrapleural pressure is \nSelect and explain. \nA. 1-2 mm Hg, B. 2-4 mm Hg, C. 9.5-10 mm Hg, D. -3 to -4 mm Hg', 'role': 'user'}, {'content': 'Answer: D \nExplanation: The normal external pressure is equal to the normal intrapleural pressure (the pressure in the chest\xa0 cavity), which is −4 mm Hg. Note in the figure that a rise\xa0in intrapleural pressure, to −2 mm Hg, shifts the entire cardiac output curve to the right by the same amount.\xa0 This\xa0 shift\xa0 occurs\xa0 because\xa0 to\xa0 fill\xa0 the\xa0 cardiac\xa0chambers\xa0 with blood requires an extra 2 mm Hg right atrial pressure to overcome the increased pressure on the outside of the heart. Likewise, an increase in intrapleural pressure to +2 mm Hg requires a 6 mm Hg increase in right atrial pressure from the normal −4 mm Hg, which shifts\xa0the entire cardiac output curve 6 mm Hg to the right.', 'role': 'assistant'}]