This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory System Pa 1, Subject name: Physiology
[{'content': 'If Paery>Palveolus>Pvein, under normal circumstances, which zone of the lung is being described? \nSelect and explain. \nA. Zone I, B. Zone II, C. Zone III, D. Zone IV', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Zone I ~ no blood flow (Palv> Pa>Pvein) Zone II ~ intermittent blood flow, i.e., during systole - Pa, cap>Palv>Pvein, Zone III ~ continuous blood flow (Pa>Pvein>Palv) These zones get created due to the interplay between alveolar pressure and the pressure in the pulmonary vessels. - In the recumbent posture, entire lung has zone III type of blood flow as hea would not have to pump blood against gravity to any region of the lung.', 'role': 'assistant'}]