This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Heart, Circulation, and Blood, Subject name: Physiology
[{'content': 'During standing, pulsatile blood flow is found in which of the zones in healthy lung? \nSelect and explain. \nA. Zone 1, B. Zone 2, C. Zone 3, D. Zone 4', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. B. Zone 2a. Intermittent or pulsatile flow is found at Zone 2 which is found in apex of healthy lungs.b. Zone 3 is base, and has continuous flow (arterial pressure > venous pressure > alveolar pressure)c. Zone 1 doesn't occur in normal physiological conditions. If found, it would be in apex. Here alveolar pressure > arterial pressure > venous pressure; so there is no blood flow. Zone 1 represents alveolar dead space. Such regions can form when pulmonary arterial pressure falls to very low (eg. In hemorrhage; Or when alveolar pressure is very high (eg. Positive pressure ventilation).d. Zone 2: arterial pressure > alveolar pressure > venous pressure: Waterfall effect = pulsatile flow", 'role': 'assistant'}]