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
During standing, pulsatile blood flow is found in which of the zones in healthy lung? 
Select and explain. 
A. Zone 1, B. Zone 2, C. Zone 3, D. Zone 4
B: Zone 2
Answer: B: Zone 2 
Explanation: 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