True statement about zones of blood flow in the lung:

Normally, in the upright posture, there is zone II (intermittent blood flow) at the apex and zone III (continuous blood flow) in the remaining lung. Reason: In the upright posture, apex of the lung is above the level hea. Thus, hea has to pump the blood against gravity to send blood toward the apex. Hence, there is low pressure and lesser blood flow at the apex of the lung. Due to low pressure in the vessels, the surrounding alveoli tend to compress the vessels. Option A is incorrect: When person goes in supine position, no pa of the lung is above the level of hea; hence, blood does not have to flow against gravity. Consequently, entire lung will have continuous blood flow (zone III). Option B is incorrect: In intermittent positive pressure ventilation (IPPV), alveolar pressure increases since ventilation is being delivered under high pressure. Since surrounding alveolar pressure increases, the blood vessels will be compressed and the zones will be conveed to lower types (III --> to II; II --> to I). Option C: In upright posture, base of the lung has zone III, as the blood flows toward gravity. Pressure in the vessels > surrounding alveolar pressure. Option D: It is a correct statement. In mitral stenosis, left atrial pressure increases (as LA has to pump vigorously to send blood through the stenosed mitral valve). Increased LA pressure is reflected backward in the pulmonary vessels due to the damning back of the blood. Increased pressure in pulmonary vessels will increase the flow, conveing zone II to zone III.