Date of download: 6/1/2016 Copyright © 2016 McGraw-Hill Education. All rights reserved. Effect of changing hydrostatic pressure on the distribution of pulmonary blood flow. Capillary blood flow in different regions of the lung is governed by three pressures: pulmonary arterial pressure, pulmonary venous pressure, and alveolar pressure. Pulmonary arterial pressure must be greater than pulmonary venous pressure to maintain forward perfusion; there are, therefore, three potential arrangements of these variables. Zone 1: P alv > P art > P ven. There is no capillary perfusion in areas where alveolar pressure is greater than the capillary perfusion pressure. Because alveolar pressure is normally zero, this only occurs where mean pulmonary arterial pressure is less than the vertical distance from the pulmonary artery. Zone 2: P art > P alv > P ven. Pulmonary arterial pressure exceeds alveolar pressure, but alveolar pressure exceeds pulmonary venous pressure. The driving pressure along the capillary is dissipated by resistance to flow until the transmural pressure is negative and compression occurs. This zone of collapse then regulates flow, which is intermittent and dependent on fluctuating pulmonary venous pressures. Zone 3: P art > P ven > P alv. Flow is independent of alveolar pressure because the pulmonary venous pressure exceeds atmospheric pressure. Zone 4: Zone of extra- alveolar compression. In dependent lung regions, lung interstitial pressure may exceed pulmonary arterial pressure. In this event, capillary flow is determined by compression of extra-alveolar vessels. The right side of the diagram shows a near-continuous distribution of blood flow from the top of the lung to the bottom, demonstrating that in the normal lung there are no discrete zones. The normal human lung at FRC spans 30 vertical centimeters, half of which distance is above the pulmonary artery and left atrium, and representative pulmonary arterial pressures are 33/11 cm H 2 O with a mean of 19 cm H 2 O. There is, therefore, no physiologic zone 1 in upright humans except perhaps in late diastole. Left atrial pressure averages 11 cm H 2 O and is sufficient to create zone 3 conditions two thirds of the distance from the heart to the apex. However, in patients undergoing positive-pressure mechanical ventilation, or in patients with airway disease creating lung units that fail to empty during the normal respiratory cycle, alveolar pressure is no longer atmospheric. Under conditions of positive end-expiratory pressure (PEEP), P alv may be as high as 15–20 cm H 2 O. This potentially shifts the entire distribution of pulmonary blood flow. (Adapted and reprinted with permission, from Hughes JM et al. Effect of lung volume on the distribution of pulmonary blood flow in man. Respir Physiol. 1968;4(1):58–72.) Legend : From: Pulmonary Disease Pathophysiology of Disease: An Introduction to Clinical Medicine, 7e, 2013 From: Pulmonary Disease Pathophysiology of Disease: An Introduction to Clinical Medicine, 7e, 2013