When cardiac output is decreased by blood loss, flow is almost absent in upper regions of the lungs ( Hughes.1991; Nu.Q.Q...,1.993). This can be attributed to a decrease in pulmonary artery pressure while alveolar pressure remains unaffected. An increased dead-space, as measured by CO 2 elimination, has been seen in dogs under such circumstances, and this observation can be explained by abolished perfusion of upper lung regions (zone I).
In contrast, blood flow is proportionally more increased in apical than in basal lung regions after an increase in cardiac output. This redistribution of blood flow can be explained by the increase in pulmonary artery pressure that can be seen during exercise. It is also worth noting that the more even distribution of blood flow during exercise is accompanied by a more even ventilation distribution (see above) and an increase in the alveolar capillary membrane area available for gas transfer, thus facilitating gas exchange.
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