Journal of Applied Physiology, Vol 62, Issue 5 1982-1988, Copyright © 1987 by American Physiological Society
Zone 2 and zone 3 pulmonary blood flow
S. L. Soohoo, H. S. Goldberg, R. Graham and A. C. Jasper
In the West model of zonal distribution of pulmonary blood flow, increases
in flow down zone 2 are attributed to an increase in driving pressure and a
decrease in resistance resulting from recruitment and distension. The
increase in flow down zone 3 is attributed to a decrease in resistance
only. Recent studies indicate that, besides the pressure required to
maintain flow through a vessel, there is an added pressure cost that must
be overcome in order to initiate flow. These additional pressure costs are
designated critical pressures (Pcrit). Because Pcrit exceed alveolar
pressure, the distinction between zones in the West model becomes less
secure, and the explanation for the increase in flow even in West zone 3
requires reexamination. We used two methods to test the hypothesis that the
Pcrit is the pertinent backpressure to flow even in zone 3, when the
pulmonary venous pressure (Ppv) exceeds alveolar pressure (PA) but is less
than Pcrit in the isolated canine left caudal lobe. First, PA was
maintained at 5 cmH2O, and pressure flow (P-Q) characteristics were
obtained in zone 2 and zone 3. Next, with PA still at 5 cmH2O, we
maintained a constant flow and measured the change in pulmonary arterial
pressure as Ppv was varied. Both techniques indicated that the pertinent
backpressure to flow was the greater of either Pcrit or Ppv and that PA was
never the pertinent backpressure to flow. Also, our results indicate no
significant change in the geometry of the flow channels between zone 2 and
zone 3. These findings refine the zonal model of the pulmonary circulation.
