to the editor: The two sides of this Point:Counterpoint (1, 3) agree that 1) gravity redistributes lung parenchyma within the chest, 2) gravity redistributes blood flow within the lung parenchyma (independent of tissue sagging), and 3) perfusion heterogeneity exists that cannot be explained by gravity alone. It is appealing to settle our debate by agreeing that our different perspectives on tissue sagging explain the differences between our groups with respect to blood flow distribution. However, to do so would steer us away from our real purpose of understanding the mechanisms by which regional blood flow is determined in the lung.
As pointed out by the letter writers (4), the frames of reference are different between the opposing sides, leading to differences in conclusions. As I opined in my first essay, the frame of reference is central to this debate. To a passenger in a moving car, the landscape appears to rush by. However, to an observer outside the car, the passenger is moving and the landscape remains stationary.
The frame of reference used by Drs. Hughes and West (3) is one looking at the entire lung from outside of the chest. If gravity could be gradually increased, they would observe increasing blood flow at the lung bases as they become more compressed. I, and others, use a point within the lung as our frame of reference. From this reference point, the volume of any lung region at the base would become smaller with increasing gravity but blood flow to this lung region would change little. The appropriate frame of reference clearly depends on the question being asked. If one is interested in the mechanisms that distribute blood flow WITHIN the lung, the reference point should be just that, within the lung. The stance that blood flow increases down the lung due to lung compression seems tantamount to saying that the trees are moving as I drive past them.
Tissue sagging certainly contributes to external observations of increasing flow down the lung, but tissue redistribution is not part of the traditional gravitational model (5). Increasing flow within zone 3 is said to be due to an increasing hydrostatic gradient that distends vessels and decreases resistance to flow. To my understanding, gravitationally induced tissue sagging is not a fundamental component of the gravitational model. On the contrary, lung compression has been invoked to explain decreased perfusion in zone 4 (2).
The principal function of the lung is to exchange respiratory gases. I am confident that all participants in this debate agree that despite the heterogeneity of both perfusion and ventilation, the lung is an efficient gas exchanger due to the close matching of regional ventilation and perfusion. The gravitational model postulates that the shared effect of gravity induces regional matching of blood flow and ventilation. The observations that perfusion and ventilation are heterogeneous within isogravitational planes and that astronauts continue to efficiently exchange gas during weightlessness, argues against this simplified model. I have great respect for the innovative and pioneering work performed by Drs. Hughes and West; however, adherence to the gravitational model will only impede efforts to understand how regional ventilation and perfusion are determined and matched.
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