Point:Counterpoint Comments The

following letters are in response to Point:Counterpoint “Gravity is/is not the major factor determining the distribution of blood flow in the human lung.” To the Editor: Glenny (1) states that the foundation of the debate should be set on two issues: 1) methods for measuring perfusion distribution and 2) lung tissue redistribution between different body positions. The former is obvious, but the latter appears not so simple. Glenny asserts that sagging of parenchyma near the base and corresponding stretching at the apex (in the upright lung) accounts for much of the total vertical perfusion gradient, because, due to sagging, there are more alveoli in a given field of view in basal than apical regions. He claims that this must be corrected for so as to obtain the effect of gravity indepen-dent of this (gravitationally induced) tissue movement. Thus, after tissue-sagging contributions to the vertical gradient are removed, what is left as a gradient is implied to be the real effect of gravity, and this is what Glenny considers. I have great trouble with this logic. Gravitationally induced tissue sagging is a fundamental part of how gravity affects perfusion (and, for that matter, ventilation) distribution. De-ducting that component is tantamount to eliminating a major, real gravitational influence on perfusion distribution. Of course this “correction ” must diminish the residual apex to base gradient. My point is that Glenny’s removal of the sagging-related gravitational component is inappropriate in this debate because it is an important effect of gravity. What Hughes and West (2) describe is the total effect of gravity without such “corrections, ” and that is what Glenny should be comparing his data with and basing his conclusions on