Pulmonary blood volume and its effects on pressure/flow relations and flow resistance in isolated lungs of rabbits

Quantitative information about the effects of pulmonary blood volume (Q(p)) on pulmonary haemodynamics is lacking since Q(p), changes inevitably with flow. To separate flow-dependent from volume-dependent changes in intravascular pressures we imposed changes in Q(p) (measured continuously) by altering outflow pressure in seven isolated, blood-perfused rabbit lungs and studied the effects of Q(p) on the relations between arteriovenous pressure gradient (Delta P) and blood flow ((Q) over dot) under two conditions: flow-dependent volume changes were either permitted or compensated. In the latter circumstances, Delta P changed more for a given change in (Q) over dot. The Delta P/(Q) over dot relations were shifted to smaller Delta P when Q(p) was increased. Hence, the calculated flow resistance (R = Delta P/(Q) over dot) decreased with increasing Q(p) at a given (Q) over dot. Assuming constant viscosity, changes in R can be predicted from changes in vessel geometry and thus Q(p). We found that R increased less than expected (by a factor of 3-7.5 instead of 9) when Q(p) was reduced to one-third. This discrepancy may be explained by a change in blood distribution within the lung despite constant Q(p) and by a change in apparent blood viscosity with (Q) over dot. Regardless of these speculations we have shown that Q(p) determines Delta P at each flow and thus flow resistance.