PULMONARY BLOOD-VOLUME AND ITS EFFECTS ON PRESSURE FLOW RELATIONS ANDFLOW 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 inevit ably with flow. To separate flow-dependent from volume-dependent chang es in intravascular pressures we imposed changes in Q(p) (measured con tinuously) by altering outflow pressure in seven isolated, blood-perfu sed rabbit lungs and studied the effects of Q(p) on the relations betw een 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 rel ations were shifted to smaller Delta P when Q(p) was increased. Hence, the calculated flow resistance (R = Delta P/(Q) over dot) decreased w ith increasing Q(p) at a given (Q) over dot. Assuming constant viscosi ty, 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 o f 3-7.5 instead of 9) when Q(p) was reduced to one-third. This discrep ancy may be explained by a change in blood distribution within the lun g despite constant Q(p) and by a change in apparent blood viscosity wi th (Q) over dot. Regardless of these speculations we have shown that Q (p) determines Delta P at each flow and thus flow resistance.