Effect of blood flow on capillary transit time and oxygenation in excised rabbit lung

We used an isolated perfused lung preparation of the rabbit to study the effect of increasing blood flow on pulmonary capillary transit time by two methods. In one method, capillary transit time was measured from fluorescent dye dilution curves from arterioles and venules of the subpleural microcirculation. Values of transit time were similar to those for the whole lung determined by dividing capillary blood volume by blood flow. Capillary transit times averaged 0.50-0.62 sec at a control blood Bow of 80 ml min^-1 kg^-1 and decreased to 0.14-0.18 sec as blood flow increased to 6 times control. To determine whether the reduced transit time would limit O₂ transport, we studied the effect of blood flow on oxygenation. Two isolated rabbit lungs were Perfused in series. Blood from one lung deoxygenated by ventilation with a N₂-C0₂ mixture was oxygenated by the test lung ventilated with air. Ventilation was matched to blood flow. P₀₂ and PC(CO2) were measured in blood flowing into and out of the test lung. At all flows, no significant alveolar gas-to-end-capillary blood Po₀₂ gradient (A-aD0₂) was measured. The isolated perfused rabbit lung showed no transit time limitation to oxygenation for blood flows that are consistent with heavy exercise in vivo.