Diameters of juxtacapillary venules determined by oil-drop method in rat lung

We report a new method for precise quantification of lung microvascular diameter. Isolated blood-perfused rat lungs (500-g Sprague-Dawley rats) at constant inflation pressure [alveolar pressure (PA)] and stopped blood flow were viewed by microscopy and video. Subpleural venules of the second and third postcapillary generations were microinjected with oil colored with Sudan Black. Vascular pressure (Pvas) was varied in steps, and at each step the horizontal diameter (DH) and the length of the oil-filled segment were determined by microcaliper measurements of the replayed video image. At PA = 5 cmH2O, a decrease in Pvas from 25 to 0 cmH2O decreased DH in the second-generation venules from 55 +/- 2 (SE) to 41 +/- 1 microns (n = 13) and in the third-generation venules from 96 +/- 6 to 73 +/- 6 microns (n = 6). The constant-volume oil-filled segment conformed to the cylinder formula in that decreases in DH correlated linearly with 1/ square root of length, thereby indicating that at all Pvas values venular geometry was constant and probably circular in cross section. The decrease in Pvas to -5 cmH2O did not further decrease DH. At Pvas = 10-25 cmH2O, an increase in PA to 15 cmH2O did not significantly increase DH, although the increase in PA did diminish the slope (compliance) of the DH-Pvas relationship in second- but not third-generation venules. We conclude that 1) lung expansion decreases compliance of juxtacapillary venules, 2) venules retain circular cross sections at Pvas between -5 and 25 cmH2O, and 3) venules are patent at subzero Pvas.