DIAMETERS OF JUXTACAPILLARY VENULES DETERMINED BY OIL-DROP METHOD IN RAT LUNG

We report a new method for precise quantification of lung microvascula r diameter. Isolated blood-perfused rat lungs (500-g Sprague-Dawley ra ts) at constant inflation pressure [alveolar pressure (PA)] and stoppe d blood flow were viewed by microscopy and video. Subpleural venules o f the second and third postcapillary generations were microinjected wi th oil colored with Sudan Black. Vascular pressure (Pvas) was varied i n steps, and at each step the horizontal diameter (D-H) and the length of the oil-filled segment were determined by microcaliper measurement s of the replayed video image. At PA = 5 cmH(2)O, a decrease in Pvas f rom 25 to 0 cmH(2)O decreased D-H in the second-generation venules fro m 55 +/- 2 (SE) to 41 +/- 1 mu m (n = 13) and in the third-generation venules from 96 +/- 6 to 73 +/- 6 mu m (n = 6). The constant-volume oi l-filled segment conformed to the cylinder formula in that decreases i n D-H correlated linearly with 1/root length, thereby indicating that at all Pvas values venular geometry was constant and probably circular in cross section. The decrease in Pvas to -5 cmH(2)O did not further decrease D-H. At Pvas = 10-25 cmH(2)O, an increase in PA to 15 cmH(2)O did not significantly increase D-H, although the increase in PA did d iminish the slope (compliance) of the D-H-Pvas relationship in second- but not third-generation venules. We conclude that 1) lung expansion decreases compliance of juxtacapillary venules, 2) venules retain circ ular cross sections at Pvas between -5 and 25 cmH(2)O, and 3) venules are patent at subzero Pvas.