Simultaneous measurement of capillary distensibility and hydraulic conductance

Capillaries are often assumed to be indistensible. Only recently has the error caused by capillary distensibility in the measurement of hydraulic conductance been considered. An oil-drop method was used to measure simultaneously distensibility and hydraulic conductance (L p) of mesenteric capillaries of 40 cranially pithed Rana pipiens. All vessels studied were distensible in the range 5–80 mm Hg, with a mean equilibrium time, t eq, of 12.4 ± 0.5 sec (SEM, n = 7). Capillary distensibility decreased with increasing pressure. Mean capillary distensibilities at 10, 30, and 60 mm Hg were 18.9 ± 4.9 × 10 −4 (mm Hg) −1, 13.5 ± 1.2 × 10 −4 (mm Hg) −1 (mean ± SEM, n = 9), and 7.6 ± 2.7 × 10 −4 (mm Hg) −1 (mean ± SEM, n = 7), respectively. Mean values of capillary L p, uncorrected and corrected for distensibility, were 0.0113 ± 0.0017 μm(sec·mm Hg) −1 and 0.0083 ± 0.0016 μm(sec·mm Hg) −1 (SEM, n = 8 capillaries), respectively. A paired t test demonstrated that uncorrected and corrected values of L p were statistically different from one another ( P < 0.005). Both distensiblity and L p varied widely between capillaries indicating the necessity of measuring and correcting for distensibility in each individual capillary when estimating L p. The finding of a slow component of vessel distensibility demonstrates that previous filtration data taken not only from single vessels, but also from whole organs, should be reinterpreted.