Capillary lengths, anastomoses, and estimated capillary transit times in skeletal muscle.

Total capillary length, capillary segment length, and number of anastomoses per capillary were measured in rat gracilis muscle at rest and after 2 min of phasic contraction. Mean values of the foregoing variables at rest (+/-SD) were, respectively, 1,012 micronm +/- 484, 409 micron +/- 274, and 0.83 +/- 1.09. Total capillary lengths are well described by the gamma distribution, number of anastomoses by the negative binomial distribution, and segment length by the Weibull distribution. Contraction has no significant effect on the means or the frequency distributions, indicating that: 1) pressure gradients between adjacent capillaries are small, and 2) intercapillary anastomoses do not improve flow distribution in exercise. Erythrocyte velocities observed in resting muscle (Burton, K. S., and P. C. Johnson. Am J. Physiol. 223: 517-524, 1972) were shown to be adequately characterized by the gamma distribution. From these velocities and the observed distribution of path lengths, we computed an estimated distribution of capillary transit times. Mean transit time was 4.29 s. The median was 2.45 s, and 11% of values exceeded 8 s. The range was 90 ms-43 s. This heterogeneity of transit times should profoundly affect calculations of O2 transport and the shape of indicator dilution curves.