Calculated dispersion of capillary transit times: significance for oxygen exchange

Probability densities for red cell velocity (V) and capillary length (L) in dog gracilis muscles were computed from mean length and mean velocity assuming two-parameter gamma distributions [Honig, Feldstein, and Frierson, Am. J. Physiol. 233 (Heart Circ. Physiol. 2): H122-H129, 1977]. The distribution of capillary transit times (L/V) was obtained from the ratio of the two gamma distributions. The lower tails of transit time distributions were compared with times thought required for O2 release from capillaries. Results indicate the following. 1) Transit time exceeds O2 release time at rest in all capillaries, regardless of assumptions in the calculation. 2) Transit time appears long enough even in moderate exercise provided mean L is about 1,000 microns and release time is about 100 ms. 3) Capillary recruitment prevents a functional O2 shunt during work at one- to two-thirds maximum O2 uptake (VO2max). 4) Recruitment is insufficient to prevent O2 shunting during exercise to VO2max. 5) Quantitative analysis of O2 transport is severely limited by lack of information about a) microvascular geometry, b) the probability distributions of parameters, and c) the kinetics of O2 release from capillaries.