Estimating transit time for capillary blood in selected muscles of exercising animals.

The mean minimal capillary transit time was estimated in muscles of various animals using a combination of physiological and morphometric methods. Radioactive microspheres were injected intravascularly in various animals running on a treadmill at maximum oxygen consumption rate (VO2,max) to label blood flow to individual muscles. The muscles were then removed and preserved by standard methods for electron microscopy. The volume density of mitochondria was measured to assess muscle oxidative capacity. Capillary densities in muscle cross-sections, capillary diameters and tortuosities were incorporated into an estimate of capillary volume per unit muscle mass. Mean capillary transit time (tc) in the exercising muscles was estimated by dividing mass-specific capillary volume by mass-specific blood flow. Estimates of tc ranged from values near 1 s in horse heart and thigh muscles to 0.2 s in duck gastrocnemius. The relationship between muscle blood flow and tc was hyperbolic. The experimental data indicate a limiting value of 0.2 s for transit times at very high blood flows. There was no correlation between tc and body-mass-specific VO2,max.