Temporal and spatial distributions of red cell velocity in capillaries of resting skeletal muscle, including estimates of red cell transit times

Microvascular blood flow in resting skeletal muscle fluctuates with time and varies from capillary to capillary. Our goal was to quantify this observed heterogeneity. We have used a television-computer method and a flying-spot technique to measure red cell velocity in capillaries in frog sartorius. Temporal distributions of velocity in 53 capillaries from 24 frogs were represented by histograms whose mean values ranged from 0.03 to 0.48 mm/sec (overall mean 0.21 mm/sec). Individual velocity values ranged from 0 to 0.83 mm/sec. At high mean velocities histograms were symmetrical, but as the mean decreased the distributions became skewed toward high velocities. We have also determined spatial distributions of velocity from 40 to 104 capillaries in each of three separate preparations. The velocity ranges and the mean were 0.07–0.82, 0.01–0.55, 0.02–0.36 mm/sec, and 0.30, 0.26, 0.12 mm/sec, respectively. In one capillary, the record of velocity vs time was used to estimate the temporal distribution of transit times; the mean and the range were 3.3 and 2.2 to 4.2 sec, respectively. In one muscle, measurements of both the capillary segment lengths (distances between successive branch points) and the red cell velocities yielded estimates of the spatial distribution of segment transit time; the mean and the range were 7.2 and 0.7 to 24.3 sec. We conclude that red cell velocity in capillaries of frog sartorius muscle varies extensively with time and in space and that one single value of mean velocity cannot represent adequately the state of blood supply to this muscle.