Wall shear rate in arterioles in vivo: least estimates from platelet velocity profiles.

Velocity profiles, as determined in vivo in rabbit mesenteric arterioles with fluorescently labeled platelets as natural flow markers, were used to calculate least estimates of the actual wall shear rate in these microvessels (17-32 micron diam). The fit of the velocity data points described the profile as close to the wall as 0.5 micron. To satisfy the no-slip condition, a thin layer of fluid with a steep velocity gradient near the wall was assumed. Least estimates of wall shear rate, as calculated from the fitted platelet-velocity profiles and using the mean velocity gradient in this layer of fluid, ranged from 472 to 4,712 s-1 with a median value of 1,700 s-1. Red blood cell center-line velocities varied between 1.3 and 14.4 mm/s (median 3.4). The wall shear rates were at least 1.46-3.94 (median 2.12) times higher than expected on the basis of a parabolic velocity distribution but with the same volume flow in the vessel. Considerable spatial differences in wall shear rate might exist even within a short segment of a vessel.