Characterization of the basal motion of RBCs in capillaries. The vessels were scanned at 2 msec per line, except for one vessel with very fast flow that was scanned at 1 msec per line. (a–c) Selected 1-sec segments of the line-scan data for flow in three different vessels. Note the increase in speed and flux in the progression from a to c. The width of the dark bands (distance between arrows) is seen to decrease with increasing flux; this implies that the profile of the RBCs relative to the central axis of the capillary decreases in the progression a to c. The Inset in a is a characterization of RBC flow. The instantaneous velocity is Δx/Δt, the flux is 1/Δt, and the linear density is 1/Δx. (d) Representative plot of the spectral power density in a 128-sec record of speed versus time; we used multitaper estimation techniques () with a half-bandwidth of 0.02 Hz. Note the peak near 0.1 Hz (∗); the peak at 3.2 Hz is aliased heart rate (6.8 Hz aliased to 3.2 Hz by the T = 100-msec window of the velocity calculation). (e) Plot of speed versus caliber for 21 vessels. The speed was determined from 20-sec intervals that included no sensory stimuli. a, b, and c refer to data for the line-scans in a, b, and c, respectively. (f) Plot of speed versus depth below the pia for flow in all 38 vessels. The solid line is a fit to the data, excluding the two points with exceptionally high speed, with slope 0.007 ± 0.0004 (mm/sec)/μm (mean ± SEM). (g) Plot of speed versus flux. The solid line is a best fit to all points in the data set. (h) Plot of linear density (Eq. ) versus flux. The drawings illustrate the change in packing that was hypothesized to occur as the RBCs shift their orientation from planar at low density to axial at high density.