Quantitative capillary topography and blood flow in the cerebral cortex of cats: an in vivo microscopic study.

In 50 anesthetized cats the microcirculation in intermediate and deeper layers of the cerebral cortex was visualized in vivo by microtransillumination, and documented by high-speed microcinephotography. The viability of the preparation was verified in a series of experiments demonstrating spontaneous vasomotion and responsiveness to chemical stimulation of pial arterioles and small arteries. Stereological methods for quantitative analysis of projected images of capillaries in a comparatively large tissue volume were employed to determine morphometric and topographical parameters of the asymmetric, highly tortuous intracortical capillary network. Capillary diameters (5.1 +/- 0.84 micrometer), radii of curvature (median 57 micrometer), total capillary lengths per tissue volume 939 +/- 338.2 mm/cu.mm), capillary volume fractions (2.1 +/- 0.51%), total capillary surface areas per tissue volume (15.3 +/- 4.85 sq.mm/cu.mm), and intercapillary distances (median 24.2 micrometer) showed significant interregional differences. The frequency distribution of the lengths of capillary segments (median 108 micrometer) was best described by a Weibull distribution. On the average 90% of all capillaries were continuously perfused. Capillary red cell flow (median velocity 1500 micrometer/sec) was predominantly unidirectional and conspicuously irregular. The variance of capillary red cell velocities (CRCVs) was significantly correlated (tau = 0.48) with capillary tortuosity. An extreme value distribution best described the observed frequency distribution of CRCVs. Flow irregularities represented both white noise and a significant stochastic periodicity at frequencies between 40 and 90 Hz.