RBC velocities in single capillaries of mouse and rat brains are the same, despite 10-fold difference in body size

Employing high-speed camera laser-scanning confocal microscopy with RBC-tracking software, we previously showed that RBC velocities in intraparenchymal capillaries of rat cerebral cortex are distributed over a wide range. In the present work, we measured RBC velocities in mice, whose body weights are less than one-tenth of that of rats. In an isoflurane-anesthetized mouse, a cranial window was opened in the left temporo-parietal region. Intravenously administered FITC-labeled RBCs were automatically recognized and tracked frame-by-frame at 500 fps, and the velocities of all RBCs recognized were calculated with our Matlab-domain software, KEIO-IS2. Among 15 241 RBCs detected in the ROI in 21 mice, 1655 were identified as flowing in capillaries. The velocities of these RBCs ranged from 0.15 to 8.6 mm/s, with a mean of 2.03 ± 1.42 mm/s. A frequency distribution plot showed that RBC velocities were clustered at around 1.0 mm/s, tailing up to 8.6 mm/s, and 59% of the RBCs in capillaries showed velocities within the range of 0.5 to 2.0 mm/s. Unexpectedly, these characteristics of RBC velocities in mice were very similar to those of rats, despite differences in RBC diameter (6.0 vs. 6.5 μm), body size (25 vs. 327 g), heart rate (461 vs. 319 bpm) and arterial blood pressure (86 vs. 84 mm Hg). We speculate that physical factors relating to oxygen exchange may constrain general RBC velocity in capillaries to a certain range for optimum oxygen exchange, regardless of species.