Capillary grouping in hamster tibials anterior muscles: flow patterns, and physiological significance

We have used fluorescein-labeled albumin and epifluorescence videomicroscopy to visualize the organization and flow patterns in the capillaries of a postural muscle, the tibialis anterior, in the pentobarbital-anesthetized hamster. Video tape records were made of overlapping microscope fields comprising portions of the muscle up to two millimeters in length and as much as 180 micron in depth. Three-dimensional reconstructions made from these tapes incorporated observations on flow patterns and geometry of the microvessels. The microcirculation in the tibialis was found to be arranged in repeating modules or 'units', consisting of approximately 15 capillaries supplied by a common arteriole and drained by a common venule. The mean distance from arteriole to venule in a unit was 855 +/- 233 micron (X +/- SD) and the mean capillary length was 805 +/- 330 micron. Flow in the capillaries within a unit was almost entirely concurrent, and there was little interchange of blood flow between the capillaries of adjacent units. The flow in capillaries of adjacent units was both countercurrent and concurrent. For each capillary, a count of the adjacent capillaries with concurrent and countercurrent flow was made. The ratio of concurrent to countercurrent flow varied from 2.4 to 4.3, being greatest at the midpoint of the unit and least at the arterial and venous ends. Our observations suggest that the capillaries of striated muscle are arranged in modules which may function as the fundamental control and distribution elements in the microcirculation.