Capillary length, tortuosity, and spacing in rat myocardium during cardiac cycle.

Microvascular geometry was evaluated in rat left ventricular midmyocardium (male Sprague-Dawley, n = 14), arrested in systole (S) or diastole (D), by bolus injections of CaCl2 or KCl, respectively. The histological method employed in this study allowed for the visualization of capillary pathways from arteriole to venule. Capillary length, as directly measured from terminal arteriole to collecting venule, was not significantly different between S and D groups, averaging 606 +/- 15 microns (pooled mean +/- SE). The capillary length tortuosity, defined as the ratio of the capillary length to the direct arteriovenous distance, was significantly increased in systolic-arrested hearts (S = 1.31 +/- 0.03; D = 1.18 +/- 0.02, P < 0.01). At the level of individual capillary segments, however, there was no increase in tortuosity in systolic-arrested hearts (S = 1.17 +/- 0.03; D = 1.16 +/- 0.02). Intercapillary spacing was significantly more uniform in systolic-arrested hearts. These data suggest that in systole, capillary length and tortuosity are generally preserved, and capillary spacing is more uniform, serving to maintain geometric conditions for oxygen supply during the cardiac cycle.