Red blood cell spacing in rat coronary capillaries during the cardiac cycle

Theoretical studies have demonstrated a pronounced effect of red blood cell (RBC) spacing on tissue oxygen supply. Our objective was to collect data regarding RBC spacing and related linear capillary hematocrit (Hct) in rat coronary capillaries, from two distinct locations within the capillary bed (proximal and distal portions), in the subendo- and midmyocardium, during systole and diastole. Hearts were rapidly frozen in situ, and tissue sections were stained in order to distinguish capillaries in proximal and distal portions of the capillary bed, as well as the RBCs within them. Morphometric data were compared according to capillary type, cardiac region, and cardiac phase. Distal portions of the capillary bed had a significantly greater capillary Hct (p < 0.001) and significantly lower RBC spacing values (P < 0.01), compared to proximal portions, irrespective of cardiac phase or region. There was a greater frequency of RBC spacing values equal to O microns in systole compared to diastole (P < 0.001) and a greater frequency of RBC spacing values greater than 40 microns in proximal portions of the capillary bed compared to distal portions (P < 0.001). These results suggest that intracapillary resistance to O2 transport is reduced, and O2 carrying capacity of the blood increased, in distal portions of the capillary bed, where PO2 in the blood is lower.