Changes in small vessel blood content of the rat heart induced by hypercapnic, hyperoxic or asphyxic conditions

The small vessel blood content (SVBC) of the ventricular walls of the heart of anesthetized closed chest rats was determined using 59FeC13 to label the plasma siderophilin. SVBC was measured breathing air, 100% O2, 5% CO2 in 21% O2 and during asphyxia. The average SVBC could be ranked: air less than 100% O2 less than 5% CO2 less than asphyxia. Only the '100% O2' values were not significantly above the 'air' values. Under control conditions, SVBC of the rat heart averaged 6.77 +/-(SE) 0.23 ml blood/100 g tissue. Inhalation of 5% CO2 increased this to 8.68 +/- 0.34, while asphyxia produced a maximal response to 14.40 +/- 0.77. Comparison of SVBC were made between various regions of the ventricular walls. The only significant difference was that the right side of the septum had a lower SVBC than the left ventricular subepicardium under all conditions except CO2. These differences in SVBC are related to the number of open capillaries in the myocardium. Thus inhalation of CO2 decreases intercapillary distance while 100% O2 has no effect.