Effect of hypoxia on small vessel blood content of rabbit brain

The small vessel blood content (SVBC) of several brain areas was determined in anesthetized rabbits exposed to various forms of hypoxia using [⁵⁹Fe-]siderophilin labeling of plasma. Under control conditions SVBC was significantly higher in cortical gray matter (1.00 ml/100 grams) than in cortical white matter (0.60), pons (0.61), or medulla (0.75). Inhalation of 10%O₂, 8%O₂ plus 5%CO₂, 1%CO in 40%O₂, or asphyxia resulted in increasing levels of average brain SVBC, in that order, from a control value of 0.77 to 2.04 during asphyxia. Estimation of regional capillary density from these data suggests that although total capillary number (i.e., during asphyxia) is similar in forebrain and hindbrain, a greater portion of total capillaries are perfused during moderate hypoxia in the forebrain than in the hindbrain.