CHANGES IN BRAIN CAPILLARY DIAMETER DURING HYPOCAPNIA AND HYPERCAPNIA

Since changes in the surface area of capillaries may be relevant to ca pillary exchange, the distensibility of brain capillaries was investig ated. Brain capillary diameters were measured after perfusion fixation of brain tissue at a constant perfusion pressure during hypo- or hype rcapnia. Sections were embedded, stained, and analyzed by light micros copy. The results showed significant differences in mean capillary dia meter between the hypocapnic and the hypercapnic group. In the eight b rain structures analyzed, capillary diameters were always larger in th e hypercapnic group. Mean capillary diameter was 4.93 +/- 0.29 mum in the hypocapnic group and 5.91 +/-0.10 mum in the hypercapnic group (me ans +/- SD). We conclude that brain capillaries exhibit a moderate deg ree of distensibility. Variations in the precapillary pressure of micr ovessels may therefore influence both capillary flow and capillary sur face area.