Correlation of O2 transport on the micro and macro scale

In the large animals used for systems physiology the microcirculation is usually hidden beneath an opaque fascia. On the other hand, small creatures suitable for microcirculatory studies are unsatisfactory models of O2 transport in large animals. These species differences are explored in detail for dog and rat. Focal anoxia on a micro scale and delivery-dependent VO2 on a macro scale are well correlated in both species. Quantitative relations between micro and bulk parameters are less consistent. An estimate of mean red cell velocity based on volume flow/gram and mean functional capillary density was an order of magnitude too small in dog gracilis, but within acceptable limits in rat gracilis. It is unclear whether rough agreement between micro and macro data in rat, and in hamster cremaster [24], is real or due to oppositely directed errors in measurements and/or assumptions. Analysis of relations among transport parameters suggests that the principal cause of all discrepancies between micro and macro data is parameter heterogeneity. Though heterogeneity is often of greater functional significance than the mean, quantitative information on the probability distributions of microcirculatory parameters and the probability structure of parameter interaction, is largely lacking. Specific suggestions for improving the microcirculatory data base are offered.