Model experiments on the effect of bifurcations on capillary blood flow and oxygen transport

O2-delivery by a single capillary is a function of the flow rate and the fraction of flow made up of red blood cells. Capillary flow rate in turn depends upon flow resistance which is determined by the fraction of capillary volume occupied by red blood cells. Experiments were carried out to study the relationship between these parameters in an in vitro model consisting of glass capillaries (I.D. 3.3--11.0 micrometer) branching from a large bore feeding channel which was perfused at variable flow rates with suspensions of human red cells with different hematocrits. Capillary flow rates ranged from 0--10(-4) mm3s-1. The results indicate that the red cell flow fraction increases with increasing capillary flow rate and with decreasing feeding vessel flow rate. Capillary volume fraction occupied by red cells similarly depends on these two parameters, but is consistently lower than the red cell flow fraction. Capillary flow resistance increases with flow rate due to increasing volume fraction of cells. If the results obtained with the model system are applicable to in vivo capillaries it must be concluded that O2-delivery by a single capillary is not linearly related to flow rate but increases more than proportionately with flow rate. Due to alteration of resistance with flow rate another type of "autoregulation" of capillary flow is proposed which tends to keep flow rate constant despite changes of driving pressure.