Determination of hematocrit from mean transit time of red blood cells and plasma in cremaster muscle of the rat

Indicator dilution methods were used to measure the mean transit time of bolus injections of fluorescently labeled red cells (MTT_rbc) and plasma (MTT_plasma) containing 5 gms% fluorescent albumin. Following injection of a bolus into the femoral artery contralateral to the cremaster muscle, intensity time curves of the bolus dispersion were obtained within functionally paired arterioles and venules of successive microvascular divisions. The intensity time curves were obtained by digitizing videodensitometric measurements recorded under fluorescence microscopy. The average microvascular hematocrit (H_m) present in the network between these two sites is evaluated assuming that the resident volume of either red cells or plasma is equal to the product of their volumetric flow and respective mean transit times, and the actual discharge hematocrit (H_d) is assumed to be equal to the systemic hematocrit. The results demonstrate a general trend of diminishing H_m/H_d as blood flows from feeding to terminal arterioles, which agrees qualitatively with estimates obtained previously by direct cell counting and spectrophotometric methods.