Experimental, theoretical, and translational studies of RBC distribution in capillary networks
Abstract
The purpose of this thesis was to evaluate the physiology of red blood cell (RBC) distribution in skeletal muscle capillary networks. Because this is the terminal site of oxygen exchange in the microcirculation, characterization of this fundamental process informs an understanding of how microvascular blood flow regulation matches oxygen supply with local tissue demand. Prior studies in this field have focused on small groups of capillaries, and have not linked capillary network structure with functional measurements, nor evaluated the temporal complexity of RBC distribution over physiologically-relevant scales. It is also unclear how the functional units called capillary modules – comprised of parallel capillaries from arteriole to venule – relate together within large capillary networks. In this thesis, we employed multiple methodologies to achieve this goal with preclinical animal models, theoretical simulations, and translational studies in human patients. First, we used intravital videomicroscopy of resting extensor digitorum longus muscle in rats and discovered that skeletal muscle capillary networks are organized into columns of interconnected capillary modules spanning thousands of microns – a structure we called the Capillary Fascicle. We showed that capillary-RBC hemodynamics are heterogeneous within a module and between modules. Next, we evaluated capillary module hemodynamics and demonstrated that RBC flow is independent of module resistance, providing evidence for regulation of driving pressure at the level of the capillary module, that requires pre- and post-capillary mechanisms of control. Using a dual-phase mathematical model of blood flow within artificial capillary module geometries, we showed that RBC flow heterogeneity is an intrinsic property of capillary module structure, and that variations to inflow hematocrit and pressure impact RBC distribution as a consequence of the rheological properties of microvascular blood flow. Finally, we used high-resolution near-infrared spectroscopy to monitor the temporal variability of hemoglobin content in skeletal muscle of patients in the intensive care unit (ICU). We showed that RBC perfusion characteristics are not consistent between patients, and that ICU interventions directly impact microvascular RBC distribution. Together, these studies support a theory of capillary networks as active participants in microvascular blood flow regulation, with structural features of capillary networks contributing to functional characteristics of RBC distribution.