Perfusion pressure and blood flow determine microvascular apparent viscosity

What is the central question of this study? The aim was to evaluate the effect of perfusion pressure on blood flow in small arterioles. The hypothesis was that blood flow regulates the thickness of the red-cell-free layer and, therefore, blood flow determines blood apparent viscosity and local vascular resistance in vascular networks with limited myogenic or metabolic regulation of blood flow. What is the main finding and its importance? Reduced perfusion pressures lowered volumetric flow rates and increased local vascular resistance, due to increased blood apparent viscosity. Thus, the local vascular resistance of small arterioles with limited myogenic or metabolic regulation of blood flow, appeared to be determined by changes in blood rheology rather than blood vessel diameter. The study of blood flow regulation is important to understand and resolve pathological conditions. As blood is a complex non-Newtonian multiphase system, the foundations of blood rheological properties have been obtained mostly in viscometers. However, blood rheological behaviour in vivo depends on the concentration of red blood cells (RBCs), their mechanical properties and the RBC hydrodynamics, including RBC migration away from the vessel wall in shear flow. This migration promotes the formation of a RBC-depleted zone, or cell-free layer (CFL), which reduces the apparent viscosity of blood. We hypothesize that perfusion pressure determines blood apparent viscosity in microvessels, as shear rate affects axial migration of RBCs by influencing the CFL thickness. In this study, we analysed the effects of perfusion pressure on blood flow in individual arterioles within the rat cremaster muscle preparation. Perfusion pressures to this microvascular bed were controlled by occlusions of the iliac artery using a pressure cuff. Blood flow measurements were obtained from direct measurements of blood flow velocity profile, as well as determination of CFL thickness using intravital microscopy. Our results indicate that perfusion pressure determines shear rates and the CFL thickness and its variations. In addition, blood flow reduction increased local vascular resistance by augmenting blood apparent viscosity rather than vascular hindrance. In conclusion, blood rheology could act as an intrinsic mechanism to further limit blood flow to tissue with limited myogenic and metabolic responses at low perfusion pressures.