Perfusion pressure and blood flow determine microvascular apparent viscosity

Area of Science:

• Physiology
• Biophysics
• Microcirculation Research

Background:

• Blood flow regulation is crucial for understanding and treating diseases.
• Blood's complex rheology in vivo is influenced by red blood cell (RBC) behavior and the cell-free layer (CFL).
• Existing research on blood rheology primarily uses viscometers, which may not fully represent in vivo conditions.

Purpose of the Study:

• To investigate the effect of perfusion pressure on blood flow dynamics in small arterioles.
• To test the hypothesis that blood flow influences the red-cell-free layer (CFL) thickness, thereby affecting apparent viscosity and vascular resistance.
• To determine if blood rheology or vessel diameter is the primary determinant of resistance in microvessels with limited regulatory capacity.

Main Methods:

• Analysis of blood flow and CFL thickness in rat cremaster muscle arterioles under controlled perfusion pressures.
• Perfusion pressure was manipulated using an iliac artery occlusion cuff.
• Blood flow velocity profiles and CFL thickness were measured using intravital microscopy.

Main Results:

• Reduced perfusion pressures led to decreased volumetric flow rates and increased local vascular resistance.
• Increased blood apparent viscosity, driven by changes in CFL thickness, was the primary cause of elevated resistance.
• Perfusion pressure was found to directly influence shear rates and CFL thickness.

Conclusions:

• Blood rheology, specifically apparent viscosity modulated by CFL thickness, acts as a key intrinsic mechanism regulating blood flow in arterioles.
• At low perfusion pressures, changes in blood rheology, rather than vessel diameter, appear to be the dominant factor limiting flow to tissues with minimal myogenic or metabolic responses.
• Findings highlight the importance of considering blood rheological properties in microvascular research and clinical applications.