The influence of oxygen tension on the local regulation of blood flow by shear stress

Abstract

Introduction and purpose. Vascular response to mechanical stimuli, namely transmural pressure (Bayliss effect) and wall shear stress (response to blood flow), play an important role in regulation of vascular tone. The purpose of the work was to study an influence of hypoxia on the vessel radius and blood flow control by response to shear stress. Methodology/approach. Mathematical simulation was used. The model is based on published data of experiments on small cerebral arteries of rats. The main assumptions of the model are: 1) the vessel is a thin wall cylinder; 2) the radius is controlled by two parameters: concentration of free calcium ions in the cytoplasm of the smooth muscle cells and concentration of nitric oxide (NO) in the smooth muscle layer; 3) the rate of NO production by endothelium is proportional to modulus of shear stress on the vessel wall. The apparent blood viscosity is calculated using the solution of the problem of two-layer flow. The numerical experiments were performed in Turbo Pascal. The main results and discussion. The dependence of vessel tone regulation by response to altered shear stress on oxygen tension is caused by dependence of NO synthesis in endothelium and NO consumption on oxygen concentration. As it follows from mathematical simulation, hypoxia reduces the role of mechanogenic regulation, and the increase of the wall sensitivity to NO makes this effect more appreciable. Calculations performed for typical value of cerebral vessel response to shear stress, show that the fall in oxygen tension from 100 to 30 per cent leads to decrease in diameter by 6 %, in blood flow rate by 11 %. The rheological factors prevent flow rate diminution, but their contribution is very small: less than 3 %. The fall in oxygen tension reduces NO production rate by endothelial cells and NO concentration in the vessel wall. At strong hypoxia (reduction in oxygen tension from 100 to 30 % and less) NO concentration in smooth muscle layer drops by more than 15 %. Conclusions. Hypoxia decreases NO-dependent vessel response to altered shear rate. This effect increases with the value of vessel response to shear stress. The rheological factors impede the decrease of this response.