Microvascular adaptation--regulation, coordination and function.

Abstract

Microvascular networks have to adapt continuously in response to changes of the local environment in order to maintain adequate function. This adaptation involves reactions to hemodynamic and metabolic stimuli. The present study analyzes fundamental requirements for vascular adaptation by combining experimental observations in microvascular networks and mathematical simulations. Angioarchitecture and flow distribution were analyzed in microvascular networks of the rat mesentery by intravital microscopy. In addition, blood flow and oxygen distribution in these networks were simulated using a mathematical model. The model was based on experimental information on blood rheology in microvessels. In addition, the diameter adaptation of vessel segments (n = 300-1000) in the networks to different sets of stimuli was simulated. The hemodynamic analysis shows that, in the experimentally observed network architecture, average wall shear stress declines consistently with intravascular pressure (from about 100 dyn/cm2 for pressures of 70 mmHg to about 10 dyn/cm2 for pressures of 15 mmHg) indicating the importance of hemodynamic factors for vascular adaptation. However, to obtain stable adaptation of microvascular networks, additional responses to the metabolic situation and information transfer from distal to proximal vessels were needed. The metabolic stimuli maintain parallel flow pathways and adequate supply of distal tissue regions, while the hemodynamic factors optimize network structure and minimize energy expenditure.