FIGURE 1

Blood flow-derived forces. Schematic representation of the mechanical forces experienced by endothelial cells due to blood circulation inside the vessels. The blood flow, measured by the volume flow rate Q, causes a shear stress σ_shear on the wall. The shear stress depends on the flow rate Q, the blood viscosity η and the vessel radius R. The relationship shown is valid in laminar flow, occurring at low Reynolds numbers. Independently of the flow, the hydrostatic pressure p causes a circumferential (hoop) stress σ_circ and an axial stress σ_axial. Contrary to shear stress, they increase linearly with the vessel radius. The circumferential stress has twice the magnitude of the axial stress. The equations hold for thin walls (D≪R). Furthermore, we assumed that no external forces are acting on the vessel, such that the pressure forces are fully counterbalanced by the stress in the walls (depending on the mechanical environment, the force-free condition may not always be fulfiled, especially in the axial direction).