Mechanically, arteriogenesis is linked to elevated pressure, which increases radial wall stress, and elevated flow, which increases endothelial surface stress. The vessel increases in diameter until the stress is normalized (Prior et al., 2004). Arteriogenesis does not occur every time there is an increase in flow, however. Most vessel networks can handle increased flow without increasing diameter due to the fact that flow is related to vessel diameter by a power of four. Initial experiments demonstrated this phenomenon in that mature vessels are unlikely to respond to increased flow by increasing diameter, but will respond to decreased flow by decreasing diameter (Brownlee & Langille, 1991). Another experiment showed that increasing shear stress caused an immediate increase in vessel expansion followed by a rapid decrease, as well as demonstrating that the mature vessels do indeed respond more favorably to decreased stress (Tuttle et al., 2001).