Fig. 2.

Schematic diagram to illustrate effects of responses to haemodynamic and metabolic stimuli on structural adaptation of diameters in microvascular networks. In each case, small arrows represent flow direction from arteriole (red) to venule (blue). Diameter changes are exaggerated for clarity. (A) Response to pressure leads to arteriovenous asymmetry: venule diameters are larger than corresponding arteriolar diameters. The response to wall shear stress has the effect of destabilizing parallel flow pathways. Whichever pathway experiences the larger wall shear stress increases in diameter, taking a larger share of the flow and thereby further increasing its wall shear stress. The other branch shrinks to zero diameter if no other stimuli are included. (B) As in (A), but including an additional local metabolic signal, such that a low-flow channel receives a growth stimulus. Inclusion of metabolic signal stabilizes parallel pathways. (C) Configuration with unequal pathway lengths. Pressure response acts as in (A). If shear and local metabolic signals are included, the short flow pathway tends to increase in diameter and take most of the flow. The long pathway receives inadequate flow. (D) As in (C), but including upstream conducted metabolic response. The long flow pathway receives a large conducted signal and increases in diameter, leading to a flow distribution that is distributed more appropriately according to tissue needs.