Figure 5. Global decrease of OTRs leads to vessel rarefaction.

For the parallel system shown in Figure 1 (C), the OTRs of the two switches are different. When the OTRs decrease with the ratio fixed, the stable co-existing equilibrium state becomes unstable. Similarly, for complex vessel systems, when there is a global decrease of the OTRs, the vessels with small OTRs may become degenerate and micro-vessel rarefaction occurs. If the OTRs decrease too much, there can be a global degeneration of the vessel system.