The two mechanisms of increase.

When a forking network (α = 1.1, μm) is pruned, perfusion () can increase, shown in (A), via Mechanism I when blood flow is rerouted from pruned vessels to other vessels, as seen in (B) where the number of perfused vessels increases and the number of hypoperfused vessels decreases. In contrast, perfusion () can also increase, shown in (A), via Mechanism II when hypoperfused vessels are pruned without any net increase in the number of perfused vessels, as evidenced in (B) by the constant number of perfused vessels.