Good collateral circulation occurs in

Collateral circulation is the alternate circulation around a blocked aery or vein another path, such as nearby minor vessels. It may occur preexisting vascular redundancy (analogous to engineered redundancy), as in the circle of willis in the brain, or it may occur new branches formed between adjacent blood vessels (neovascularisation), as in the eye after a retinal embolism. Its formation may be provoked by pathological conditions such as high vascular resistance or ischemia . An example of the usefulness of collateral circulation is a systemic thromboembolism in cats. This is when a thrombotic embolus lodges above the external iliac aery (common iliac aery), blocking the external and internal iliac aeries and effectively shutting off all blood supply to the hind leg. Even though the main vessels to the leg are blocked, enough blood can get to the tissues in the leg the collateral circulation to keep them alive. Ref - sciencedirect.com