This is often a result of anastamoses, branches formed between adjacent blood vessels. Collateral circulation can be established in the venous system (between veins) or in the arterial system (between arteries). It may arise from pathological conditions such as congestion or ischaemia.An example of the usefulness of collateral circulation is a systemic thrombo-embolism in cats.This is when a thrombus lodges above the external iliac artery (common iliac artery), blocking the external and internal iliac arteries and effectively shutting off all blood supply to the hind leg.nother example (in humans) is where a person suffers an acute myocardial infarction (heart attack). Collateral circulation in the heart tissue will sometimes bypass the blockage in the main artery and supply enough oxygenated blood to enable the cardiac tissue to survive and recover.