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Collateral circulation is circulation in an area of tissue or an organ with a number of different pathways for blood to reach it. 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. Even though the main vessels to the leg are blocked, enough blood can get to the tissues in the leg via the collateral circulation to keep them alive.
Cerebral collateral circulation Blood flow to the brain is maintained via a network of collateral arteries that anastomose in the circle of Willis, which lies at the base of the brain. In the circle of Willis so-called communicating arteries exist between the front (anterior) and back (posterior) parts of the circle of Willis, as well as between the left and right side of the circle of Willis.
Cardiac collateral circulation Another 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.
Collateral Circulation in the Venous System Hepatic Cirrhosis arising from chronic congestion in the portal vein may give rise to collateral circulation between branches of the portal and caval veins of the liver, or between the two caval veins. Consequences of newly established venous collaterals arising from portal hypertension include oesophageal varices and hemorrhoids (porto-caval collateral circulation).
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