Alternative pathways taken by blood when the normal route through an artery or vein is blocked or has not developed properly. Because of the obstruction the alternative pathways (collateral’s) may increase in size and eventually may become nearly as large as the original artery or vein, which is blocked. In some conditions (e.g. Pulmonary Atresia) large collateral’s may form before birth, carrying blood to the lung circulation from the aorta (sometimes referred to as MAPCAs Major Aorto Pulmonary Collateral Arteries).
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