Collateral Blood Flow
Collateral vessels can play a significant role in supplying oxygen to an organ, particularly when oxygen delivery is limited by disease in the normal vasculature. Collateral vessels can be pre-existing vessels (usually small arteries and arterioles) that normally have little or no blood flow depending on the distribution of pressures within the vascular bed. Acute occlusion of normal vessels (e.g., thrombosis of a large artery) can cause a redistribution of pressures within the vascular bed thereby causing blood flow to occur in pre-existing collateral vessels. Conditions of chronic stress (e.g., endurance exercise training or chronic ischemia) can cause new blood microcirculatory vessels to form by angiogenesis.
Collateral blood vessels are particularly important in skeletal muscle and coronary circulations. Conditions of chronic limb ischemia, caused by arterial disease, for example, can lead to the development of collateral vessels. These vessels are in parallel with the diseased vessels and therefore can partially take over the function of suppling blood flow to the limb. These collateral vessels, along with flow autoregulation, can help to maintain normal blood flow in the resting limb, although maximal flow capacity is ordinarily reduced despite the presence of the collaterals. In the heart, collateral vessels can help to supply blood flow to ischemic regions caused by stenosis or occlusion of epicardial arteries. Collateral blood flow may be an important mechanism helping to limit infarct size in the heart when a large coronary artery suddenly becomes occluded by a thrombus. Current research is directed towards stimulating the formation of collateral blood vessels by treating the heart with drugs that stimulate angiogenesis.