Development of Collateral Circulation-A Phenomenon of Long- Term Local Blood Flow Regulation
When an artery or a vein is blocked in virtually any tissue of the body, a new vascular channel usually develops around the blockage and allows at least partial resupply of blood to the affected tissue. The first stage in this process is dilation of small vascular loops that already connect the vessel above the block-age to the vessel below. This dilation occurs within the first minute or two, indicating that the dilation is simply a neurogenic or metabolic relaxation of the muscle fibers of the small vessels involved. After this initial opening of collateral vessels, the blood flow often is still less than one quarter that needed to supply all the tissue needs. However, further opening occurs within the ensuing hours, so that within 1 day as much as half the tissue needs may be met, and within a few days often all the tissue needs.
The collateral vessels continue to grow for many months thereafter, almost always forming multiple small collateral channels rather than one single large vessel. Under resting conditions, the blood flow usually returns very near to normal, but the new channels seldom become large enough to supply the blood flow needed during strenuous tissue activity. Thus, the development of collateral vessels follows the usual principles of both acute and long-term local blood flow control, the acute control being rapid neurogenic and metabolic dilation, followed chronically by manifold growth and enlargement of new vessels over a period of weeks and months.
The most important example of the development of collateral blood vessels occurs after thrombosis of one of the coronary arteries. Almost all people by the age of 60 years have had at least one of the smaller branch coronary vessels close. Yet most people do not know that this has happened because collaterals have devel-oped rapidly enough to prevent myocardial damage. It is in those other instances in which coronary insuffi-ciency occurs too rapidly or too severely for collater-als to develop that serious heart attacks occur.