How does atherosclerosis cause disease?
Atherosclerosis causes disease in two ways. 1) Atherosclerosis can limit the ability of the narrowed arteries to increase delivery of blood and oxygen to the tissues of the body during times when oxygen demand needs to be increased, for example, during exertion; or 2) complete obstruction of an artery by a thrombus or embolus (thrombus and embolus are forms of blood clots), which results in tissue death (necrosis). Exertional angina and intermittent claudication are two examples of insufficient delivery of blood and oxygen to meet tissue demand; whereas strokes and heart attacks are examples of death of tissue caused by complete artery obstruction by blood clots.
There are many similarities between coronary heart disease (or coronary artery disease, which is atherosclerosis involving the arteries of the heart) and peripheral artery disease, and the two conditions may coexist in the same individual. For example, patients with exertional angina typically have no symptoms at rest. However, during exertion, the critically narrowed coronary arteries are incapable of increasing blood and oxygen delivery to meet the increased oxygen needs of the heart muscles. Lack of blood and oxygen causes chest pain (exertional angina). Exertional angina typically subsides when the patient rests. In patients with intermittent claudication, the narrowed arteries in the lower extremities (for example, a narrowed artery at the groin) cannot increase blood and oxygen delivery to the calf muscles during walking. These patients experience pain in the calf muscles that will only subside after resting.
Patients with unstable angina have critically narrowed coronary arteries that cannot deliver enough blood and oxygen to the heart muscle even at rest. These patients have chest pain at rest and are at imminent risk of developing heart attacks. Patients with severe artery occlusion in the legs can develop rest pain (usually in the feet). Rest pain represents such severe occlusion that there is the insufficient blood supply to the feet even at rest. They are at risk of developing foot ulcers and gangrene.
When atherosclerosis narrows the arteries, blood tends to clot in the narrowed areas, forming a thrombus, a type of blood clot (plural thrombi). Sometimes pieces of the blood clot break off and travel in the bloodstream until they are trapped in a narrower point in the artery beyond which they cannot pass. A thrombus or piece of thrombus that travels to another point is called an embolus. Thrombi and emboli can cause sudden and complete artery blockage, leading to tissue necrosis (death of tissue due to lack of oxygen).
For example, complete blockage of a coronary artery by a thrombus causes heart attack, while complete blockage of the carotid or cerebral artery causes ischemic stroke. Emboli originating from atherosclerosis in the aorta (the main artery delivering blood to the body) can obstruct small arteries in the feet, resulting in painful and blue (cyanotic) toes, foot ulcers, and even gangrene.
What are collateral vessels or collateral circulation?
Sometimes, despite the presence of a severe blockage in an artery, the involved area does not become painful or lose its blood supply due to the presence of collateral vessels. Collateral circulation means that the particular area is supplied by more than one artery so that the blockage of a single vessel does not result in a severe degree of blood loss. Collateral circulation can develop over time to help provide oxygenated blood to an area where an artery is narrowed. Doctors believe that regular supervised exercise can stimulate the growth and development of collateral circulation and relieve symptoms of intermittent claudication.