An embolus is a detached, traveling mass carried by the bloodstream that ultimately lodges in a blood vessel too narrow for it to pass through. This blockage, known as an embolism, is a serious medical event because it abruptly cuts off blood flow to downstream tissues or organs. This sudden lack of oxygen and nutrients can quickly lead to tissue damage and organ failure. Because an embolus travels, it can originate in one part of the body and cause devastating effects far away from its source.
Anatomy of an Embolus: Composition and Origin
The term embolus describes a mobile object, which clearly differentiates it from a thrombus, a blood clot that remains stationary at its site of formation, such as in a deep vein or the heart chamber. The vast majority of emboli are fragments of a thrombus that have broken off, leading to a condition called a thromboembolism. These clots are typically composed of a meshwork of fibrin, platelets, and trapped blood cells.
While blood clots are the most common source, an embolus can be formed from a variety of non-blood materials that enter the circulation. Fat emboli often arise following severe trauma or fractures of long bones, where fat globules from the bone marrow are released into the veins. Cholesterol emboli can detach from atherosclerotic plaques, which are fatty deposits lining the walls of larger arteries.
Air or gas emboli can occur during surgical procedures or from rapid decompression in diving, where bubbles form in the blood and act as a physical obstruction. Other rare forms include septic emboli, which are clumps of bacteria or infected material, and amniotic fluid emboli, which occur when amniotic fluid enters the mother’s bloodstream during childbirth. Common sites of origin for thromboemboli include the deep veins of the legs (DVT), or the left side of the heart in people with irregular heart rhythms like atrial fibrillation.
The Circulatory Path: Where Emboli Travel and Lodge
An embolus travels through the body’s circulatory system until it encounters a vessel whose diameter is too small to permit its passage. The location where the embolus finally lodges dictates the type of embolism that occurs, broadly categorized into the venous or arterial system. The venous circulation carries deoxygenated blood from the body back to the heart’s right side, which then pumps it into the lungs.
Emboli originating in the venous system, such as from DVT in the legs, travel to the right side of the heart and are then propelled into the pulmonary arteries of the lungs. This blockage of a lung artery is known as a pulmonary embolism (PE), and it is the most frequent type of embolism.
Conversely, systemic emboli typically originate in the left side of the heart or the major arteries and travel through the arterial system, which carries oxygenated blood out to the rest of the body. These systemic emboli can lodge in any organ, but they commonly travel to the brain, causing an ischemic stroke, or to the extremities, leading to acute limb ischemia. In rare instances, an embolus from the venous system can cross over to the arterial system through a structural defect in the heart, like a patent foramen ovale (PFO), a condition known as a paradoxical embolism.
Immediate Health Consequences
The most serious consequence of an embolus lodging in a vessel is the sudden deprivation of blood and, therefore, oxygen to the downstream tissue. This lack of blood flow is termed ischemia, which, if sustained, rapidly leads to tissue death, or infarction. The clinical signs that manifest depend entirely on the organ affected by this lack of perfusion.
In the case of a pulmonary embolism, the blockage in the lung’s arteries can cause sudden shortness of breath, sharp chest pain that worsens with deep breathing, and a rapid heart rate. If a systemic embolus travels to the brain, it causes an embolic stroke, with symptoms appearing abruptly. These symptoms often include sudden weakness or numbness, slurred speech, confusion, or difficulty with vision.
When the blockage occurs in an artery supplying a limb, the consequences are recognized by specific signs of severe ischemia. These peripheral emboli cause sudden, severe pain, paleness of the skin (pallor), lack of a pulse below the blockage (pulselessness), and potentially paralysis. Timely recognition of these symptoms is important, as the longer the tissue is starved of oxygen, the greater the extent of irreversible damage.