A thrombus is a blood clot that forms inside a blood vessel, often causing a blockage of the flow of blood through the vascular system.

The process of thrombus formation, called “thrombosis”, results from the activation of the normal blood clotting system. In pathological thrombosis, the process of coagulation far exceeds the body’s capacity to dissolve the clot. As additional layers of platelets and fibrin are deposited, the thrombus continues to grow larger until a mass is formed. This thrombus can eventually block the space within a blood vessel called the “lumen”, resulting in complete blockage.

An embolus is the sudden blockage of an artery by a mass that is carried through the bloodstream, eventually lodging in a blood vessel that is too small to allow its passage. The mass can be composed of fat, cancer cells, air, or even a blood clot. For example, a thromboembolism is a thrombus that detached and traveled through the circulatory system eventually blocking a distant artery.

When a long bone is fractured, or during orthopedic surgery, damaged fat cells in the bone marrow may release fragments of fat into the bloodstream – that can form a fat embolism. Similarly, large fragments of bone marrow including hematopoietic tissue may be released into the bloodstream, which can cause a bone marrow embolism. Clumps of cancer cells may break free into the circulation to form tumor emboli.

An air embolus can form as a result of the inadvertent introduction of air bubbles during a catheterization procedure. There are two main types of embolus depending on whether it lodges in the pulmonary or systemic circulation. A pulmonary embolus originates in peripheral veins and is carried to the heart where it enters the pulmonary circulation, eventually becoming lodged in the pulmonary arterial system. A systemic embolus originates in the heart and travels via the aorta to the systemic arterial circulation. They commonly affect the arteries leading to the brain, kidneys, spleen, gut, and lower limbs.