An embolus is a particle that travels in the blood vessels, includig veins or arteries. Most emboli are clots of blood cells. A blood clot that travels in the blood stream is called a thromboembolus.
When an embolus travels through the blood vessels, it lodges there, blocking up the blood that comes up from behind. The cells that normally receive their blood supply via this passage are starved of oxygen (develop ischemia) and die. This condition is called an embolism.

Pulmonary embolism: an embolus, usually formed in the leg (sometimes referred to as deep vein thrombosis (DVT)), becomes lodged in one of the pulmonary arteries. Many embolisms are dissolved by the body and disappear spontaneously, but severe pulmonary embolism can cause death.

Cerebral embolism: If a blood clot travels to the brain, it causes an ischemic stroke or transient ischemic attack.

Retinal embolism: Small clots that block a major artery can block the smaller blood vessels that nourish the retina at the back of the eye. The result is a sudden loss of vision in one eye.

Septic embolism: it occurs when the particles generated by infection of the body reaches the bloodstream and block blood vessels.

Amniotic fluid embolism: emboli are not all made of coagulated blood. During pregnancy, the uterus is filled with amniotic fluid, which protects the fetus. The amniotic fluid can form an embolus that can travel to the lungs, causing pulmonary amniotic embolism.

Air Embolism: the deep-sea divers who ascend too rapidly to the surface may develop an air embolism caused by air bubbles in the blood that can block blood flow in arteries.

Fat embolism: if particles or even fat from the bone marrow is introduced into the bloodstream, they can block blood vessels in the same way as a blood clot or air bubble.

The main cause of most pulmonary embolisms is deep vein thrombosis (DVT). It is a condition where blood clots develop in the leg veins. Natural agents in the blood often dissolve small clots. However, some clots are too large to be dissolved and large enough to block major blood vessels in the lungs or brain.

Factors that slow blood flow in the legs can contribute to clot formation. DVT or pulmonary emboli may develop in people who have been sitting for too long during a long flight, or as a result of immobilization of a leg in a cast or prolonged bed rest without moving the legs. Other factors associated with DVT or pulmonary embolism, are metastatic cancer, previous surgery, a broken leg or hip, or a genetic predisposition of blood cells to clot.

Symptoms of pulmonary embolism can be mild or severe. Some people have many small emboli detectable only by special radiographic techniques. Severe obstruction, however, may cause serious breathing problems or death.

Symptoms of pulmonary embolism appear suddenly and include:
• shortness of breath, rapid breathing or wheezing;
• bloody sputum;
• cough;
• a light-headedness, dizziness, fainting;
• severe pain in the chest or back.