health

The most common type of embolism results from the clotting of blood.
When blood clots form inside blood vessels, which is a process called
thrombosis, they may break free and travel to the pulmonary artery. The
pulmonary artery carries blood from the right side of the heart into the
lungs. In the lungs, blood disposes of carbon dioxide and picks up more
oxygen. When blood clots get stuck in the pulmonary artery, they can
prevent blood from picking up oxygen. This condition is a medical emergency
that causes symptoms similar to a heart attack.
Thrombophlebitis (throm-bo-fluh-BI-tis) can occur when a blood clot
causes inflammation inside a vein (usually in the legs). If the clot forms in a
vein found in the muscle, it causes what is known as a deep vein thrombosis
(DVT). A DVT may break free, becoming an embolus that travels inside the
blood vessel to the lungs and eventually causes blockage of the pulmonary
artery inside the heart. When this occurs, it is called a pulmonary embolism.

The following are substances, although less common, that may cause
embolism:

• Bubbles. Sometimes bubbles occur in the blood vessels of underwater
  divers as they ascend; as compressed nitrogen bubbles out of
  solution, divers develop joint pain, called the bends, from decompression
  sickness. Bubbles can also occur during injections of fluids
  into veins or arteries, which is one reason why doctors and nurses
  squeeze the air out of needles before using them on patients.
  
• Tumors. These growths may block blood flow or a piece of a tumor
  may break off into the bloodstream and get caught in another part
  of the body.
  
• Fat. An embolism may occur when fat breaks off in place and travels
  through the blood vessel. For example, when a serious injury
  occurs to an organ such as the liver, fat may break off and travel
  through the bloodstream.
  
• Bone fragments. A bone chip from a broken arm or leg may become
  lodged in a blood vessel.

An embolism may occur in any artery. However, it often occurs in the
pulmonary arteries because all the blood returning to the heart from the
body is pumped through the pulmonary arterial system first.