Blood clots are formed by the process of coagulation - the blood’s natural tendency to clump and plug an injured blood vessel. Blood clots are made of blood cells and fibrin strands. They serve a valuable function in wound healing and stopping the flow of blood after an injury.

For example, when a person is cut, blood flows from the injured blood vessel for a short period. However, the clotting process begins almost immediately. During this process, blood platelets first gather at the site of the injury and form a loose plug. These platelets release a number of chemicals that enhance and promote blood clotting. Once the loose platelet plug is in place, a mesh of fibrin forms to create a stronger blood clot. This blood clot will remain in place as the tissue injury heals. After it heals, still other chemicals are responsible for dissolving the clot. This process of forming and dissolving blood clots is called hemostasis.

Blood clots are rarely dangerous on their own. As part of the body’s natural healing mechanism, clots that have formed for whatever reason are usually dissolved (lysis) and reabsorbed by the body without danger or need for intervention.

They can be dangerous, however, when they form within arteries and veins and obstruct the normal flow of blood within the body. Dangerous blood clots can form after surgery or in response to traumatic injury, such as auto accidents. 

They can also form as a result of the gradual buildup of plaque within an artery (e.g., atherosclerosis) or other diseases. Blood clots can form in any vein or artery in the body, including the coronary arteries that supply the heart with blood, the veins in the legs and the arteries that supply the brain with blood (e.g., carotid arteries).

A blood clot that forms in a location where it partially blocks a vein or artery is known as a thrombus. In some cases, a blood clot breaks off from the site it was created and travels through the bloodstream until it becomes lodged in a smaller blood vessel, blocking the blood supply. This is known as an embolus.

A blood clot can be life-threatening if it either forms or becomes lodged within any of the following arteries:

• The pulmonary artery, which travels from the heart to the lungs. Blockage in this artery is called a pulmonary embolism and may result in damage to the lung and, in some cases, death. These may be caused by an abnormal heart rhythm called atrial fibrillation or by a peripheral venous disorder in the deep veins of the legs (e.g., deep vein thrombosis).
  
  
• One of the coronary arteries, which are located on the surface of the heart and supply the heart muscle with fresh, oxygen-rich blood. Blockage of any of the coronary arteries could trigger a heart attack. Blood clots usually form in this circumstance in response to plaque rupture, which occurs when a plaque deposit on the inner wall of a coronary artery fractures, triggering the clotting process. 
  
  
• The carotid arteries, which lie in the neck and supply the brain with oxygen-rich blood. Blockage could trigger a stroke or a mini-stroke (transient ischemic attack).

  

• The femoral artery in the leg. Blockage could cause pain (claudication), lack of color in that area and weakness. If left untreated, it could result in tissue death (necrosis) and infection (gangrene).
  
  
• An artery in the abdomen. Blockage could cause pain, vomiting and/or diarrhea.