Blood clots are a collection of sticky blood cells that form when a blood vessel is damaged. The body creates blood clots as a normal response to blood vessel damage. The main job of a blood clot is to seal the leak in a damaged blood vessel. This prevents the blood from leaking out and protects the person from bleeding.
Clots (or thrombi) that block the arteries and prevent flow of blood and oxygen to an organ can lead to areas of tissue damage (infarcts). When blood clots break (called an embolism) away from the area they're meant to protect, they can endanger other organs.
Clots that block blood flow are the main culprits in most heart attacks and strokes. They can also damage other organs:
- When a blood clot (thrombus) forms in one or more arteries that supply blood to the heart, it blocks the blood flow to a part of the heart muscle, reducing or completely cutting off the oxygen supply to the cells in that area. As a result, the part of the heart muscle that is deprived of oxygen dies, and a heart attack occurs.
- Clots that block the flow of oxygen to the brain are the primary cause of strokes.
- Clots that form in the eye may cause sudden blindness.
Presence of an obstructing blood clot (thrombus) is referred to as thrombosis. Thrombosis in a vein is almost always associated with phlebitis (inflammation of a vein). Thrombophlebitis is an inflammation of a vein in the area where a blood clot has formed.
Thrombophlebitis is classified as either superficial or deep. In other words, thrombosis can affect either superficial (surface) or deep (below the surface) veins, causing thrombophlebitis.
Superficial thrombophlebitis occurs when a blood clot affects veins near the skin surface, or superficial veins.
Deep venous thrombosis (DVT) occurs when a blood clot affects deeper, larger veins, such as those in the lower legs and thighs. DVT is more worrisome than superficial thrombophlebitis. These clots can break away (called emboli) from a blood vessel and cause a pulmonary embolism if they travel to the lung. (For more information on pulmonary embolism, see the section "Symptoms and Complications.") DVT is more common for people over 40 years of age.