Blood Clots

Deep Vein Thrombosis · Thrombophlebitis · Phlebitis

The Facts

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 (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 (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.


Ischemic strokes (strokes caused by blood clots blocking an artery that supplies the blood to the brain) can be due to blood clots that have formed in the heart as a result of rhythm disorders known as atrial fibrillation and atrial flutter.

Atrial fibrillation is a type of irregular heartbeat (arrhythmia) that triggers a rapid quivering beat in the upper chambers of the heart (the atria). This irregular pumping may cause some blood to remain in the heart chamber and form clots, which can then travel to the brain. Emboli can also form at the site of artificial heart valves, after a heart attack, or as a result of heart valve disorder or heart failure.

Cardiogenic embolism refers to clots that form inside the heart and travel to the brain. A stroke may occur in someone who has suffered a heart attack. A heart that has been damaged by a heart attack doesn't pump blood properly, which can cause formation of a blood clot that travels to the brain. The presence of artificial replacement heart valves can also lead to blood clot formation. By taking blood-thinning medications, people with artificial heart valves help to prevent clots from forming.

Some blood clots may form in a narrowed artery as a result of atherosclerosis, commonly known as hardening of the arteries. In this case, the arterial walls slowly thicken, harden, and narrow over time until blood flow is reduced. As a result, these arteries become vulnerable to injury. If they tear, a blood clot may form, completely blocking the already narrowed artery and shutting off oxygen to part of the heart or brain.

Blood clotting problems and rare blood disorders also cause blood clots to form, but the cause of blood clots is not always known.

For some women, the use of oral contraceptives (birth control pills) may increase the risk blood clot formation. The risk is higher for women over 35 years of age who smoke or have a history of previous blood clots.

A variety of things can cause inflammation of a superficial vein. One common cause is due to trauma or injury, for example from solutions or medications given intravenously (into a vein) in hospitals. Piercing the vein to give the solution or medication can cause irritation. Any trauma to a vein (such as an injury from a car accident) will trigger inflammation in the area, leading to pain, discomfort, redness, and swelling. During this process, there is increased blood flow to the injury, and a blood clot often forms in the inflamed or injured area of the vein. Superficial thrombophlebitis is an uncomfortable condition but rarely causes serious problems.

Sometimes, thrombophlebitis is caused by a bacterial infection in the vein. The usual culprit is a bacteria called Staphylococcus, commonly found on the skin.

In certain cases, thrombophlebitis develops without an obvious reason. It may develop in the leg veins of pregnant women, in people with varicose veins, and in some people with cancer in the abdomen (particularly the pancreas). Women over the age of 35 years who smoke and take oral contraceptives (birth control pills) are at a higher risk of developing blood clots.

DVT occurs when blood clots form in the deep veins of the legs or pelvis, and is often caused by:

  • prolonged sitting or bed rest
  • surgery or trauma (especially hip surgery, gynecological surgery, heart surgery)
  • medications such as estrogen, and birth control pills with higher levels of estrogen
  • injury to the leg or immobilization (such as casting after a broken bone)

Certain inherited conditions can make DVT more likely to occur. Blood flow in the veins depends on contraction of surrounding muscles, and with inactivity, such as extended bed rest, the blood starts to collect and blood clots can easily form.

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The contents of this health site are for informational purposes only. Always seek the advice of your physician or other qualified healthcare provider regarding any questions you may have about a medical condition.

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