Medically reviewed on January 11, 2018.
Blood clots are gel-like clumps of blood. They are beneficial when they form in response to an injury or a cut, plugging the injured blood vessel, which stops bleeding.
Some blood clots form inside your veins without a good reason and don't dissolve naturally. These may require medical attention, especially if they are in your legs or are in more critical locations, such as your lungs and brain. A number of conditions can cause this type of blood clot.
Blood clots form when platelets (blood components) and plasma proteins thicken, forming a semisolid mass. This process may be triggered by an injury or it can sometimes occur inside blood vessels that don't have an obvious injury.
Once these clots form, they can travel to other parts of your body, causing harm. Factors and conditions that can cause troublesome blood clots, as well as serious conditions that are associated with blood clots, include:
- Antiphospholipid syndrome
- Arteriosclerosis / atherosclerosis
- Certain medications, such as oral contraceptives and hormone therapy drugs
- Deep vein thrombosis (DVT)
- Factor V Leiden
- Family history of blood clots
- Heart arrhythmias (heart rhythm problems)
- Heart attack
- Heart failure
- Peripheral artery disease
- Polycythemia vera
- Prolonged sitting or bed rest
- Pulmonary embolism (blood clot in an artery in the lung)
When to see a doctor
Seek emergency care if you experience:
- Cough that produces bloody sputum
- A fast heartbeat
- Difficult or painful breathing
- Chest pain or tightness
- Pain extending to your shoulder, arm, back or jaw
- Sudden weakness or numbness of your face, arm or leg
- Sudden difficulty speaking or understanding speech (aphasia)
- Sudden changes in your vision
Consult your doctor if you develop these signs or symptoms in an area on an arm or leg:
To reduce your risk of developing blood clots, try these tips:
- Avoid sitting for long periods. If you travel by airplane, walk the aisle periodically. For long car trips, stop and walk around frequently.
- Move. After you've had surgery or been on bed rest, the sooner you get up and move around, the better.
- Drink plenty of fluids when traveling. Dehydration can contribute to the development of blood clots.
- Change your lifestyle. Lose weight, lower high blood pressure, stop smoking and exercise regularly.