Once these clots form, they can travel to other parts of the body where they can cause harm. There are a variety of factors and conditions that can cause troublesome blood clots, and there are some serious conditions that are associated with blood clots. These include:

• Antiphospholipid syndrome
• Arteriosclerosis / atherosclerosis
• Cancer
• Certain medications, such as oral contraceptives and hormone therapy drugs
• Coronavirus disease 2019 (COVID-19)
• Deep vein thrombosis (DVT)
• Factor V Leiden
• Family history of blood clots
• Heart arrhythmia
• Heart attack
• Heart failure — a condition in which the heart doesn't pump blood as well as it should.
• Obesity
• Peripheral artery disease (PAD)
• Polycythemia vera
• Pregnancy
• Prolonged sitting or bed rest
• Pulmonary embolism — a blood clot in an artery in the lung
• Smoking
• Stroke
• Surgery

Blood clots are gel-like clumps of blood. When they form in response to a cut or other injury, they stop the bleeding by plugging the injured blood vessel. These blood clots help the body heal.

But some blood clots form inside the veins without a good reason. They don't dissolve naturally. These clots may require medical attention, especially if they are in the legs, lungs or brain. A number of conditions can cause this type of blood clot.

Blood clots are made when substances in the blood thicken and form a semisolid mass. This process may be triggered by an injury. Sometimes it occurs inside blood vessels that don't have an obvious injury.

Seek emergency care if you experience:

• Cough that produces bloody sputum.
• A fast heartbeat.
• Lightheadedness.
• Difficult or painful breathing.
• Chest pain or tightness.
• Pain that spreads to the shoulder, arm, back or jaw.
• Sudden weakness or numbness of the face, arm or leg.
• Sudden difficulty speaking or understanding speech.

Consult your health care provider if you develop these symptoms in an area on an arm or leg:

• Swelling.
• Change in skin color, such as an area on the leg that looks unusually red or purple.
• Warmth.
• Pain.

To reduce the risk of developing blood clots, try these tips:

• Avoid sitting for long periods. If you travel by airplane, walk the aisle now and then. For long car trips, stop frequently and walk around.
• 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 increase the risk for blood clots.
• Change your lifestyle. Lose weight, lower high blood pressure, stop smoking and exercise regularly.