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.

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 rhythm problems)
• Heart attack
• Heart failure
• 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.

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.

Self-care measures

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.