What is a Blood Clot?

Updated January 2022

Blood clotting, also known as coagulation, is the body’s first line of defense against bleeding. When we hurt ourselves, our clotting system forms a “plug” or “seal” to protect us from losing too much blood. Our bodies often break down the clot after we’ve healed – but sometimes, clots form inappropriately or fail to dissolve after an injury. A blood clot that forms and stays in a blood vessel is called a thrombus.

Other medical terms used to describe blood clots include:

Thrombosis: When a thrombus forms in a blood vessel

Embolus or Embolism: A clot that detaches and travels through blood vessels to another part of the body

There are two main types of thrombosis: 

Arterial thrombosis refers to a blood clot that blocks an artery. Arteries carry blood away from the heart to other parts of the body. Arterial blood clots can block blood flow to the heart and brain, often resulting in a heart attack or stroke.

Venous thrombosis, also known as venous thromboembolism or VTE, refers to a blood clot in a vein. Veins carry blood to the heart from other parts of the body. VTE is a condition that includes deep vein thrombosis (DVT) and pulmonary embolism (PE).

What is deep vein thrombosis (DVT)?

DVT occurs when a blood clot forms in a major vein. The clot halts blood flow in the vein, which can lead to pain and swelling in the area of the clot. DVTs usually develop in the legs but can also form in the arms or pelvis. DVTs are often diagnosed with ultrasounds.

What is pulmonary embolism (PE)?

A PE occurs when a clot elsewhere in the body travels through the bloodstream to the lungs. PEs block blood flow to the lungs, decrease the amount of oxygen in the blood, and affect a person’s ability to breathe. PEs can be life-threatening.

Know the symptoms


  • Pain, discomfort, heaviness, or tenderness in affected area; sometimes pain is described as aching, throbbing, or cramping
  • Swelling, itchiness, or warmth near the clot
  • Skin changes, such as discoloration or thickening


  • Sudden shortness of breath
  • Chest pain
  • Rapid or irregular heartbeat
  • Coughing up blood

Seek immediate medical attention if you experience these symptoms.

Am I At Risk for a Blood Clot?

Blood clots can affect people of any age, gender, race, or ethnicity.

Risk factors are things that increase your chances of developing a disease. Known risk factors for blood clots include:

  • A hospital stay
  • Surgery
  • Major trauma, such as a car accident, fall, or head injury
  • Infection
  • Inflammatory or autoimmune disease
  • Active cancer/chemotherapy
  • Estrogen-containing birth control pills and hormone replacement therapies
  • Pregnancy
  • Obesity
  • Leg paralysis
  • History of heart attack or stroke
  • Prior blood clot(s)
  • Family history of blood clots
  • Genetic or acquired clotting disorders
  • Immobility (limited movement), including
    • Being on bedrest
    • Being sedentary, meaning that you sit most of the day and are not physically active
    • Traveling for long periods of time (4+ hours on a plane, car, or train)

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