Both are conditions affecting the flow of blood through blood vessels. But thrombosis develops because of a blood clot, and embolism results from an object or bodily substance obstructing blood flow.

Thrombosis and embolism share many similarities, but they are unique conditions. Thrombosis occurs when a thrombus, or blood clot, develops in a blood vessel and reduces the flow of blood through the vessel. Embolism occurs when a piece of a blood clot, foreign object, or other bodily substance becomes stuck in a blood vessel and largely obstructs the flow of blood.

A similar condition, thromboembolism, refers to a reduction in blood flow that’s specifically caused by an embolism from a blood clot.

Many people develop blood clots, and there are many types and causes of thrombosis and embolism. A block in blood flow in a deep vein, large artery, or pulmonary (lung) blood vessel carries the greatest health risk. As many as 100,000 Americans die each year from deep vein thrombosis (DVT) or pulmonary embolism.

Read on to learn more about these conditions.

The symptoms of thrombosis and embolism depend on the:

  • type of blood vessel involved
  • location
  • impact on blood flow

Small thrombi and emboli that don’t significantly block blood vessels may not cause symptoms. Around 50 percent of people with DVT have no signs of the condition at all. However, large obstructions can starve healthy tissues of blood and oxygen, causing inflammation and eventually tissue death.

Venous thrombosis

Veins are the blood vessels responsible for returning blood to the heart for recirculation. When a clot or embolus blocks a major or deep vein, blood pools behind the obstruction, causing inflammation. Though they can occur anywhere, most cases of venous thrombosis develop in the deep veins of the lower legs. Blockages that occur in the small or superficial veins tend not to cause major complications.

Common symptoms of venous thrombosis include:

  • pain and tenderness
  • redness or discoloration
  • swelling, often around the ankle, knee, or foot

The affected area will also be warm to the touch.

Pulmonary embolism

Pulmonary embolism (PE) occurs when a piece of a blood clot breaks free and travels through the blood stream to the lungs. It then becomes lodged in a blood vessel. It’s commonly associated with DVT.

Pulmonary embolism can be very dangerous and develop extremely rapidly. In about 25 percent of pulmonary embolism cases, sudden death is the first symptom. Seek immediate medical attention if you suspect PE.

Common symptoms of PE include:

  • trouble breathing
  • rapid breathing
  • dizziness and light-headedness
  • rapid heart rate
  • chest pain that gets worse when breathing in
  • coughing up blood
  • passing out

Arterial thrombosis

Arterial thrombosis is often associated with atherosclerosis. Atherosclerosis is the development of plaques, or fatty hardenings, on the inner wall of an artery. Plaques cause the artery to narrow. This increases the amount of pressure in the blood vessel. If this pressure becomes intense enough, the plaque can become unstable and rupture.

Sometimes when a plaque ruptures the immune system overreacts. This can lead to the development of a large clot and a life-threatening condition, like a heart attack or stroke.

Seek immediate medical attention if you have symptoms of arterial thrombosis including:

  • chest pain that often comes on randomly, such as when you’re resting, and won’t respond to medication
  • shortness or loss of breath
  • sweating
  • nausea
  • a limb or area of skin that has become cool, lighter in color than normal, and very painful
  • unexplained loss of muscle strength
  • lower portion of the face slumps to one side

When a blood vessel wall is injured, blood cells, called platelets and proteins, form a solid mass over the wound. This mass is called a thrombus, or blood clot. The clot helps seal off the injury site to limit bleeding and protect it during healing. This is similar to a scab on an external wound.

Once the wound has healed, blood clots typically dissolve on their own. Sometimes, however, blood clots form randomly, won’t dissolve, or are very large. This can lead to serious health risks by reducing blood flow and causing damage or death to the involved tissue that it supplies.

Embolisms can also occur when other substances are trapped in blood vessels, like air bubbles, fat molecules, or bits of plaque.

There is no specific test used to diagnose thrombosis and embolism, although duplex ultrasound, or the use of sound waves to create images of flowing blood, is commonly used.

Other tests that may be used to help diagnose or assess abnormal blood clots or obstructions include:

  • magnetic resonance imaging (MRI), or computed tomography (CT) scans
  • blood tests
  • venography, when the blood clot is thought to be in a vein
  • arteriogram, when the blockage is thought to be in an artery
  • heart and lung functioning tests, such as arterial blood gasses or ventilation perfusion lung scans

In most cases, medical treatment depends on the type, extent, and location of the blood clot or obstruction.

Common medical therapies used to treat thrombosis and embolism include:

  • thrombolytic medications that help dissolve clots
  • anticoagulant medications that make it harder for clots to form
  • catheter-directed thrombolysis, which is surgery where a long tube, called a catheter, delivers thrombolytic medications directly to the clot
  • thrombectomy, or surgery to remove the clot
  • inferior vena cava filters, or small bits of mesh surgically placed over the clot to catch emboli and prevent them from spreading to the heart and then the lungs

Certain lifestyle changes or preventative medications can help treat clots or reduce your risk of developing them.

The following may help prevent blood clots or obstructions:

  • maintain a healthy weight and diet
  • quit smoking and alcohol use
  • exercise
  • stay hydrated
  • avoid prolonged periods of sitting or inactivity
  • treat chronic inflammatory conditions
  • manage unhealthy blood sugar levels
  • take blood pressure and cholesterol medications as prescribed by your doctor
  • talk to your doctor about stopping the use of estrogen-based medications
  • use mechanical devices like compression socks or intermittent pneumatic compression devices
  • keep your legs elevated while sitting
  • make sure your doctor knows about a history or family history of clots or clotting conditions
  • stretch your foot and leg muscles daily
  • wear loose-fitting clothes

Complications associated with both thrombosis and embolism vary depending on:

  • the extent of the blockage
  • the location of the clot
  • how it was stuck
  • underlying health conditions

Embolism is often considered more dangerous than mild to moderate thrombosis because embolism tends to obstruct the entire blood vessel.

Complications of moderate to severe cases of thrombosis and embolism include:

  • swelling
  • pain
  • dry, scaling skin
  • skin discoloration
  • dilated or enlarged veins, such as spider-web or varicose veins
  • tissue damage
  • heart attack or stroke
  • organ failure
  • loss of limb
  • brain or heart damage
  • ulcers

For mild cases of thrombosis and embolism, symptoms may resolve within a few days to weeks of medication and lifestyle changes. The outlook for more severe cases depends mostly on the type, extent, and location of the clot or obstruction.

About 50 percent of people with DVT have long-term complications, generally related to reduced blood flow. Around 33 percent of people with a combination of DVT and PE develop new clots within 10 years.