What Are Blood Clots?

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A blood clot, also called a thrombus, is blood that has coagulated or clotted. While clotting is crucial in some circumstances—healing a skin wound by forming a scab, for example—blood clots that occur within arteries or veins can be dangerous and even life-threatening if they block the flow of blood to essential organs, including the heart, lungs, and brain.

What Is a Blood Clot?
Verywell / Emily Roberts

Blood Clot Symptoms

The signs and symptoms of a blood clot depend on whether the clot is in an artery or a vein and wherein the body it is located. A clot in an artery may bring on moderate to severe pain that develops quickly, for example. If a vein is affected, the pain is likely to be milder and to increase in severity over several hours or even days.

Any blood clot may cause swelling, tingling, tenderness, or a feeling of warmth.

If an artery that leads to the brain is clogged, neurological symptoms such as confusion or paralysis can occur, possibly indicating a stroke. A blood clot in the leg might cause the leg to swell so that it's noticeably larger than the other leg and may be a sign of DVT. If a blood clot forms in the coronary artery, symptoms of a heart attack—including tightness in the chest or arms, dizziness—may occur. 


All injuries cause damage to blood vessels. When you get a bruise, it's because a blood vessel was damaged, causing blood to leak out and become visible under the skin. A clot then forms inside the blood vessel; without this process, minor injuries could cause uncontrolled bleeding.

Blood clots are made up of two elements: platelets and fibrin. Platelets are cells produced in the bone marrow that travel throughout the bloodstream. When bleeding occurs, the platelets become sticky, allowing them to adhere to each other and the blood vessel walls.

Fibrin is a substance that resembles a long, sticky string. Strands of fibrin stick to blood vessel walls and clump together to form a web-like complex that red blood cells become caught up in. A blood clot consists of platelets and fibrin strands, as well as trapped red blood cells. The strands of fibrin bind the platelets together and essentially tighten the clot to make it stable.

The clotting mechanism also can cause clots to form in ways that are harmful—a condition called thrombosis.

If a blood clot blocks an artery to the heart, the result can be a heart attack. If blood to the brain is blocked, the result can be a stroke. 

Arteries get smaller and smaller as they move away from the heart, so a clot that starts near the heart will eventually lodge in a smaller vessel. This prevents oxygenated blood from reaching any areas fed by that artery. Embolic strokes, the most common type of stroke, for example, are caused by blood clots traveling to the brain and starving brain tissue of blood and oxygen.

Veins, on the other hand, get larger as they return blood to the heart, so blood clots that form in veins can travel all the way to the heart and then get pumped into the lungs, where they can create a life-threatening condition called a pulmonary embolism. They can also lodge in blood vessels, most commonly in the legs; when this happens, it is called deep vein thrombosis (DVT).

There are numerous risk factors that can predispose you to develop a potentially dangerous blood clot, including:

  • Atrial fibrillation, a condition that causes an irregular heartbeat
  • Atherosclerosis, a buildup of plaque in the arteries
  • Certain genetic disorders, such as factor V Leiden mutation (FVL)
  • Certain medications, such as oral contraceptives and hormone therapy drugs
  • Heart arrhythmias (heart rhythm problems)
  • Heart failure
  • Obesity
  • Peripheral artery disease
  • Pregnancy
  • Prolonged sitting or bed rest
  • Smoking
  • Surgery


A variety of diagnostic tests are used to detect blood clots, depending on your symptoms and the likely location of the clot. They include: 

  • D-dimer blood test: This measures a substance in the blood that can detect whether there is abnormal clotting activity somewhere in the bloodstream. 
  • Cardiac biomarker blood test: This is a blood test that can detect damage to the heart muscle and is used to diagnose a heart attack.
  • Compression ultrasound: This is a noninvasive test that can be performed at the bedside and is often very useful in diagnosing DVT. 
  • V/Q scan: A ventilation-perfusion scan (V/Q scan) uses a radioactive dye to examine blood flow to the lungs and can detect whether a pulmonary blood vessel has been blocked by a pulmonary embolus.
  • CT scan: This is often the first test used to diagnose a stroke. It is also useful for confirming a pulmonary embolus.
  • MRI scan: MRI scans can be used to detect clots in blood vessels.
  • Angiography or venography: These are catheterization techniques in which a dye is injected into a blood vessel where a clot is suspected; X-rays are then taken to detect the clot.
  • Echocardiography: Echocardiograms use sound waves to get images of your heart and are often used in patients who have had embolisms affecting an artery—especially in people who have had an embolic stroke. To get into an artery, in almost every case, an embolism will have to either originate within the heart or travel through the heart.


Prescription medications are the mainstay of blood clot prevention and treatment, though a surgery may be considered for some people. Drugs used to treat blood clots include:

  • Anticoagulant drugs: These inhibit one or more of the clotting factors, a group of blood proteins that are responsible for blood clotting.
  • Anti-platelet drugs: These medications are used to reduce the "stickiness" of platelets, the tiny blood elements that form the nucleus of a blood clot. By inhibiting the ability of platelets to clump together, these drugs inhibit blood clotting. 
  • Thrombolytic drugs: These powerful drugs, also known as fibrinolytic agents or "clot busters," are given intravenously to dissolve blood clots that are in the process of forming. For the most part, their use is limited to patients who are within the first few hours of an acute heart attack or stroke in an attempt to re-open a blocked artery and prevent permanent tissue damage. 


Some of the strategies for preventing blood clots are those recommended for overall health and wellness.

Get routine exercise, keep your weight in a healthy range, and avoid smoking. The latter recommendation is particularly important when it comes to blood clots since it can cause inflammation that promotes thrombosis.

Make sure any chronic conditions you have are being properly managed, especially cardiovascular concerns, and—as much as possible—avoid sitting for long periods of time.

A Word From Verywell

Blood clots can be dangerous, so if you experience any symptoms that you think may indicate a clot, call your healthcare provider immediately or go to the emergency room. Fortunately, there are many medications that can prevent and treat blood clots effectively.

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. American Society of Hematology. Blood Clots.

  2. Litvinov RI, Weisel JW. What Is the Biological and Clinical Relevance of Fibrin?. Semin Thromb Hemost. 2016;42(4):333-43. doi:10.1055/s-0036-1571342

  3. American Heart Association. Understand Your Risk for Excessive Blood Clotting.

  4. Adivitiya, Khasa YP. The evolution of recombinant thrombolytics: Current status and future directions. Bioengineered. 2017;8(4):331-358. doi:10.1080/21655979.2016.1229718

Additional Reading
Richard N. Fogoros, MD

By Richard N. Fogoros, MD
Richard N. Fogoros, MD, is a retired professor of medicine and board-certified in internal medicine, clinical cardiology, and clinical electrophysiology.