Blood Clots: Everything You Need to Know

Medically Reviewed by Poonam Sachdev on November 19, 2023
7 min read

Blood clots are your body's way of stopping you from bleeding too much. Blood has a seemingly impossible job: it must flow continuously and smoothly throughout your body for an entire lifetime but quickly shut off to avoid spills when you get a cut or injury.

What do blood clots do?

A type of blood cell called platelets teams up with plasma, which is the liquid part of your blood, to stop bleeding by creating a clot over any injury you have. Once the injury is healed, your body should break down the blood clot.

Blood clots are healthy and lifesaving when they stop bleeding. But they can also form when they aren't needed and cause a heart attack, stroke, or other serious medical problems.

The life cycle of a normal blood clot depends on a series of chemical interactions.

1. Platelets form a plug. Platelets get “turned on” by triggers released when a blood vessel is damaged. They stick to the walls in the area and each other, changing shape to form a plug that fills in the broken part to stop blood from leaking out.

When activated, platelets also release chemicals to attract more platelets and other cells, setting off the next step.

2. The clot grows. Proteins in your blood called clotting factors signal each other to cause a rapid chain reaction. It ends with a dissolved substance in your blood turning into long strands of fibrin. These get tangled up with the platelets in the plug to create a net that traps even more platelets and cells. The clot becomes much tougher and more durable.

3. Reactions stop its growth. Other proteins offset extra clotting factor proteins, so the clot doesn't spread farther than needed.

4. Your body slowly breaks it down. As the damaged tissue heals, you don't need the clot anymore. The tough fibrin strands dissolve, and your blood takes back the platelets and cells of the clot.

Blood clots can form in veins or arteries, the vessels that help move blood around your body. They usually form inside an injured blood vessel to help your body heal. Sometimes, they form without an injury and don't dissolve on their own. You'll need to see a doctor right away, especially for blood clots in the arteries or veins of your legs, lungs, or brain.

Arterial clot vs. venous clot

Veins are low-pressure vessels responsible for moving blood away from your organs and directing it back to your heart. When an abnormal clot forms, it can potentially hinder normal blood flow to the heart. This can lead to pain and swelling as blood pools behind the clot.

Meanwhile, arteries are robust, high-pressure vessels that carry oxygen- and nutrient-rich blood from your heart to other parts of your body. When your doctor checks your blood pressure, the results show the pressure in your arteries. A clot in your artery usually means you have a condition called atherosclerosis, which involves the hardening of the artery. It is related to the buildup of plaque that constricts the vessel's inner wall.

Blood clots in period

You may notice blood clots during your period. Small ones are normal, but if they're larger than a grape, see a doctor. They could mean you have a serious medical problem.

Blood clots in the legs, arms, groin, or behind the knee

A blood clot in the veins of the leg, arm, groin (called the femoral vein), or behind the knee (called the popliteal vein) could be a condition called deep vein thrombosis (DVT). DVT happens when you have a medical problem that impacts how the blood clots. A clot in this area can also form if you sit or lay for too long, like when you're on bed rest or taking a long-distance trip. Around 1 to 3 in every 1,000 adults form a DVT or pulmonary embolism (PE) in the U.S. every year.

Blood clots in lungs

PE, or blood clots in your lungs, is a dangerous condition that happens when clots break free from vessels in other parts of your body and get wedged in the blood vessels of your lungs.

Blood clots in brain

When a blood vessel in your brain narrows, it slows blood flow to other parts of your brain. The result is pain, seizures, headaches, and sometimes even death. Hemorrhagic stroke and ischemic stroke are two types of blood clots in the brain. Brain blood clots are rare, affecting around 1 in every 100,000 people.

Blood clots in urine

Called hematuria, blood in your urine most commonly happens with a bladder infection, prostate infection, or urinary tract stones. These conditions can cause you to pee blood and even form a clot, which can block the flow of urine. If you have a blood clot, this usually means your urinary tract is injured. Other symptoms include pain in your side, back, or lower stomach and trouble peeing.

Blood clots in poop

Sometimes blood clots can also form when you have blood in your stool (rectal bleeding). This type of bleeding usually happens with hemorrhoids, anal fissures, inflammatory bowel disease (IBD), and diverticulitis. If you notice large blood clots in your poop, see a doctor right away.



There are three types of blood clots:

  • Venous thromboembolism happens in your veins, most often in your legs.
  • A pulmonary embolism happens when a blood clot travels to your lungs and blocks an artery.
  • Coronary thrombosis is when a blood clot blocks an artery in your heart, causing a possible heart attack.

Thrombus and embolus

A blood clot that forms in your vein is a thrombus, while an embolus is a blood clot that breaks away and travels through your blood vessel until it can no longer move because the vessel is too small.

The process begins whenever flowing blood comes into contact with specific substances in your skin or in blood vessel walls. When they touch, it usually means the skin or blood vessel wall is broken.

Waxy cholesterol plaques that form in arteries have these things inside, too. If the plaque breaks open, they'll start the clotting process. Most heart attacks and strokes happen when a plaque in your heart or brain suddenly bursts.

Blood clots can also form when your blood doesn't flow properly. If it pools in your blood vessels or heart, platelets are more likely to stick together. Atrial fibrillation and deep vein thrombosis (DVT) are two conditions where slowly moving blood can cause clotting problems. Blood clots can also form:

  • When you sit or lie for long periods
  • During and after pregnancy
  • When you take birth control pills or estrogen hormones
  • When you use an IV catheter for a long time
  • After an injury

Certain medical conditions can put you at risk for blood clots, including:

  • Cancer
  • Obesity
  • Liver disease
  • Kidney disease
  • Conditions passed down through families (Factor V Leiden and prothrombin G20210A)
  • Rare conditions (protein C, protein S, and antithrombin III deficiencies)

Some drugs stop platelets from signaling each other so they won't stick together.

  • Aspirin
  • Clopidogrel (Plavix)
  • Dipyridamole (Persantine)
  • Prasugrel (Effient)
  • Ticagrelor (Brilinta)
  • Ticlopidine (Ticlid)

Medicines called blood thinners make it hard for your body to make clotting factors, or they prevent proteins in the clot-forming process from working.

  • Apixaban (Eliquis)
  • Dabigatran (Pradaxa)
  • Edoxaban (Savaysa)
  • Heparin
  • Rivaroxaban (Xarelto)
  • Warfarin (Coumadin)

Clot-dissolving drugs such as alteplase, streptokinase, and tenecteplase activate the protein that breaks down the fibrin strands. Sometimes, doctors prescribe it as a treatment for heart attacks or strokes.

If you have a blood clotting condition passed down through your family, you won't be able to stop blood clots from happening or reverse the illness. But there are ways to lower your chances of forming blood clots, such as:

  • Visiting your doctor for a yearly checkup

  • Getting regular cancer screenings

  • Keeping a healthy weight

  • Not smoking

  • Drinking plenty of water

  • Taking birth control or hormone therapy without estrogen

  • Getting regular exercise

Your body can quickly form clots to stop bleeding if you're injured. But they can also form when you don't need them, causing serious medical conditions such as heart attack and stroke. Unless you were born with an illness that makes you prone to blood clots, there are ways to prevent them through lifestyle changes.

What are the signs of having a blood clot?

Symptoms of a blood clot include:

  • Pounding or cramping pain
  • Swelling
  • Color changes to your skin
  • Your arm or leg becomes warm
  • You're suddenly out of breath
  • Sharp chest pain that gets worse when you inhale
  • Cough
  • Coughing up blood