What Causes Blood Clots? Symptoms and Causes

By Sabina Rebis, MD
Medically reviewed checkmarkMedically reviewed
May 18, 2022

Our blood’s ability to clot is a life-saving mechanism that prevents us from bleeding out every time we get injured.

However, if blood clots at a time when it’s not supposed to, it can cause a medical emergency such as a heart attack, stroke, or pulmonary embolism. 

If you think you are experiencing a blood clot, seek immediate medical care. 

This article goes over what a blood clot is and how it forms.

Next, I’ll discuss what symptoms you may get if you have a blood clot and what can cause them to develop.

Lastly, we’ll talk about how a blood clot is diagnosed and treated and how you can prevent one from forming. 

What Is a Blood Clot?

A blood clot is a clump of blood cells, platelets, and proteins.

When you are injured, your blood vessels first sense this and constrict. Platelets and proteins in the blood then pick up on this and arrive at the seat of the injury to seal it. 

However, there are problems that can happen with clotting when:

  • Blood clots form in places they shouldn’t
  • Your body makes too many blood clots or creates abnormal ones
  • The blood clots don’t break down like they are supposed to

When the clotting mechanism goes awry, blood clots can travel to other places in the body (e.g., heart, lungs, brain, kidneys) and cause significant health problems by partially or entirely blocking blood vessels. 

The Centers for Disease Control and Prevention (CDC) estimates that 900,000 people are affected by blood clots each year, and many go undiagnosed.  

How does blood clot?

There are several steps to forming a blood clot.

It all starts when a blood vessel is injured and sends out chemicals notifying the rest of the body of the injury. 

These are the steps that follow:

  • First, the chemicals trigger the blood vessel to narrow and reduce the amount of blood flowing through it. 
  • Platelets then travel from the spleen (where they are stored) to where the injury is. The vessel walls become sticky to help grab the platelets when they come. 
  • The platelets start to change shape and become stickier. Finally, they clump together to form a plug on the vessel wall.
  • Clotting factors in your blood are typically turned off (to prevent abnormal clots from forming). The platelets release chemicals that turn on the other clotting factors into the blood. Fibrin, a long sticky protein, forms a mesh that holds the clot in place and traps red blood cells in a clump. 

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Sometimes an abnormal blood clot can form in a vessel and block blood flow, or it can travel to other places in the body and impede flow there.

Symptoms depend on where the clot is clogging blood flow. 

Blood clots in the leg or arm

A deep venous thrombosis (DVT) is when a blood clot forms in a vein, usually in the leg but sometimes in the arm. 

Common symptoms of a DVT include:

  • Pain
  • Swelling of the affected area
  • Tenderness
  • Redness of the skin

Blood clots in the heart

When a blood clot clogs the blood flow to the heart muscle, it can cause a heart attack. 

Symptoms of a heart attack include:

A heart attack is a medical emergency. If you or a family member are experiencing the above symptoms, call 911.

To learn more about heart attacks, click here. 

Blood clots in the lung

A blood clot in the lung is called a pulmonary embolism. 

Symptoms include:

A pulmonary embolism is a medical emergency.

If you or a family member are experiencing the above symptoms, call 911.

Blood clots in the brain

A blood clot in the brain may cause an ischemic stroke. 

Symptoms of a stroke include:

  • Loss of balance, headache, or dizziness
  • Blurred vision
  • Drooping on one side of the face
  • Arm or leg weakness
  • Speech difficulty

A stroke is a medical emergency. 

If you or a family member are experiencing the above symptoms, call 911.

To learn more about strokes, click here. 


Several causes may result in the formation of abnormal blood clots. 

Prolonged inactivity may be one such cause.

This may be a result of an injury such as a broken bone (hip, pelvis, or leg), a recent surgery, being confined to a bed or chair, paralysis, or prolonged sitting while traveling long distances. 

Other causes can include:

  • Being over the age of 65
  • Taking hormones or birth control pills 
  • Cancer 
  • An inherited clotting disorder
  • Obesity 
  • Heart problems
  • Family history of blood clots
  • Having varicose veins 


If you believe you are experiencing a blood clot, seek emergency care.

The healthcare team will first perform a physical exam and ask you about your medical history and what medications you are currently taking.

They will also ask questions about your symptoms.

Lab tests

When a clot starts to dissolve, it releases a protein called a D-dimer.

Doing a D-dimer blood test looks to see if there is D-dimer in your blood. If your D-dimer is not elevated, it is unlikely that a blood clot is present. 

A troponin lab test shows whether there has been recent injury to the heart muscle, possibly due to blood flow restriction to the heart secondary to a blood clot.

Troponin is a protein that is released by injured heart muscle. During a heart attack, the troponin level in your blood starts to rise. 


Several imaging tools are available to visualize blood clots to determine precisely where it is. 

  • Ultrasound helps find blood clots in the legs and arms. 
  • Angiography (arteries) or venography (veins) are imaging techniques used to visualize blood flow using injected dyes. 
  • CT scans use imaging technology to take clear pictures that may show an area with decreased blood flow to indicate the location of the clot. 
  • MRIs use magnets and radio waves to visualize decreased blood flow secondary to a  blood clot in your brain. 


Treatment depends on where the blood clot is.

To restore blood flow, the medical team may use a combination of medications and medical procedures.


Blood thinners are medications that prevent blood clots from forming

These medications are taken orally or through needle injection. 

If you are at a higher risk of blood clots, your healthcare team will regularly test your blood to keep your clotting factor in a specific range. 

When there is a severe clot, thrombolytic medications help dissolve them to restore blood flow. 

Medical Procedures

A thrombectomy is a procedure that removes the blood clot from a vessel. 

A stent is then placed to open up the vessel and allow blood to flow again.

Cardiac catheterization is a procedure that clears or opens up a narrow blocked artery leading to the heart. 

Who is at risk for blood clots?

Anyone can be affected by blood clots regardless of gender, race, or age. 

However, certain risk factors can make you more vulnerable. 

Risk factors include:

  • Recent surgery or hospitalization
  • Pregnancy
  • Cancer
  • Family history of blood clots
  • Atrial fibrillation
  • COVID-19
  • Smoking


Staying active may help prevent blood clots. 

Some other things you can do to decrease your risk of blood clots are:

  • Wear compression stockings if you stand for long periods throughout the day. 
  • Change positions frequently when on a long trip.
  • Wear loose-fitting clothing.
  • Eat less salt.
  • Try not to cross your legs while you sit.
  • Move around as soon as safely possible after surgery or bed rest.
  • Talk to your doctor about anticoagulant medications if you are at risk.
  • Stop smoking.
  • Maintain a healthy weight. 

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When To Seek Medical Attention

If you believe you are experiencing a blood clot, call your healthcare provider right away. 

Call 911 if you experience:

  • Shortness of breath
  • Feeling faint
  • Pressure or pain in your chest
  • Difficulty speaking
  • Extreme weakness 
  • Facial droop on one side 

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What are the first signs of a blood clot?
The first signs will depend on where the blood clot is. If the clot is in your arm or leg, you may feel pain, the skin may get warm, and there may be some swelling. If the clot is in your heart you may feel chest pain or pressure, clammy skin, pain in your left arm, and have trouble breathing. If the clot is in your lung, you may suddenly feel short of breath or have pain when you take a deep breath. If the clot is in your brain, you may experience trouble with your balance, vision, and ability to speak. You may also have trouble walking or feel weak on one side of your body.
What does a blood clot feel like?
A blood clot in your arm or leg may cause some pain and the skin may feel extra warm in that area. A blood clot in your lung or heart could cause chest pain and make you feel short of breath.
Are blood clots serious?
Blood clots can be serious and life-threatening. If you feel you are experiencing one, you need to seek medical treatment right away.
Does a blood clot go away on its own?
Occasionally, a blood clot may go away on its own, but if you are experiencing symptoms that may be due to a blood clot, it is very important to ensure you do not require intervention with oral medications or a procedure. While you have a blood clot, you are at higher risk for developing another blood clot and you may need to take anticoagulant medication. This medication can help stop another clot from forming and prevent the clot you have from getting bigger.

K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.

Sabina Rebis, MD

Dr. Sabina Rebis is a board certified family medicine physician with over 5 years of primary care and urgent care experience.

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