what causes a blood clot in the lung

Pulmonary blood clots (pulmonary embolisms) usually start as clots in the legs or pelvis that break off and travel to the lungs, where they block an artery and disrupt blood flow and oxygenation.

What actually is a “blood clot in the lung”?

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

It happens when a clump of material—most often a blood clot—gets stuck in a lung artery and blocks blood flow to part of the lung.

Think of it like a traffic jam in a major highway to your lungs: the blockage is small in size but can cause a big problem quickly.

Main direct cause: clot traveling from elsewhere

Most clots in the lung do not start in the lung itself—they come from somewhere else in the body.

Typical chain of events:

1. A clot forms in a deep vein, usually in the leg or pelvis (this is called deep vein thrombosis, or DVT).

2. A piece of that clot breaks off and travels through the veins to the right side of the heart.

3. The heart pumps it into the lung arteries, where it gets lodged and blocks blood flow.

Less commonly, clots can come from deep veins in the arms or other deep veins in the body.

Why do these clots form? (Risk factors)

Clots form more easily when blood is slow, sticky, or vessels are damaged. Several situations and conditions increase that risk.

1. Reduced movement or immobility

• Long flights or car rides where you sit still for many hours.

• Long hospital stays, especially after surgery.

• Being on bed rest, in a cast, or otherwise unable to move much.

When muscles in your legs are not moving, blood flow slows, and clots can form more easily.

2. Surgery, injury, or trauma

• Major surgery, especially hip, knee, abdominal, or pelvic surgery, irritates blood vessels and often involves long recovery times in bed.

• Serious fractures (especially large bones like the femur) both limit movement and can release fat droplets that rarely act like emboli.

• Any significant trauma can damage vessel walls, making clots more likely.

3. Medical conditions that increase clotting

• Certain cancers , especially of the brain, ovary, pancreas, colon, stomach, lung, kidney, and cancers that have spread, strongly increase clot risk.

• Heart disease , particularly heart failure, makes blood flow more sluggish and favors clot formation.

• Inherited clotting disorders (like factor V Leiden, prothrombin gene mutations, or antithrombin III deficiency) make blood more prone to clot even in younger people.

4. Hormones and pregnancy

• Use of birth control pills or estrogen therapy can make blood more likely to clot, especially in people who smoke or already have risk factors.

• Some forms of hormone replacement therapy (HRT) and medications like tamoxifen also raise clot risk.

• Pregnancy and the period shortly after giving birth naturally increase clotting tendency and pressure in the pelvic veins.

5. Lifestyle and general risk factors

• Obesity/overweight increases pressure in leg and pelvic veins and is often linked with less activity.

• Smoking (noted in many clinical resources even if not always listed in every summary) damages blood vessels and interacts with hormones to increase clot risk.
• Getting older also raises the likelihood of clots, partly because of more medical issues and less mobility.

6. Prior history and family history

• A previous blood clot (DVT or PE) is a strong predictor of future clots.

• A family history of blood clots or known clotting disorders suggests inherited risk.

Less common causes of “clots” in the lung

Sometimes the blockage in the lung is not a classic blood clot, but something else that behaves like one in the artery.

These rarer causes include:

• Fat emboli from a severely broken long bone (like the thigh), where fat droplets from bone marrow enter the bloodstream.

• Air bubbles entering the circulation (for example, from certain medical procedures or trauma) that travel to lung arteries.

• Amniotic fluid embolism around childbirth, when amniotic fluid enters the mother’s bloodstream.

• Tumor cells breaking off from cancers and lodging in lung vessels.

These are far less common than a standard clot from a DVT but are medically serious.

When should someone worry?

A clot in the lung can be life‑threatening and is treated as an emergency.

You cannot diagnose this at home—symptoms overlap with other conditions like pneumonia or anxiety, so medical evaluation is essential.

Seek urgent or emergency care (call local emergency services) if someone has:

• Sudden shortness of breath, especially if it came out of nowhere.

• Sharp chest pain that worsens with deep breaths or coughing.

• Coughing up blood.

• Fast heartbeat, feeling faint, lightheaded, or actually collapsing.

• Leg pain, swelling, warmth, or redness (often in the calf), which can be a warning sign of DVT.

Quick “story” example

Imagine someone who:

• Has knee replacement surgery.
• Spends several days mostly in bed, then takes a long car ride home.
• A week later, notices one calf is swollen and sore but ignores it.
• Suddenly develops severe shortness of breath and chest pain while walking to the bathroom.

In that kind of scenario, a DVT in the leg may have broken off and traveled to the lungs, causing a pulmonary embolism.

Bottom note

Information gathered from public forums or data available on the internet and portrayed here.

If you or someone around you is having symptoms that might be a blood clot in the lung, contact emergency medical services or go to an emergency department immediately.