A blood clot is a gel-like mass of blood cells and protein fibers that forms to stop bleeding after an injury. This process is essential for survival, but clots that form inside blood vessels without an obvious injury, or that don’t dissolve on their own, can block blood flow and cause serious damage. Up to 900,000 people in the United States are affected by dangerous blood clots each year, and an estimated 60,000 to 100,000 die from them.
How Blood Clots Form
When a blood vessel is damaged, the body launches a rapid chain reaction called the coagulation cascade. Damaged tissue releases a signal that activates a series of clotting proteins in the blood, which work together to produce small amounts of a key enzyme called thrombin. Thrombin does two things at once: it activates platelets (tiny cell fragments in your blood) and it converts a dissolved protein called fibrinogen into solid fibrin strands.
Activated platelets rush to the injury site and clump together, forming an initial plug. They also provide a surface where clotting proteins can assemble more efficiently, dramatically amplifying thrombin production. This burst of thrombin generates large amounts of fibrin, which weaves into a mesh that reinforces the platelet plug. A separate protein then cross-links those fibrin strands, making the mesh stronger and more stable.
The finished clot is more than just platelets and fibrin. Red blood cells pack tightly into the structure, changing shape into compressed forms called polyhedrocytes that seal gaps. White blood cells contribute additional structural and immune components. The fibrin network acts as the backbone, giving the clot its elasticity and stability. Once the vessel heals, the body gradually breaks the clot down.
When Clots Become Dangerous
Problems arise when clots form inside blood vessels that aren’t injured, grow too large, or break loose and travel through the bloodstream. These abnormal clots fall into two main categories, and the distinction matters because their causes, symptoms, and consequences are different.
Venous clots form in veins, most commonly the deep veins of the legs, pelvis, or arms. This condition is called deep vein thrombosis (DVT). DVT itself does not cause heart attacks or strokes, but the clot can break free and travel through the right side of the heart into the lungs, a potentially fatal event called a pulmonary embolism (PE). When a clot lodges in the lung’s blood vessels, it blocks oxygen exchange and forces the heart to work harder. The result is a sudden drop in blood oxygen, increased strain on the heart, and in severe cases, cardiovascular collapse. Sudden death is the first symptom in roughly 25% of people who experience a PE.
Arterial clots form in arteries, usually in the heart or brain. Because arteries carry oxygen-rich blood to organs, a blockage can directly cause a heart attack or stroke by cutting off the blood supply to tissue that needs it.
Risk Factors for Abnormal Clots
Blood clots result from a combination of genetic tendencies and situational triggers. The most common inherited risk factor is a genetic variant called Factor V Leiden. Women who carry one copy of this variant have a 4 to 7 times higher risk of developing a venous clot compared to the general population. On its own, that elevated risk is manageable for most people. But when genetic and environmental factors combine, the numbers multiply. A woman with Factor V Leiden who also takes oral contraceptives faces a 34-fold increase in risk, compared to just 3-fold for oral contraceptives alone.
Major environmental risk factors include:
- Surgery and trauma: Tissue damage activates the clotting system, and post-operative bed rest compounds the problem.
- Prolonged immobility: Sitting or lying still for long periods slows blood flow in the legs, giving clots an opportunity to form. This includes hospitalization, long car rides, and flights lasting more than 4 hours.
- Hormone therapy and oral contraceptives: Estrogen-containing medications increase clotting protein levels. Postmenopausal hormone therapy carries a similar relative risk increase as birth control pills.
- Cancer: Many cancers release substances that activate the clotting system.
- Obesity: Excess weight increases pressure on veins in the legs and pelvis.
- Advancing age: Clot risk rises steadily with age.
- Pregnancy: The body naturally increases clotting ability during pregnancy to prepare for delivery, which also raises the risk of unwanted clots.
Symptoms of a Blood Clot
A DVT in the leg typically causes swelling, pain or tenderness (often starting in the calf), warmth in the affected area, and skin that looks red or discolored. These symptoms usually affect one leg, not both. Some DVTs produce no noticeable symptoms at all, which is part of what makes them dangerous.
A pulmonary embolism produces different, more urgent symptoms: sudden shortness of breath, sharp chest pain that may worsen with deep breaths, a rapid heartbeat, coughing (sometimes with blood), and lightheadedness or fainting. Because PE can be fatal within minutes, these symptoms call for emergency medical attention.
Arterial clots present differently depending on where they form. A clot in the coronary arteries causes the classic signs of a heart attack: chest pressure, pain radiating to the arm or jaw, and shortness of breath. A clot in a brain artery causes stroke symptoms: sudden numbness or weakness on one side of the body, confusion, difficulty speaking, or vision changes.
How Blood Clots Are Diagnosed
Doctors use a combination of clinical scoring and lab tests to evaluate suspected clots. For DVT, the process often starts with a risk assessment tool called the Wells score, which assigns points based on symptoms and risk factors. Patients with a low risk score then receive a blood test called a D-dimer, which detects fragments of dissolved fibrin. A negative D-dimer in a low-risk patient has a 99.1% chance of correctly ruling out DVT, making it a reliable screening tool for that group.
The picture changes for higher-risk patients. When the clinical suspicion is stronger (a Wells score of 2 or higher), the D-dimer’s reliability drops, with its ability to rule out DVT falling to about 89%. For these patients, doctors skip straight to imaging, typically an ultrasound of the affected limb that can directly visualize the clot. For suspected pulmonary embolism, a CT scan with contrast dye is the standard imaging test.
Treatment: Preventing Growth vs. Breaking Clots
Treatment depends on the type, size, and location of the clot, but it generally falls into two categories.
The most common approach uses anticoagulants, often called blood thinners. These medications don’t actually dissolve an existing clot. Instead, they stop the clot from growing larger and prevent new clots from forming, giving the body time to break down the clot naturally. Most people with DVT are treated with anticoagulants, typically taken as a pill for several months. During this time, you’ll need periodic monitoring to ensure the medication is working and not causing excessive bleeding, which is the main side effect.
For life-threatening clots, such as a large pulmonary embolism or a stroke-causing blockage in the brain, doctors may use thrombolytics. These powerful medications actively dissolve clots by activating the body’s own clot-dissolving system. They work fast but carry a significant risk of serious bleeding, so they’re reserved for emergencies where the benefit of restoring blood flow outweighs that risk. In some cases, doctors use mechanical devices threaded through blood vessels to physically extract the clot.
Travel and Prolonged Sitting
Any trip lasting more than 4 hours increases your risk of a blood clot, whether you’re flying, driving, or riding a bus or train. The mechanism is straightforward: sitting still for hours slows blood flow in the deep veins of your legs, creating conditions where clots can form. The risk is highest for people who already have other risk factors, but it applies to anyone on a long journey.
Simple precautions make a meaningful difference. Getting up to walk every hour or two, flexing and extending your ankles while seated, staying hydrated, and avoiding alcohol all help keep blood moving. Compression stockings, which gently squeeze the legs to support circulation, are another option for people at higher risk. If you have a history of blood clots or multiple risk factors, talk to your doctor before a long trip about whether a short course of preventive medication makes sense.