What does it mean when you have blood clots? A clear medical explanation

By Sina Hartung, MMSC-BMI, Harvard Medical SchoolReviewed by Eureka Health Medical Group

Published: July 9, 2025Updated: July 9, 2025

Key Takeaways

A blood clot is a semi-solid mass of blood that forms when platelets and proteins stick together. Inside a cut this stops bleeding, but inside a vein or artery it can cut off oxygen to the legs, lungs, brain, or heart. Such clots—called deep-vein thrombosis, pulmonary embolism, stroke, or heart attack—are medical emergencies that need prompt imaging, blood thinners, and close follow-up.

What exactly is happening inside your body when a blood clot forms?

A clot starts when the inner lining of a vessel is irritated or blood flow slows. Platelets pile up, fibrin strands weave through, and the gel-like plug hardens. That plug can stay put—deep-vein thrombosis (DVT)—or break loose and travel—pulmonary embolism (PE) or stroke. “Think of a clot as biological cement: useful on a sidewalk, disastrous in plumbing,” says the team at Eureka Health.

Clots act like internal plugsWhen the plug forms in a deep leg vein, it blocks drainage and pressure builds, causing swelling and pain.
Traveling clots can be deadlyRoughly 1 in 3 untreated leg clots migrate to the lungs, where they can stop oxygen exchange within minutes.
Size does not predict dangerA clot the width of a pencil lead can still lodge in a coronary artery and trigger a heart attack.
Body usually dissolves small clotsNatural enzymes like plasmin break down micro-clots daily, but larger clots outpace this cleanup crew.
Three key conditions drive dangerous clotsNIH News in Health lists “abnormalities in the blood, improper blood flow, and damage to the lining of blood vessels” as the three factors that most often spark a harmful thrombosis, the classic Virchow’s triad clinicians watch for. (NIH)
Clot-building cascade starts within moments of vessel injuryThe Merck Manual explains that a cut vessel first constricts, then platelets stick and release chemicals that turn on clotting factors; a fibrin net quickly traps more cells so the hole is sealed in just a few minutes. (Merck)

Which blood clot symptoms should send you to the ER right now?

Some signs signal that a clot is already blocking a vital vessel. “Shortness of breath paired with sharp chest pain is the classic PE alarm bell,” notes Sina Hartung, MMSC-BMI. Call 911 instead of waiting to “see if it gets better.”

Sudden one-sided leg swellingA calf circumference difference of more than 2 cm doubles the likelihood of DVT in clinical studies.
Chest pain that worsens on inhalationUp to 85 % of PE patients report pleuritic pain that sharply increases when they breathe in.
New, unexplained shortness of breathAny drop in oxygen saturation below 94 % on a home pulse oximeter warrants urgent evaluation.
Blurred vision or drooping faceThese are hallmark stroke signs; intravenous clot-busting therapy is only approved within 4.5 hours.
Rapid heartbeat over 100 bpm at restTachycardia appears in 70 % of patients with significant PE and reflects the heart’s struggle to pump past the blockage.
Coughing up blood (hemoptysis)World Thrombosis Day cautions that coughing up blood, especially with chest pain or breathlessness, signals a pulmonary embolism and demands immediate emergency care. (WTD)
Sudden dizziness or faintingThe Mayo Clinic lists lightheadedness and fainting among the emergency warning signs of a dangerous clot, advising that they warrant a 911 call without delay. (Mayo)

Who is most likely to develop dangerous clots and why?

Risk accumulates when blood is thick, flow is slow, or the vessel wall is damaged—Virchow’s triad. The team at Eureka Health explains, “Surgery, cancer, pregnancy, and long flights each hit at least one corner of that triad, sometimes all three.”

Major surgery increases DVT risk 20-foldTissue injury activates clotting factors, and postoperative immobility slows venous return.
Cancer creates a pro-clotting bloodstreamMalignancies release substances like tissue factor; 1 in 5 clot events occur in people with active cancer.
Estrogen shifts coagulation toward clottingCombined oral contraceptives raise VTE risk from 1 per 10 000 women-years to about 3-4.
Genetic thrombophilias stack the oddsFactor V Leiden carriers have a lifetime DVT risk of roughly 10 %; double that if they also smoke.
Extended travel immobilizes calf musclesFlights over 4 hours increase clot risk by 26 % compared with shorter trips.
Half of all clots arise during or soon after a hospital stayThe CDC notes that roughly 50 % of DVT/PE events are linked to recent hospitalization or surgery, underscoring immobility and tissue injury as critical contributors. (CDC)
VTE strikes up to 900 000 Americans yearly and kills as many as 100 000According to CDC estimates, venous thromboembolism affects 300 000–900 000 people in the United States each year, leading to 60 000–100 000 deaths. (CDC)

How can you lower your clot risk at home starting today?

Many preventive steps cost nothing and start working immediately. “Simple calf pumps during a flight can cut venous pressure in half,” says Sina Hartung, MMSC-BMI.

Move every 1–2 hoursTen heel-to-toe raises increase venous flow by 200 % in ultrasound studies.
Stay hydratedDrinking 2 liters of water daily keeps plasma viscosity low and reduces clot likelihood during long travel.
Wear graduated compression stockings20–30 mm Hg knee-high stockings lower postoperative DVT rates from 21 % to 9 % in meta-analysis.
Know your family historyIf a first-degree relative had an unprovoked clot before age 50, ask your doctor about thrombophilia testing.
Stop smokingNicotine thickens blood and damages vessels; quitting reduces clot risk within 4 weeks.
Keep your weight in a healthy rangeHealthline notes that obesity raises clot risk by increasing abdominal pressure and inflammation, so pairing calorie-balanced eating with regular activity removes a major trigger. (Healthline)
Discuss estrogen-based birth control alternativesCombined oral contraceptives raise deep-vein thrombosis odds three- to six-fold, according to the DVT overview on Wikipedia, so asking about lower-dose or non-hormonal options can lower risk. (Wikipedia)

Which tests and prescription drugs do doctors use to diagnose and treat clots?

Diagnosis relies on imaging and blood work; treatment focuses on stopping growth and preventing new clots. The team at Eureka Health states, “Modern anticoagulants cut recurrence rates below 3 % per year when taken correctly.”

D-dimer screens for fibrin breakdownA level under 500 ng/mL can safely rule out DVT in low-risk patients, sparing unnecessary scans.
Ultrasound maps leg veins in real timeCompression ultrasound has 95 % sensitivity for above-knee DVT, making it first-line imaging.
CT pulmonary angiography confirms PEThis scan visualizes clots in the lung arteries within minutes and guides immediate therapy.
Anticoagulants thin blood, not dissolve clotsMedications like direct oral anticoagulants reduce fatal PE risk by 90 % but require daily adherence.
Thrombolytics reserved for massive eventsAlteplase can dissolve clots within hours, yet carries a 2–5 % risk of major bleeding and is only used in life-threatening cases.
Ventilation–perfusion scan detects PE without contrastWhen iodinated contrast is unsafe, a V/Q lung scan can pinpoint perfusion defects characteristic of pulmonary embolism, providing a vital diagnostic option. (CDC)
Typical anticoagulation course is at least three monthsNHS guidance states that oral agents such as warfarin or rivaroxaban are usually continued for a minimum of 3 months after a DVT to lower recurrence risk. (NHS)

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