A blood clot is a gel-like mass of blood that forms when your body converts liquid blood into a semi-solid clump. This process is normal and essential: every time you get a cut or scrape, clotting stops the bleeding and starts healing. But when clots form inside blood vessels without an injury, or don’t dissolve when they should, they can block blood flow and become dangerous. Up to 900,000 people in the United States are affected by problematic blood clots each year.
How Blood Clots Form
Clotting happens in two stages. The first stage is fast. When a blood vessel is damaged, a sticky protein called von Willebrand factor acts like glue, grabbing circulating platelets and pressing them against the wound. These platelets pile on top of each other to form a temporary plug within seconds. Think of it like pressing a finger over a leak.
The second stage reinforces that plug so it actually holds. Your blood contains a series of proteins (called clotting factors) that activate one another in a chain reaction. The end result: a protein called thrombin converts a dissolved substance in your blood, fibrinogen, into fibrin. Fibrin strands weave through the platelet plug like threads through fabric, then another factor cross-links those strands into a tough mesh. What started as a loose pile of platelets becomes a stable, contractile seal over the wound.
Once the vessel heals, your body gradually breaks the clot down and reabsorbs it. Problems arise when this system activates where it shouldn’t, builds clots faster than the body can dissolve them, or sends clot fragments traveling through the bloodstream.
Where Dangerous Clots Happen
Not all clots are the same. The two broad categories are venous clots (in veins) and arterial clots (in arteries), and they cause different emergencies.
Venous clots form in veins, most commonly in the deep veins of the legs. This is called deep vein thrombosis, or DVT. Because veins carry blood back to the heart and lungs, a piece of a DVT can break loose, travel through the bloodstream, and lodge in a lung artery. That’s a pulmonary embolism (PE), which can be life-threatening. Together, DVT and PE are referred to as venous thromboembolism, or VTE.
Arterial clots form in arteries, which carry oxygen-rich blood to organs. When an arterial clot blocks blood flow to the heart, it causes a heart attack. When it blocks flow to the brain, it causes a stroke. Arterial clots tend to be rich in platelets and are closely linked to conditions like atherosclerosis, where fatty deposits narrow the arteries and create rough surfaces where clots can start.
Symptoms of a DVT
A deep vein clot in the leg often announces itself with a cluster of signs that are easy to dismiss as a pulled muscle. The classic symptoms include swelling in one leg (rarely both), pain or cramping that typically starts in the calf, warmth over the affected area, and a change in skin color to red or purple. Some people notice the swelling first, others feel a deep soreness that gets worse when they stand or walk. In some cases, a DVT produces no noticeable symptoms at all, which is part of what makes it dangerous.
Symptoms of a Pulmonary Embolism
A pulmonary embolism often feels dramatically different from a leg clot, because now the problem is in your lungs. The hallmark symptom is sudden shortness of breath that appears even at rest and worsens with any physical activity. Many people describe sharp chest pain that intensifies when they breathe in deeply, cough, or bend over. Some say it feels like a heart attack.
Other warning signs include a rapid or irregular heartbeat, dizziness or lightheadedness, fainting, coughing up blood-streaked mucus, excessive sweating, and skin that looks pale or bluish. PE symptoms can range from mild to severe depending on the size of the clot and how much lung tissue it affects. A large embolism can cause a sudden drop in blood pressure and loss of consciousness.
Who Is at Higher Risk
Several factors raise the likelihood of developing a blood clot. The most common thread is immobility. Sitting in the same position for hours, whether during a long flight, a road trip, or recovery from surgery, slows blood flow in the legs and gives clots a chance to form. Surgery itself is a major risk factor, particularly hip and knee replacements and abdominal procedures, because tissue damage activates the clotting system.
Other well-established risk factors include:
- Hormonal changes. Oral contraceptives, hormone replacement therapy, and pregnancy all raise clotting protein levels in the blood.
- Cancer and chemotherapy. Many cancers release substances that promote clotting, and some chemotherapy drugs increase risk further.
- Family history. Inherited conditions that make blood clot more easily, such as Factor V Leiden, run in families. If a close relative has had a DVT or PE, your own risk is higher.
- Obesity. Excess weight increases pressure in the veins of the legs and pelvis.
- Age. Risk rises significantly after age 60, though clots can occur at any age.
Having more than one of these factors at the same time multiplies your risk. A person on birth control who also takes a long-haul flight, for example, faces a higher combined risk than either factor alone would suggest.
How Blood Clots Are Treated
The main goal of treatment is to stop an existing clot from growing and prevent new clots from forming. For most venous clots, this means anticoagulant medications, commonly called blood thinners. Despite the nickname, these drugs don’t actually thin your blood. They interfere with the chain reaction that builds fibrin mesh, making it much harder for clots to form or expand.
The oldest blood thinner still in use, warfarin, works by depleting vitamin K, which several clotting factors need to function. It’s effective, but it requires regular blood tests to keep the dose in the right range, and it interacts with many foods and medications. Newer oral anticoagulants (sometimes called DOACs) work differently: they directly block either thrombin or factor Xa, two key enzymes in the clotting chain. These newer drugs have more predictable effects and typically don’t require routine blood monitoring.
In emergency situations, such as a large pulmonary embolism causing dangerously low blood pressure, doctors may use clot-dissolving drugs or procedures to physically remove the clot. For most people, though, treatment means taking an anticoagulant for several months. Some people with recurring clots or ongoing risk factors stay on blood thinners indefinitely.
Reducing Your Risk
Movement is the single most effective thing you can do to prevent blood clots. During long flights or car rides, the CDC recommends getting up and walking every two to three hours. Choosing an aisle seat on a plane makes this easier. When you can’t stand, simple seated exercises help: raise and lower your heels while keeping your toes on the floor, then reverse it by raising and lowering your toes. Tightening and releasing your calf muscles also keeps blood moving.
Compression stockings apply gentle pressure to the lower legs, helping veins push blood back toward the heart. They’re especially useful during travel or after surgery. Staying well hydrated also supports healthy blood flow, since dehydration thickens the blood slightly and may contribute to clot formation.
If you’re recovering from surgery or illness and can’t move much, hospitals typically use compression devices on the legs and sometimes prescribe a short course of blood thinners to cover the highest-risk period. The combination of mechanical compression and medication significantly lowers the chance of a post-surgical clot.