Thrombosis is a serious medical condition where blood clots form inside blood vessels or the heart chambers, potentially blocking normal blood flow. These clots can stay in place and grow, or break free and travel through the bloodstream to vital organs. When a clot reaches critical areas like the lungs, brain, or heart, it can trigger life-threatening emergencies such as stroke, heart attack, or pulmonary embolism. Understanding the warning signs and risk factors can help protect you and your loved ones.

How Common Is Thrombosis

Thrombosis is extremely common worldwide and represents a major public health challenge. According to medical data, thrombosis is the underlying cause of 1 in every 4 deaths globally, making it one of the leading causes of death in developed countries.^[1] This staggering statistic reflects how thrombosis leads to serious conditions like heart attacks, strokes, and pulmonary embolisms, which together account for the highest proportion of thrombosis-related deaths in the United States.^[2]

Thrombosis becomes more common with age, particularly in people over 60 years old. The condition affects both men and women, though certain factors like hormone use and pregnancy can increase risk in women. While anyone can develop a blood clot, the likelihood increases significantly among people with specific medical conditions or lifestyle factors. People with conditions such as atrial fibrillation (an irregular heartbeat), cancer, coronary artery disease, diabetes, and certain clotting disorders face higher risks of developing thrombosis.^[1]

The two main forms of thrombosis—venous and arterial—affect different populations to varying degrees. Venous thromboembolism (blood clots in veins) is often underdiagnosed but represents a serious and frequently preventable medical condition.^[7] Many people who develop deep vein thrombosis have no symptoms at all, which means the true prevalence may be even higher than reported statistics suggest.

What Causes Thrombosis

Blood clots form when the body’s normal clotting system becomes unbalanced. Under healthy conditions, blood clots serve a protective function—they stop bleeding when you’re injured by sealing damaged blood vessels. However, when clots form inappropriately inside blood vessels, they become dangerous. The formation of these abnormal clots involves complex interactions between blood cells, platelets, plasma proteins, coagulation factors, inflammatory factors, and the inner lining of blood vessels.^[2]

Thrombosis can develop through several mechanisms. In arterial thrombosis, clots typically form after the rupture of atheroma—fatty deposits in the blood vessel wall. This type of clotting is often referred to as atherothrombosis and is closely linked to conditions like high cholesterol and high blood pressure.^[5] Arterial clots can also originate from the heart, particularly in people with atrial fibrillation, where irregular heartbeats cause blood to pool and clot inside the heart chambers.^[5]

Venous thrombosis develops through different processes. When blood flow slows down or becomes stagnant—such as during long periods of sitting or bed rest—the risk of clotting increases. Injury to the inner lining of veins, whether from surgery, trauma, or medical procedures, can trigger clot formation. Additionally, certain inherited or acquired conditions make blood more likely to clot, creating what doctors call a hypercoagulable state.^[2]

Sometimes thrombosis occurs without any identifiable trigger. These cases are called “unprovoked” or idiopathic blood clots, meaning doctors cannot pinpoint a specific cause despite thorough investigation.^[21] Even when no obvious reason exists, these clots require the same serious attention and treatment as those with known causes.

Risk Factors for Developing Thrombosis

Certain groups of people face significantly higher risks of developing blood clots. Age plays an important role—being 65 or older increases your chances of thrombosis.^[12] Your family history matters too; if close relatives have had blood clots, your risk may be elevated due to inherited clotting disorders or shared lifestyle factors.^[6]

Medical conditions that increase thrombosis risk include cancer, which can affect blood clotting mechanisms through multiple pathways. Heart failure and inflammatory disorders like Crohn’s disease also raise the likelihood of clot formation. People with rheumatologic disorders such as rheumatoid arthritis experience increased clotting risk as well.^[20] Smoking significantly increases the danger of thrombosis, particularly when combined with other risk factors like birth control pill use.^[6]

Obesity and being overweight contribute to thrombosis risk by affecting blood flow and increasing inflammation throughout the body. Physical inactivity represents another major risk factor—people who cannot move around well or sit for extended periods face higher chances of developing clots.^[12] This explains why long flights, car trips lasting more than three hours, or prolonged bed rest after surgery or illness create dangerous conditions for clot formation.^[13]

Recent surgery dramatically increases thrombosis risk, particularly operations on the legs, hips, abdomen, or pelvis. Hospital stays in general raise clot risk because patients often have limited mobility and may have multiple medical conditions requiring treatment.^[13] Dehydration can thicken blood and slow circulation, making clots more likely to form.^[13] Varicose veins, which indicate problems with blood flow in leg veins, also increase the likelihood of developing deep vein thrombosis.^[13]

Warning Signs and Symptoms

The symptoms of thrombosis vary dramatically depending on where the blood clot forms or travels within your body. Understanding these warning signs can save your life, because early recognition and treatment significantly improve outcomes. Blockages most commonly occur in areas with smaller blood vessels, particularly the lungs, brain, and the lower parts of arms and legs.^[1]

When a blood clot forms in the deep veins of your leg—a condition called deep vein thrombosis or DVT—you may experience throbbing pain in one leg, usually in the calf or thigh. This pain typically worsens when walking or standing. The affected leg often swells noticeably, and the skin around the painful area may feel warm to touch. You might notice redness or darkening of the skin, though these color changes can be harder to see on brown or black skin. The veins may become swollen and feel hard or sore when touched.^[13] However, about half of people with DVT have no symptoms at all, which makes this condition particularly dangerous.^[7]

If a blood clot travels to your lungs—a pulmonary embolism—you may suddenly develop sharp chest pain that worsens when you breathe deeply or cough. Breathing becomes difficult, whether you’re active or resting. Your heartbeat may become faster than normal or irregular. Some people cough up blood. You might feel lightheaded or even faint due to very low blood pressure.^[1] It’s crucial to understand that you can have a pulmonary embolism without any prior symptoms of DVT in your legs.^[7]

When thrombosis affects the brain or arteries in the neck, it can cause a stroke. Warning signs include weakness or drooping on one side of your face, arm, or leg. You may have difficulty speaking or understanding what others say. Dizziness, loss of balance, blurred vision, and difficulty swallowing can all signal a stroke. These symptoms require immediate emergency attention, no matter how briefly they last.^[3]

If a blood clot blocks arteries supplying the heart, you may experience a heart attack. Symptoms include chest pain or discomfort that may feel like heavy pressure or squeezing. Shortness of breath, weakness, nausea, cold sweats, anxiety, and dizziness can accompany the chest discomfort. If these symptoms worsen quickly or last longer than 10 minutes, call emergency services immediately.^[3]

How Thrombosis Affects Your Body

When a blood clot forms inside a blood vessel, it disrupts the carefully balanced system that keeps blood flowing smoothly throughout your body. Blood needs to circulate freely to deliver oxygen and nutrients to organs and tissues while removing waste products. A clot creates a physical barrier that can partially or completely block this essential flow.^[1]

Two dangerous scenarios can unfold when thrombosis occurs. First, the clot may stay where it formed and continue growing larger. As it expands, it increasingly restricts blood flow through that vessel. If the blockage becomes severe enough, tissues downstream from the clot begin suffering from lack of oxygen—a condition called ischemia. When ischemia continues, tissue damage or death, called necrosis, can occur.^[5]

The second scenario involves the clot breaking loose from its original location. When this happens, the clot becomes what doctors call an embolus—a traveling clot carried through the bloodstream. This mobile clot may flow through increasingly smaller blood vessels until it gets stuck, creating a blockage far from where it first formed. This process, called embolism, explains how a clot in your leg can end up blocking blood flow in your lungs, or how a clot from your heart can cause a stroke in your brain.^[1]

In arterial thrombosis, the blockage typically causes sudden, severe problems because arteries carry oxygen-rich blood away from the heart to vital organs. When an artery becomes blocked, the affected tissue immediately begins suffering from oxygen starvation. This explains why arterial thrombosis in the heart causes heart attacks and why blockages in brain arteries cause strokes—these organs cannot function without constant oxygen supply.^[6]

Venous thrombosis creates different problems. Since veins carry blood back toward the heart, blockages in veins cause blood to pool and back up in tissues. This leads to swelling, pain, and potential damage to the affected area. The most dangerous complication occurs when a venous clot breaks free and travels through the heart into the pulmonary arteries supplying the lungs. Even if a pulmonary embolism doesn’t cause immediate death, it can lead to long-term lung damage and increased pressure in lung blood vessels—a serious condition called chronic thromboembolic pulmonary hypertension.^[7]

After recovering from deep vein thrombosis, between one-third and one-half of patients develop lasting complications called post-thrombotic syndrome. This happens because the clot damages the tiny valves inside veins that normally prevent backward blood flow. People with post-thrombotic syndrome experience chronic swelling, pain, skin discoloration, and in severe cases, scaling or ulcers in the affected limb.^[7]

Preventing Blood Clots

The good news about thrombosis is that it’s often preventable, even for people with risk factors. Maintaining a healthy weight helps reduce your risk by improving circulation and decreasing inflammation throughout your body. Regular physical activity keeps blood moving through your vessels and strengthens your cardiovascular system. Doctors recommend trying to get at least 30 minutes of activity on most days of the week.^[1]

Staying well-hydrated by drinking plenty of fluids helps prevent blood from becoming too thick. Dehydration increases clotting risk, so this simple step can make a meaningful difference, especially during travel or hot weather.^[13] If you smoke, quitting represents one of the most important actions you can take to reduce thrombosis risk. Smoking damages blood vessels and affects blood clotting mechanisms. Your doctor can help you find stop-smoking programs and medications that work.^[1]

Avoiding prolonged sitting is crucial for clot prevention. When you must sit for extended periods, make a point to stand up and move around every hour or two. While seated, you can exercise your lower leg muscles by pumping your feet up and down—pulling your toes toward your knees then pointing them downward. This simple movement helps keep blood flowing through your legs.^[19] Never cross your legs while sitting if you’ve previously had a DVT, as this position can restrict circulation.^[20]

When traveling for three or more hours by plane, train, or car, take special precautions. On long drives, stop the vehicle and walk around every hour. On planes or trains, get up and walk down the aisle regularly if possible. Wear loose, comfortable clothing during travel. Drink plenty of water and avoid excessive alcohol consumption, which can lead to dehydration.^[13]

If your doctor prescribes compression stockings, wear them as directed. These specially fitted garments apply consistent pressure to your legs, preventing blood from pooling and reducing clot risk. They’re particularly important if you’ve had a previous blood clot.^[1] For people with a history of thrombosis, getting out of bed and moving as soon as your doctor allows after surgery or illness is one of the best prevention strategies. Being active helps blood circulate properly and reduces the risk of new clots forming.^[19]