Blood clots can form in veins or arteries and cause serious health complications if they block blood flow. Some clots occur in large veins of the legs, pelvis or arms while others develop in the arteries of the heart, brain, lungs, or limbs. Knowing where clots are most likely to develop can help assess personal risk factors and prevent dangerous clotting episodes.
What is a blood clot?
A blood clot, also called a thrombus, is a solid mass that forms in a blood vessel when platelets, proteins and red blood cells clump together. Clots serve a useful purpose by stopping bleeding from an injury, but clots can also form inside intact blood vessels for a variety of reasons. These clots may partially or completely block normal blood flow, which deprives tissues of oxygen and nutrients.
Two main types of blood clots
There are two primary types of abnormal blood clots that form inside blood vessels:
- Venous clots – Clots that form in the veins are called venous thromboembolism (VTE). This includes deep vein thrombosis (DVT) which occurs in the deep veins of the leg, pelvis or arm and pulmonary embolism which happens if a clot breaks off and travels to the lungs.
- Arterial clots – Clots that form inside arteries are called arterial thromboembolism (ATE). This includes heart attack from a clot in a coronary artery, stroke from a clot in an artery in the brain, and peripheral arterial disease from clots in the arteries of the limbs.
Where do most venous blood clots occur?
Venous blood clots, or VTE, most often develop in the large veins of the legs, pelvis or arms.
Deep vein thrombosis
Up to 80 percent of venous clots occur as deep vein thrombosis (DVT) which forms in the deep veins of the leg or pelvis. The most common sites include:
- Calf veins (peroneal, tibial, gastrocnemius)
- Popliteal vein behind the knee
- Femoral vein in the groin/upper thigh
- Iliac veins of the pelvis
Any type of prolonged immobility increases the risk for DVT in the legs. This includes long trips, surgery, hospitalization, and sitting for long periods without moving. The right leg is slightly more prone to DVT than the left leg. DVT blood clots cause pain, swelling, warmth and redness in the affected area.
Upper extremity DVT
About 10 percent of DVT occurs in veins of the arms, known as upper extremity DVT or Paget-Schroetter syndrome. The most common sites are:
- Basilic vein in the upper arm
- Axillary vein near the armpit
- Subclavian vein under the collarbone
Risk factors for upper arm DVT include IV catheters, surgery, trauma, strenuous upper arm activity, andthoracic outlet syndrome which compresses the subclavian vein. Upper arm DVT can cause pain, swelling, and discoloration of the arm.
Other venous clot locations
While less common, DVT blood clots may also occasionally develop in:
- Cerebral sinuses (dural sinus thrombosis)
- Hepatic vein (Budd-Chiari syndrome)
- Portal vein (portal vein thrombosis)
- Renal vein (renal vein thrombosis)
These types of DVT require specialized testing for diagnosis and have specific risk factors involved.
Where do most arterial blood clots occur?
Arterial blood clots, or ATE, often develop inside arteries of the heart, brain, lungs, or limbs. The most frequent locations include:
Coronary artery clots
A blood clot that blocks a coronary artery causes a heart attack, or myocardial infarction. This is the #1 cause of death worldwide. Up to 65% of heart attacks involve clotting inside a coronary artery, while the remainder stem from severe arterial plaques. Coronary clots most often form at the sites of ruptured plaques.
Carotid or cerebral artery clots
A blood clot blocking a carotid artery in the neck or cerebral artery in the brain leads to an ischemic stroke. Over 80% of strokes occur when a clot forms locally inside a brain artery versus traveling from elsewhere. The middle cerebral artery and internal carotid artery are most often involved.
Pulmonary artery clots
A pulmonary embolism occurs when a DVT clot breaks off and travels to the lungs, blocking a pulmonary artery. While a PE starts as a venous clot, it lodges in the arterial side of the lungs to cause blockage. PE is the third most common acute cardiovascular illness after heart attack and stroke.
Peripheral arterial clots
Clots in the arteries of the arms or legs cause acute limb ischemia. Blockages usually occur at narrow spots where atherosclerosis has built up. The popliteal artery behind the knee and tibial arteries of the calf are common sites. Diabetes, smoking and PAD increase risk.
What are the most common blood clot locations?
Overall, combining venous and arterial clots, the general order of frequency by location is:
- Legs – DVT causes clots in the calf veins, popliteal vein, femoral vein and iliac veins. Leg clots are the most common overall.
- Coronary arteries – Heart attack from clots in the coronary arteries is also extremely common, the #1 cause of death.
- Brain – Ischemic stroke from a cerebral artery clot is the second leading cause of death globally.
- Lungs – Pulmonary embolism from leg DVT clots traveling to the lungs is #3.
- Arms – Upper extremity DVT is less common than leg DVT but still substantial.
- Peripheral arteries – Clots cause acute limb ischemia.
So in summary, the veins of the legs and arteries of the heart and brain are the most frequent sites of dangerous blood clot formation. Knowing the locations where clots are likely to occur based on symptoms can help speed diagnosis and lifesaving treatment.
What conditions increase blood clotting risk?
A variety of medical conditions can increase the risk for developing harmful blood clots, either venous or arterial.
Common clotting risk factors
Conditions that frequently raise overall clot risk include:
- Obesity
- Smoking
- High blood pressure
- Diabetes
- Metabolic syndrome
- Family history of clotting
- Older age (after 40)
- Hospitalization and surgery
- Cancer and chemotherapy
Clotting disorders
Inherited and acquired clotting disorders that increase risk include:
- Factor V Leiden mutation
- Prothrombin gene mutation
- Protein C or S deficiency
- Antiphospholipid antibody syndrome
- Heparin-induced thrombocytopenia (HIT)
- Paroxysmal nocturnal hemoglobinuria (PNH)
Pregnancy
Pregnancy raises the likelihood of developing clots by 5-fold due to hormone changes and blood vessel compression. Clots most often form in the calf or iliac veins but can also occur in the placenta (placental abruption).
Birth control pills
Oral contraceptives with estrogen increase the chance of blood clots, especially in women over 35 who smoke. Progestin-only pills do not seem to raise clotting risk.
Sitting immobility
Prolonged sitting without getting up on long flights or car trips promotes blood stagnation which raises venous clot risk. Being immobile in a hospital bed or after surgery also increases odds of DVT.
Cancer
Having an active cancer doubles the rate of developing blood clots. Certain cancers like pancreatic, brain, lung, colon and hematologic cancers are high-risk. Chemotherapy further increases likelihood of clots.
Conclusion
Dangerous blood clots most often arise in the deep veins of the legs as DVT due to immobility and injury. Clots also frequently develop in the arteries of the heart, brain and lungs which can lead to major cardiovascular events and stroke. Knowing the common locations for clot formation based on symptoms allows for quicker diagnosis and life-saving treatment. Individual risk depends on medical conditions like obesity, smoking, cancer, and genetic factors that impact clotting tendencies. Preventive measures are key for those at increased risk. With proper awareness and preventive care, major clotting events can often be avoided.
| Location | Type of Clot | Underlying Condition | Symptoms |
|---|---|---|---|
| Calf veins | DVT | Immobility, injury | Calf pain, swelling, warmth |
| Popliteal vein | DVT | Knee surgery, trauma | Pain behind the knee |
| Femoral vein | DVT | Hip/pelvis surgery, injury | Upper leg swelling, pain |
| Iliac veins | DVT | Pelvic mass, pregnancy | Groin pain, pelvic swelling |
| Upper arm veins | DVT | IV catheters, repetitive activity | Arm pain, swelling, discoloration |
| Coronary arteries | Arterial clot | Atherosclerosis, plaque rupture | Chest pain, shortness of breath |
| Carotid/cerebral arteries | Arterial clot | Atherosclerosis, AFib | Stroke symptoms like weakness, numbness, difficulty speaking |
| Pulmonary artery | Arterial clot (DVT origin) | Prior DVT, immobilization | Rapid breathing, chest pain, cough, racing heart |
| Peripheral arteries | Arterial clot | Diabetes, atherosclerosis | Limb pain, pallor, numbness |