What Causes a Blood Clot?
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Medically reviewed by Isabel Casimiro, MD, PhD
Blood clotting—also known as coagulation—is a normal process that helps you stop bleeding when your blood vessels become injured or damaged. When a blood vessel is damaged, blood platelets and proteins in the plasma (the liquid portion of your blood) join forces to create a blood clot. Normally your body will break down the blood clot and get rid of it naturally. But sometimes, blood clots can form inside the blood vessels even without injury or may not go away on their own.
Blood clots are clumps of blood that can form in your arteries and veins (which are two different types of blood vessels). Several factors can influence your risk of having a blood clot, including an injury to a blood vessel, an underlying health condition where the blood clots more than normal, and the slowing or blockage of blood flow.
Primary Causes of Blood Clots
Blood clots that form in the arteries (blood vessels that transport blood from your heart to the rest of your body) typically occur when a plaque buildup inside an artery bursts. Plaques are deposits of cholesterol, calcium, and fatty substances in your blood vessels. This process can lead to blockages that interrupt blood flow. But when a clot forms abnormally in a vein, it can block blood flow back to your heart and cause pain and swelling.
There are three main causes of blood clots, which include:
Injury to the inner lining of your blood vessels
An underlying health condition where blood clots more easily than normal
Slow or stagnant blood flow in your arteries or veins
These causes are collectively referred to as "Virchow's triad," named after the scientist who first identified them.
Injury to Inner Lining of Blood Vessels
There are several culprits to consider when the lining of your blood vessels becomes damaged. Potential causes of injury can include catheter placement, trauma, and surgery. When a blood clot is caused by a catheter, the blood clot can occur superficially (just below the skin's surface) or deep in your vein. Both superficial and deep vein clots can cause pain, redness, and tenderness near the injured vein.
The likelihood of developing a blood clot as a result of trauma can vary based on the severity and the type of trauma you experience. You can also reduce your risk or prevent a blood clot by using strategies, like wearing compression socks, exercising your legs, or taking blood-thinning medications (if your healthcare provider recommends them). Keep in mind: despite these prevention measures, there is still a small chance that you develop a blood clot.
Abnormal Blood Clot Formation
When your blood clots more easily than it should, it could be due to an underlying health condition you're born with (inherited) or something that develops later in life (acquired). Some conditions can make it easier for blood clots to form.
There are fewer genetic or inherited conditions that can increase your risk of a blood clot. It's more common for acquired health conditions or risk factors to cause blood clotting problems. For example, using certain medications like birth control pills or hormone replacement therapy, smoking tobacco, and experiencing pregnancy, surgery, injuries, infections, and inflammation can all cause your blood to clot more easily than normal. Long-term health conditions like obesity, arthritis, or ulcerative colitis can also lead to this condition.
Slow Blood Flow
When the blood flow in your arteries and veins slows down or stops, a blood clot can develop more easily. A stoppage or slowing down of blood flow is medically known as "stasis." There are two types of stasis that can increase your risk of a blood clot: arterial (occurring in the arteries) and venous (occurring in the veins). Long periods of sitting (such as on long flights or car rides), pregnancy, and a history of blood clots can slow down your blood flow and increase your risk of a blood clot.
Are Blood Clots Hereditary?
Blood clots as a result of inherited conditions are less frequent and typically stem from genetic mutations in your DNA. These abnormalities usually affect the proteins that help your blood clot properly. These gene mutations can also affect how your body breaks down and gets rid of blood clots naturally. Examples of these genetic changes include having a deficiency in proteins that aid the blood clotting process, such as antithrombin III and proteins C and S.
You may have an inherited blood clotting issue if you:
Have a family history of blood clots
Had several blood clots before the age of 40
Have a personal history of miscarriages during pregnancy
Who Gets Blood Clots?
The chances of developing a blood clot can vary based on several factors. The likelihood of developing a blood clot in the deep veins, for example, increases with age. People assigned female at birth are more likely to experience these conditions than people assigned male at birth. Those who have a history of cancer are also at an increased risk. Research also shows that African Americans experience blood clots at higher rates than whites.
Risk Factors
Blood clots can develop for a variety of reasons. The following factors can all increase your risk:
Being over the age of 65
Using medications like birth control pills and hormone therapy
Receiving a diagnosis for health conditions such as cancer, obesity, heart disease, diabetes, high blood pressure, high cholesterol, autoimmune diseases, blood disorders, varicose veins, and stroke
Undergoing chemotherapy
Living a sedentary lifestyle or not getting enough physical activity
Experiencing a hip, pelvis, or leg fracture and excessive bleeding or bruising
Having a personal or family history of blood clots
Frequently taking long trips (longer than an hour) by car, plane, bus, or train where you're sitting for extended periods of time
Becoming pregnant
Recovering from a surgery
Enduring trauma or injury to a blood vessel
A Quick Review
Blood clots are clumps of blood that can form in your veins and arteries. The main causes of blood clots include injury to the blood vessels, health conditions that cause abnormal blood clot formations, and stoppages in blood flow. A history of underlying conditions like cancer or diabetes, living a sedentary lifestyle, and experiencing pregnancy may also increase your risk of a blood clot.
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