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What Causes Excessive Blood Clotting?

To understand what causes excessive blood clotting, it helps to understand the body's normal blood clotting process.

Normally, blood clots form to seal small cuts or breaks on blood vessel walls and stop bleeding. After the bleeding has stopped and healing has occurred, the body breaks down and removes the clots.

Blood clotting is a complex process, but it mainly involves:

  • The inner layer of cells lining the blood vessels. These cells play a major role in causing blood clots to form.
  • Clotting factors. These proteins help create a network of fibrin—another type of protein. Fibrin acts like glue to hold blood clots together.
  • Platelets (PLATE-lets). These small blood cell fragments can stick together to form clots.

Excessive blood clotting can occur if the body's clotting process is altered or wrongly triggered. Blood clots can form in, or travel to, the arteries or veins in the brain, heart, kidneys, lungs, and limbs. (Arteries and veins are the blood vessels that carry blood to your heart and body.)

Certain diseases and conditions, genetic mutations, medicines, and other factors can cause excessive blood clotting.

Diseases and Conditions

Many diseases and conditions can cause the blood to clot too much or prevent blood clots from dissolving properly. Certain diseases and conditions are more likely to cause clots to form in certain areas of the body.

Antiphospholipid Antibody Syndrome

This condition, also called APS, is an autoimmune disorder. If you have APS, your body makes antibodies (proteins) that attack phospholipids (fos-fo-LIP-ids)—a type of fat. Phospholipids are found in all living cells and cell membranes, including blood cells and the lining of blood vessels.

In APS, the antibodies trigger blood clots to form in the body's arteries and veins. These blood clots can lead to many health problems, including frequent miscarriages.

APS is more common in women and people who have other autoimmune or rheumatic disorders, such as lupus. ("Rheumatic" refers to disorders that affect the joints, bones, or muscles.)

Bone Marrow Disorders

Some bone marrow disorders can cause your body to make too many blood cells that can lead to blood clots. Examples include polycythemia vera (POL-e-si-THE-me-ah VE-ra), or PV, and thrombocythemia (THROM-bo-si-THE-me-ah).

PV is a rare blood disease in which your body makes too many red blood cells. These extra red blood cells make your blood thicker than normal. This slows the flow of blood through your small blood vessels, which can cause blood clots to form.

Thrombocythemia is a condition in which your body makes too many platelets. The platelets can stick together to form blood clots.

Thrombotic Thrombocytopenic Purpura and Disseminated Intravascular Coagulation

Two rare, but serious conditions that can cause blood clots are thrombotic thrombocytopenic purpura (throm-BOT-ik throm-bo-cy-toe-PEE-nick PURR-purr-ah), or TTP, and disseminated intravascular coagulation (ko-ag-u-LA-shun), or DIC.

TTP causes blood clots to form in the body's small blood vessels, including vessels in the brain, kidneys, and heart.

DIC is a rare complication of pregnancy, severe infections, or severe trauma. DIC causes tiny blood clots to form suddenly throughout the body.

Problems With Blood Clot Breakdown

After a blood clot has done its job, the body normally breaks down the fibrin that holds the clot together.

Several rare genetic and acquired conditions affect the fibrin network that holds blood clots together. Thus, the clots don't break down properly, and they remain in the body longer than needed.

In one condition, for example, the body's fibrin is abnormal and resists being broken down. In another condition, the body has a decreased amount of plasmin. This protein helps break down fibrin.

Excessive Blood Clotting That Mainly Affects the Heart and Brain

Any condition that damages the smooth inner surface of the blood vessels can trigger blood clotting. Many of these conditions are acquired. However, some genetic problems also can damage the inner surface of the blood vessels.

Certain diseases, conditions, or factors can trigger excessive blood clotting mainly in the arteries and veins of the heart and brain.

Atherosclerosis. Atherosclerosis is a disease in which a waxy substance called plaque builds up inside your arteries. Over time, the plaque may rupture (break open). Platelets clump together to form blood clots at the site of the damage. Atherosclerosis is a major cause of damage to the blood vessel walls.

Vasculitis. Vasculitis (vas-kyu-LI-tis) is a disorder that causes the body's blood vessels to become inflamed. Platelets may stick to areas where the blood vessels are damaged and form blood clots. Vasculitis also is a major cause of damage to the blood vessel walls.

Diabetes. Diabetes increases the risk of plaque buildup in the arteries, which can cause dangerous blood clots. Nearly 80 percent of people who have diabetes will eventually die of clot-related causes. Many of these deaths are related to complications with the heart and blood vessels.

Heart failure. Heart failure is a condition in which the heart is damaged or weakened. As a result, it can't pump enough blood to meet the body's needs. Heart failure slows blood flow, which can cause blood clots to form.

Atrial fibrillation. Atrial fibrillation (A-tre-al fih-brih-LA-shun), or AF, is the most common type of arrhythmia (ah-RITH-me-ah). An arrhythmia is a problem with the rate or rhythm of the heartbeat. AF can cause blood to pool in the upper chambers of the heart. This can cause blood clots to form.

Overweight and obesity. Overweight and obesity refer to body weight that's greater than what is considered healthy for a certain height. These conditions can lead to atherosclerosis, which increases the risk of blood clots.

Metabolic syndrome. Some research shows that people who have metabolic syndrome are at increased risk for excessive blood clotting. Metabolic syndrome is the name for a group of risk factors that increases your chance of having heart disease and other health problems.

Excessive Blood Clotting That Mainly Affects the Limbs

Blood clots can form in the veins deep in the limbs. This condition is called deep vein thrombosis (DVT). DVT usually affects the deep veins of the legs.

A blood clot in a deep vein can break off and travel through the bloodstream. If the clot travels to the lungs and blocks blood flow, the condition is called pulmonary embolism (PULL-mun-ary EM-bo-lizm), or PE.

Certain diseases, conditions, or factors can trigger excessive blood clotting mainly in the deep veins of the limbs. Examples include:

  • Hospitalization for major surgery, a serious medical illness, trauma, or broken bones. If you're not able to be active for long periods, blood flow in your veins slows down. As a result, blood clots may form in your legs.
  • "Coach class." This term refers to a situation in which you must stay in one position for a long time, such as in a car or plane. Staying in one position can slow blood flow and lead to blood clots in the legs.
  • Cancer growth and cancer treatment.

Genetic Mutations

Genetic causes of excessive blood clotting most often are due to gene mutations (changes to a normal gene).

Common mutations include Factor V Leiden and Prothrombin G20210A mutations. Fairly rare genetic mutations include proteins C and S deficiencies and antithrombin III deficiency.

Medicines

Some medicines can disrupt the body's normal blood clotting process. Medicines containing the female hormone estrogen are linked to an increased risk of blood clots. Examples of medicines that may contain estrogen include birth control pills and hormone therapy.

Heparin is a medicine commonly used to prevent blood clots. But, in some people, the medicine can cause blood clots and a low platelet count. This condition is called heparin-induced thrombocytopenia (HIT).

HIT rarely occurs outside of a hospital because heparin usually is given in hospitals. In HIT, the body's immune system attacks the heparin and a protein on the surface of the platelets. This attack prompts the platelets to start forming blood clots.

Other Factors That Can Alter the Blood Clotting Process

Many other factors also can alter the clotting process and lead to excessive blood clotting. Common examples include:

  • Smoking. Smoking raises the risk of unwanted blood clots and makes it more likely that platelets will stick together. Smoking also damages the lining of the blood vessels, which can cause blood clots to form.
  • Increased homocysteine levels. Homocysteine is an amino acid linked to a high risk of vascular disease. Increased levels of this substance may damage the inner lining of the arteries.
  • Pregnancy. Women are more likely to develop blood clots when they're pregnant. This is because they have more platelets and clotting factors in their blood during pregnancy. Also, during pregnancy, the uterus compresses the veins. This slows blood flow, which can lead to blood clots.
  • HIV and HIV treatments. The risk of blood clots is highest in HIV patients who have infections, are taking certain medicines, have been hospitalized, or are older than 45.
  • Dehydration. This is a condition in which your body doesn't have enough fluids. This condition causes your blood vessels to narrow and your blood to thicken, which raises your risk for blood clots.
  • Organ transplants and implanted devices, such as central venous catheters and dialysis shunts. Surgery or procedures done on blood vessels may injure the vessel walls. This can cause blood clots to form. Also, catheters and shunts have a man-made surface that may trigger blood clotting.
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May 30, 2012 Last Updated Icon

The NHLBI updates Health Topics articles on a biennial cycle based on a thorough review of research findings and new literature. The articles also are updated as needed if important new research is published. The date on each Health Topics article reflects when the content was originally posted or last revised.