Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

How Blood Clots


Michael B. Streiff

, MD, Johns Hopkins University School of Medicine

Reviewed/Revised Oct 2023
Topic Resources

Hemostasis is the body's way of stopping injured blood vessels from bleeding. Hemostasis includes clotting of the blood.

  • Too little clotting can cause excessive bleeding from minor injury

  • Too much clotting can block blood vessels that are not bleeding

An abnormality in any part of the system that controls bleeding can lead to excessive bleeding Bruising and Bleeding Bruising or bleeding after an injury is normal (see also How Blood Clots). However, some people have disorders that cause them to bruise or bleed too easily. Sometimes people bleed without any... read more Bruising and Bleeding or excessive clotting Excessive Clotting Excessive clotting (thrombophilia) occurs when the blood clots too easily or excessively. Inherited and acquired disorders can increase blood clotting. Clots in larger blood vessels cause legs... read more , both of which can be dangerous. When clotting is poor, even a slight injury to a blood vessel may lead to severe blood loss. When clotting is excessive, small blood vessels in critical places can become clogged with clots. Clogged vessels in the brain can cause strokes Overview of Stroke A stroke occurs when an artery to the brain becomes blocked or ruptures, resulting in death of an area of brain tissue due to loss of its blood supply (cerebral infarction). Symptoms occur suddenly... read more , and clogged vessels leading to the heart can cause heart attacks Acute Coronary Syndromes (Heart Attack; Myocardial Infarction; Unstable Angina) Acute coronary syndromes result from a sudden blockage in a coronary artery. This blockage causes unstable angina or a heart attack (myocardial infarction), depending on the location and amount... read more Acute Coronary Syndromes (Heart Attack; Myocardial Infarction; Unstable Angina) . Pieces of clots from veins in the legs, pelvis, or abdomen can travel through the bloodstream to the lungs and block major arteries there (pulmonary embolism Pulmonary Embolism (PE) Pulmonary embolism is the blocking of an artery of the lung (pulmonary artery) by a collection of solid material brought through the bloodstream (embolus)—usually a blood clot (thrombus) or... read more ). The body has control mechanisms to limit clotting and dissolve clots that are no longer needed.

Hemostasis involves three major processes:

Blood Clots: Plugging the Breaks

When an injury causes a blood vessel wall to break, platelets are activated. They change shape from round to spiny, stick to the broken vessel wall and each other, and begin to plug the break. They also interact with other blood proteins to form fibrin. Fibrin strands form a net that entraps more platelets and blood cells, producing a clot that plugs the break.

Blood Clots: Plugging the Breaks
Blood Clots: Plugging the Breaks

Blood vessel factors

An injured blood vessel constricts so that blood flows out more slowly and clotting can start. At the same time, the accumulating pool of blood outside the blood vessel (a hematoma) presses against the vessel, helping prevent further bleeding.

Platelet factors

As soon as a blood vessel wall is damaged, a series of reactions activates platelets Platelets The main components of blood include Plasma Red blood cells White blood cells Platelets read more Platelets so that they stick to the injured area. The "glue" that holds platelets to the blood vessel wall is von Willebrand factor, a large protein produced by the cells of the vessel wall. The proteins collagen and thrombin act at the site of the injury to induce platelets to stick together. As platelets accumulate at the site, they form a mesh that plugs the injury. The platelets change shape from round to spiny, and they release proteins and other substances that entrap more platelets and clotting proteins in the enlarging plug that becomes a blood clot.

Blood clotting factors

Formation of a clot also involves activation of a sequence of blood clotting factors, which are proteins produced mainly by the liver. There are over a dozen blood clotting factors. They interact in a complicated series of chemical reactions that ultimately generate thrombin. Thrombin converts fibrinogen, a blood clotting factor that is normally dissolved in blood, into long strands of fibrin that radiate from the clumped platelets and form a net that entraps more platelets and blood cells. The fibrin strands add bulk to the developing clot and help hold it in place to keep the vessel wall plugged.

Stopping clotting

The reactions that result in the formation of a blood clot are balanced by other reactions that stop the clotting process and dissolve clots after the blood vessel has healed. Without this control system, minor blood vessel injuries could trigger widespread clotting throughout the body—which actually happens in some diseases (see Bruising and Bleeding Bruising and Bleeding Bruising or bleeding after an injury is normal (see also How Blood Clots). However, some people have disorders that cause them to bruise or bleed too easily. Sometimes people bleed without any... read more Bruising and Bleeding ).

Medications and blood clots

Some people are at high risk of forming blood clots and are intentionally given medications to decrease the risk. Medications may be given that reduce the stickiness of platelets, so that they will not clump together to block a blood vessel. Aspirin, ticlopidine, clopidogrel, prasugrel, abciximab, and tirofiban are examples of medications that interfere with the activity of platelets.

Other people at risk of forming blood clots may be given an anticoagulant Anticoagulation Pulmonary embolism is the blocking of an artery of the lung (pulmonary artery) by a collection of solid material brought through the bloodstream (embolus)—usually a blood clot (thrombus) or... read more , a medication that inhibits the action of blood proteins called clotting factors. Although often called "blood thinners," anticoagulants do not really thin the blood. Commonly used anticoagulants are warfarin, given by mouth, and heparin, given by injection. Direct oral anticoagulants (DOACs) directly inhibit thrombin or activated factor X, potent proteins that are needed for clotting to occur. Examples of DOACs include dabigatran, apixaban, edoxaban, and rivaroxaban.

People who take warfarin or heparin must be under close medical supervision. Doctors monitor the effects of these medications with blood tests that measure the time it takes for a clot to form, and they adjust the dose on the basis of test results. Doses that are too low may not prevent clots, while doses that are too high may cause severe bleeding. Other types of anticoagulants, such as a class of medications called low-molecular-weight heparins (such as dalteparin, enoxaparin, and tinzaparin), do not require as much supervision. People taking DOACs do not require frequent laboratory testing of coagulation.

If a person already has a blood clot, a thrombolytic (fibrinolytic) medication can be given to help dissolve the clot. Thrombolytic medications, which include streptokinase and tissue plasminogen activators, are sometimes used to treat heart attacks and strokes caused by blood clots. These medications may save lives, but they can also put the person at risk of severe bleeding. Heparin, a medication given to reduce the risk of clot formation, sometimes has an unintended, paradoxical activating effect on platelets that increases the risk of clotting (heparin-induced thrombocytopenia-thrombosis).

Estrogen, alone or in oral contraceptives, can have the unintended effect of causing excessive clot formation. Certain medications used to treat cancer (chemotherapy drugs), such as asparaginase, can also increase the risk of clotting.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Anacin Adult Low Strength, Aspergum, Aspir-Low, Aspirtab , Aspir-Trin , Bayer Advanced Aspirin, Bayer Aspirin, Bayer Aspirin Extra Strength, Bayer Aspirin Plus, Bayer Aspirin Regimen, Bayer Children's Aspirin, Bayer Extra Strength, Bayer Extra Strength Plus, Bayer Genuine Aspirin, Bayer Low Dose Aspirin Regimen, Bayer Womens Aspirin , BeneHealth Aspirin, Bufferin, Bufferin Extra Strength, Bufferin Low Dose, DURLAZA, Easprin , Ecotrin, Ecotrin Low Strength, Genacote, Halfprin, MiniPrin, St. Joseph Adult Low Strength, St. Joseph Aspirin, VAZALORE, Zero Order Release Aspirin, ZORprin
Coumadin, Jantoven
Hepflush-10 , Hep-Lock, Hep-Lock U/P, Monoject Prefill Advanced Heparin Lock Flush, SASH Normal Saline and Heparin
Xarelto, Xarelto Granules, Xarelto Starter Pack
quiz link

Test your knowledge

Take a Quiz!