In this lesson, you will learn how blood clots form. You will discover why blood sometimes clots when it shouldn’t, and you will learn some of the symptoms associated with abnormal blood clots. A quiz at the end of the lesson will test your knowledge.

The Importance of Blood

Blood is a specialized fluid that carries oxygen and nutrients to every cell in your body and carries carbon dioxide and metabolic wastes away from those same cells. Blood transports immune cells, antibodies, and hormones to body regions where they’re needed. By transferring heat from your internal organs to your skin, your blood even plays a role in regulating your temperature.

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Your body goes to great lengths to conserve this vital fluid called blood. Your body stores iron so new red blood cells can be quickly manufactured when they’re needed. If you sustain a major injury, your blood vessels constrict and shunt blood away from the trauma site. Most importantly, your body preserves blood through a process called clotting, or coagulation. Two distinctly different but closely linked mechanisms are responsible for clotting your blood: platelet activation and the coagulation cascade.

Platelet Activation

Platelets are fragments of large cells called megakaryocytes (meaning ‘large cells’) that are found in your bone marrow. More than 100 billion platelets break away from their parent megakaryocytes and enter your circulation every day. When platelets come into contact with an injured blood vessel wall, they become ‘activated.’ Activated platelets release chemicals that attract and activate more platelets. As activated platelets clump together, or aggregate, they trap fibrinogen, which is a protein found in your bloodstream. Fibrinogen adheres to the surface of activated platelets, creating a sticky meshwork that is the beginning of a clot, or thrombus.

Because activated platelets immediately form a plug at the site of an injury, platelet aggregation is sometimes called primary hemostasis, meaning ‘first to stop bleeding.’ In truth, platelet aggregation occurs almost simultaneously with and overlaps the coagulation cascade, which is called secondary hemostasis.

Coagulation Cascade

Even as activated platelets form a plug at the site of an injury, a group of proteins called coagulation factors begin organizing and solidifying the thrombus. Coagulation factors are manufactured in your liver and released to circulate freely in your bloodstream where they remain inactive until they encounter an injured vessel wall or a plug of activated platelets. When that occurs, a series of reactions (collectively known as the coagulation cascade) leads to the formation of a mature thrombus.

The coagulation cascade is a complicated chemical pathway. It helps to envision the coagulation cascade as a row of falling dominoes whose first member gets pushed over by an injury to a blood vessel. However, rather than being a single row of dominoes, the coagulation cascade has several ‘arms’ that enter and exit the pathway. Tipping any of these arms over can trigger the cascade that leads to thrombus formation. And, even as the dominoes are all tilting toward the formation of a thrombus, other arms are leading off to create substances that lead back into the cascade and stop the dominoes from falling. This is important because uncontrolled clotting, or thrombosis, can be dangerous.

Thrombosis – An Abnormal State of Clotting

Thrombosis is a term used to describe the spontaneous formation of blood clots in places where they should not form. Pathologic (abnormal) clots can block the vessels where they form, or they can break free from the wall of the vessel and become mobile, creating an embolus. When an embolus travels to another area of your body and becomes lodged, it is called an embolism.

A number of issues can contribute to thrombosis. Certain inherited diseases, such as Factor V Leiden, are characterized by defective coagulation factors that inappropriately trigger the coagulation cascade. (Coagulation factors are typically designated by Roman numerals. Factor V Leiden, which was first identified in Leiden, Holland, is one of the most common genetic disorders associated with thrombosis.) Certain cancers produce their own coagulation factors that are stickier than normal coagulation factors; many cancer patients actually die due to thrombosis and embolism, rather than their underlying cancer.

The tendency to clot too easily can occur in previously healthy people, too. If you sit for prolonged periods, if you undergo pelvic or leg surgery, if you sustain an injury to your leg or foot, or if you are immobilized due to an injury or illness, you can develop clots in the veins of your legs or pelvis. (The veins of your arms, legs, and groin are the most likely sites for thrombosis to occur.) If a thrombus in your legs or pelvis releases fragments and produces an embolus, it can travel through your bloodstream, return to your heart, and get pushed into a lung where it could cause a pulmonary embolism.

The deep veins of the legs, arms, and groin are the most common sites for thrombosis.
DVT Lower Extremity

Symptoms of Thrombosis

The signs and symptoms of thrombosis vary a great deal, depending on whether the clot occurs in an artery or vein and what body region or organ is affected. Arterial clots tend to cause abrupt, severe symptoms, while clots occurring in veins often generate minimal symptoms until they cause an embolism.

For example, an arterial embolism or thrombus within your brain can lead to a stroke, which could be manifested by slurred speech, numbness, weakness, difficulty walking, poor balance, paralysis, or sudden unconsciousness. A thrombus in one of the arteries of your heart could trigger chest pain, an abnormal heart rhythm, a heart attack, or sudden death. An arterial embolism or thrombus in one of your extremities would cause severe pain in the extremity ‘downstream’ from the clot, and the skin of the extremity would turn cold and blue.

Symptoms due to venous thrombosis tend to be more subtle, at least at first. You could have a fairly large thrombus in your leg veins and be completely unaware of it until you develop a pulmonary embolism. Even then, your symptoms could include mild anxiety or slight air hunger, severe shortness of breath, chest pain, sudden collapse, and death. Many people who develop clots in their leg veins complain of leg heaviness, pain, or swelling, but not everyone develops these symptoms.

Similarly, a clot in an arm vein – which can also lead to a pulmonary embolism – may only cause subtle symptoms: arm heaviness or ‘fullness,’ swelling, and prominent surface veins are commonly seen in people with a thrombus in an arm.

Lesson Summary

Blood performs several vital functions, including delivery of oxygen and nutrients and removal of carbon dioxide and metabolic wastes. Two principal mechanisms – platelet activation and the coagulation cascade – are responsible for clotting your blood. Thrombus formation can be lifesaving after an injury, but abnormal clotting, or thrombosis, can be dangerous or even life-threatening.

Thrombosis can be due to inherited conditions, such as Factor V Leiden, or it can result from acquired conditions, such as cancer. Even healthy people can develop thrombosis under certain conditions, such as prolonged immobilization. Symptoms of thrombosis are variable, depending on whether an artery or a vein is blocked and where in your body the clot occurs. Arterial thrombosis tends to cause sudden, severe symptoms, while the symptoms of venous thrombosis often develop gradually. Both arterial and venous thrombosis can be fatal.

Learning Outcomes

After this lesson, you’ll have the ability to:

  • Recall the functions of blood
  • Describe platelet activation and the coagulation cascade
  • Differentiate between normal and abnormal thrombus formation
  • Explain what an embolus is
  • Identify the causes, risk factors, signs, and symptoms of thrombosis

Medical Disclaimer: The information on this site is for your information only and is not a substitute for professional medical advice.