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How Blood Clots - Platelets and the Coagulation Cascade

Updated on November 11, 2015
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Linda Crampton is a teacher with an honours degree in biology. She enjoys writing about human biology and the science of health and disease.

Red blood cells are the most common type of cell in our blood. They pick up oxygen from our lungs and carry it to our tissue cells.
Red blood cells are the most common type of cell in our blood. They pick up oxygen from our lungs and carry it to our tissue cells. | Source

Blood Clotting or Coagulation

Blood clotting or coagulation is a biological process that stops bleeding. It's vital that blood clots when we have a surface injury that breaks blood vessels. Clotting can prevent us from bleeding to death and protect us from the entry of bacteria and viruses. Clots also form inside our body when a blood vessel is injured. Here they prevent blood loss from the circulatory system.

Our body can both make clots and break them down once they've done their job. In most people, a healthy balance is maintained between these two activities. In some people abnormal blood clotting occurs, however, and their body may not be able to break clots down. A large clot inside a blood vessel is potentially dangerous because it can block blood flow in the vessel. Internal clots that form without an obvious injury or clots that travel through blood vessels are also dangerous.

Coagulation of blood is a fascinating and complex process that involves many steps. Proteins made by the liver and sent into the bloodstream are an essential part of the process. The proteins circulate around the body in our blood, ready for action at any time. An external or internal injury is the trigger that activates the proteins and sets the blood clotting process in motion.

From left to right, a red blood cell, an activated platelet and a white blood cell
From left to right, a red blood cell, an activated platelet and a white blood cell | Source

Formed Elements in Blood

Red blood cells (or erythrocytes) carry oxygen to cells. The five types of white blood cells (leukocytes) fight infections in various ways. Platelets (thrombocytes) play an essential role in the blood clotting process. They develop a spiky appearance when they're activated.

Hemostasis

Hemostasis is the process in which bleeding is stopped. It involves three steps.

  • Vasoconstriction: narrowing of damaged blood vessels to reduce blood loss. This is caused by contraction of the smooth muscle in the wall of vessels.
  • Activation of platelets: activated platelets stick to each other and to collagen fibres in the broken walls of blood vessels, forming a platelet plug that temporarily blocks blood flow. The platelets also release chemicals that attract other platelets and stimulate further vasoconstriction.
  • Formation of a blood clot: the clot contains fibres that trap the platelets and is stronger and longer-lasting than the platelet plug.

The Process of Platelet Activation

Platelet Agglutination, Activation and Aggregation

Platelets are small cell fragments in our blood. They have a somewhat irregular form but are roughly disk shaped. They are produced by budding off from a larger cell in bone marrow called a megakaryocyte. Platelets play an important role in the initiation of a blood clot.

The first step in healing a wound is the activation of platelets. When platelets touch the damaged wall of a blood vessel, encounter turbulence in blood flowing around a wound or encounter specific chemicals in the blood, they become "sticky". They bind to the injured cells in a wound as well as to each other. During this activation process, the platelets become more rounded in shape and develop spikes.

Activated platelets form a mesh, or a platelet plug, that covers and fills a wound. The plug temporarily stops bleeding and is a very helpful emergency response to a wound. It's quite weak, however, and may be removed by flowing blood unless it's strengthened by a blood clot. The activated platelets in a plug release chemicals that are needed by the blood clotting process.

Blood Clotting Summary

A prothrombin activator converts prothrombin into thrombin. Thrombin is an enzyme that converts fibrinogen into fibrin. Prothrombin and fibrinogen are proteins that are always present in our blood.
A prothrombin activator converts prothrombin into thrombin. Thrombin is an enzyme that converts fibrinogen into fibrin. Prothrombin and fibrinogen are proteins that are always present in our blood. | Source

Summary of the Blood Clotting Process

The blood clotting process is complex and involves many reactions. However, the process can be summarized in three steps.

  • A complex known as a prothrombin activator is produced by a long sequence of chemical reactions.
  • The prothrombin activator converts a blood protein called prothrombin into another protein called thrombin.
  • Thrombin converts a soluble blood protein called fibrinogen into an insoluble protein called fibrin.
  • Fibrin exists as solid fibres which form a tight mesh over the wound. The mesh traps platelets and other blood cells and forms the blood clot.

Prothrombin and fibrinogen are always present in our blood, but they aren't activated until a prothrombin activator is made when we're injured.

Blood Clotting Overview

The Coagulation Cascade - Blood Clotting in More Detail

Blood clotting occurs in a multi-step process known as the coagulation cascade. The process involves many different proteins. The cascade is a chain reaction in which one step leads to the next. In general, each step produces a new protein which acts as an enzyme, or catalyst, for the next step.

The coagulation cascade is often classified into three pathways - the extrinsic pathway, the intrinsic pathway and the common pathway.

The extrinsic pathway is triggered by a chemical called tissue factor that is released by damaged cells. This pathway is "extrinsic" because it's initiated by a factor outside the blood vessels. It's also known as the tissue factor pathway.

The intrinsic pathway is triggered by blood coming into contact with collagen fibers in the broken wall of a blood vessel. It's "intrinsic" because it's initiated by a factor inside the blood vessel. It's sometimes called the contact activation pathway.

Both pathways eventually produce a prothrombin activator. The prothrombin activator triggers the common pathway in which prothrombin becomes thrombin followed by the conversion of fibrinogen to fibrin.

Although dividing the coagulation process into extrinsic and intrinsic pathways is a useful approach to the topic and is a widely used tactic, scientists say that it's not completely accurate. For many students of this complex process, however, it's the best solution for understanding blood clotting.

The Classical Blood Coagulation Pathway

A summary of the intrinsic and extrinsic pathways in the coagulation cascade; recent studies have found that additional reactions and clotting factors are involved in the pathways, but this diagram gives a general idea of the process
A summary of the intrinsic and extrinsic pathways in the coagulation cascade; recent studies have found that additional reactions and clotting factors are involved in the pathways, but this diagram gives a general idea of the process | Source

Clotting or Coagulation Factors

The Roman numerals in a coagulation cascade diagram represent clotting or coagulation factors. These factors are chemicals that are required in the chain of reactions that make up the blood clotting process.

Hemostasis Mechanism

Clotting Factors

The chemicals involved in the coagulation cascade are called clotting or coagulation factors. There are twelve clotting factors, which are numbered with Roman numerals and given a common name as well. The factors are numbered according to the order in which they were discovered and not according to the order in which they react.

Other chemicals are needed for blood clotting in addition to those in the coagulation cascade. For example, vitamin K is an essential chemical in the blood clotting process.

Names and Sources of the Clotting or Coagulation Factors

Coagulation Factor
Common Name
Source
Factor l
fibrinogen
liver
Factor ll
prothrombin
liver
Factor lll
tissue factor or thromboplastin
Damaged tissue cells release tissue thromboplastin. Platelets release platelet thromboplastin.
Factor lV
calcium ions
bone, and absorption through the lining of the small intestine
Factor V
proaccelerin or labile factor
liver and platelets
Factor Vl (unassigned)
No longer used after it was discovered that this chemical is Factor Va
N/A
Factor Vll
proconvertin or stable factor
liver
Factor Vlll
anti-hemophilic factor
platelets and the lining of blood vessels
Factor lX
Christmas factor
liver
Factor X
Stuart Prower factor
liver
Factor Xl
plasma thromboplastin antecedent
liver
Factor Xll
Hageman factor
liver
Factor Xlll
fibrin stabilizing factor
liver

Coagulation Cascade Animation

Studying the Blood Clotting Process

At the high school level the discussion of blood clotting often begins with the prothombin activator and the previous steps before its formation are ignored or summarized very briefly. At the college or university level a more detailed knowledge of the process may be needed.

Students sometimes find that studying the coagulation cascade is a challenge, especially when reactions in the cascade must be memorized. Videos from a reliable source can be helpful because they show the blood clotting process visually and can be paused and replayed as necessary. It may be useful to make notes based on a video and then ask an instructor for clarification if necessary. Making frequent diagrams of the cascade can also help a student to memorize the reactions.

Sometimes different sources present slightly different versions of the coagulation cascade. This is due to our lack of precise knowledge of some of the steps or the fact that a version hasn't been updated with the latest discoveries. If you're studying blood clotting at an educational institution, the version of coagulation that your instructor gives you will be the "official" version.

A Summary of the Hemostasis Process

A summary of the blood clotting process
A summary of the blood clotting process | Source

Anti-Clotting Mechanisms in the Body

When a blood clot has served its function and the tissue underneath the clot has been repaired, the clot needs to be removed. In addition, it's important that any clots inside a blood vessel don't become large enough to block the vessel. The body has anti-clotting mechanisms to deal with these problems.

The endothelium is the layer of cells that lines the inside of a blood vessel wall. The smooth surface of the endothelium discourages clot formation when there is no injury. In addition, there is no exposed collagen inside a blood vessel. Collagen is a fibrous protein that provides strength to tissues. When blood contacts collagen the clotting process is stimulated.

Another factor that prevents unwanted clots from forming is the fact that the clotting proteins in the blood are present in an inactive form. They only become active when the body is wounded.

A chemical called Protein C acts as an anticoagulant by inactivating two of the activated coagulation factors (Factor Va and Factor Vllla). Protein S helps Protein C do its job. The two proteins are very useful for preventing blood clotting.

Stabilization of the fibrin network over a wound by Factor Xlll. Fibrin must be broken down once it's done its job.
Stabilization of the fibrin network over a wound by Factor Xlll. Fibrin must be broken down once it's done its job. | Source

Removing Blood Clots

Once a clot forms, the body has a mechanism to break it down. Fibrinolysis is the process in which fibrin is destroyed by an enzyme called plasmin, as well as by other enzymes. Plasmin cuts the fibrin threads up into smaller pieces, which can then be further broken up by other enzymes and removed from the body in the urine.

A healthy body protects us by clotting blood when we're injured, removing clots when they're no longer needed and preventing clots from growing too big. The normal blood clotting process is certainly complicated, but it's also amazing. Learning more about the process may help researchers discover ways to improve coagulation as well as prevent it from occurring inappropriately.

A Blood Clotting Quiz

© 2013 Linda Crampton

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    • billybuc profile image

      Bill Holland 3 years ago from Olympia, WA

      You do such a great job with these medical hubs. They are so detailed and yet you make them easy to understand. Great job and thank you for the information.

    • AliciaC profile image
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      Linda Crampton 3 years ago from British Columbia, Canada

      Thank you, Bill. I appreciate the comment very much!

    • Austinstar profile image

      Austinstar 3 years ago from Somewhere in the universe

      You could definitely write technical books for medical students and nurses! Great job!

      We need to save your hubs in case of the zombie apocalypse.

    • AliciaC profile image
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      Linda Crampton 3 years ago from British Columbia, Canada

      Thank you so much for the lovely comment, Austinstar. I hope that zombie apocalypse never arrives!

    • Vickiw 3 years ago

      Hi Alicia, a really interesting Hub, made very understandable and user friendly with your diagrams and pictures. Quality. Couldn't get your quiz though, maybe because I am on myiPad ?

    • AliciaC profile image
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      Linda Crampton 3 years ago from British Columbia, Canada

      Thank you very much for the comment, Vicki. I think the problem with the quiz appearing is probably because you were using an iPad to read the hub. I'll have to see if the quiz appears on my iPad!

    • aviannovice profile image

      Deb Hirt 3 years ago from Stillwater, OK

      Our body really does do a lot of amazing things. Thanks for the great explanation on clotting.

    • AliciaC profile image
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      Linda Crampton 3 years ago from British Columbia, Canada

      Thanks for the visit and the comment, Deb. Our bodies are definitely amazing!

    • drbj profile image

      drbj and sherry 3 years ago from south Florida

      Your explanation of blood clotting is so detailed and complete, Alicia, you might want to re-title your hub: "Everything in the World You Would Ever Want to Know and Then Some About Blood Clotting." :)

    • AliciaC profile image
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      Linda Crampton 3 years ago from British Columbia, Canada

      Thank you very much, drbj! I appreciate the comment!!

    • teaches12345 profile image

      Dianna Mendez 3 years ago

      Your posts are always a great read. Again, your detailed photos, charts and research makes an excellent article. Thanks for the interesting education of blood clots.

    • AliciaC profile image
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      Linda Crampton 3 years ago from British Columbia, Canada

      Thank you very much for the comment, Dianna. I appreciate your visit, as always!

    • ryanjhoe profile image

      ryanjhoe 3 years ago from Somewhere over the rainbow

      This article is very helpful for me to get the information about blood clot and I never really understand about it before. Thanks for sharing this!

    • AliciaC profile image
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      Linda Crampton 3 years ago from British Columbia, Canada

      Thank you very much for the visit and the comment, ryanjhoe!

    • Sheri Faye profile image

      Sheri Dusseault 3 years ago from Chemainus. BC, Canada

      Now that was interesting and well written! Thank you!

    • AliciaC profile image
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      Linda Crampton 3 years ago from British Columbia, Canada

      Thank you very much, Sheri Faye!

    • Porschia 2 years ago

      great information

    • AliciaC profile image
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      Linda Crampton 2 years ago from British Columbia, Canada

      Thank you very much for the comment, Porschia.

    • charity sekyi 2 years ago

      It has help me to know much for what am looking for

    • AliciaC profile image
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      Linda Crampton 2 years ago from British Columbia, Canada

      Thank you, charity sekyi. I'm glad the article was helpful for you.

    • mehwish 2 years ago

      Very easy way for understsnding blood clotting...

      Thanks

    • AliciaC profile image
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      Linda Crampton 2 years ago from British Columbia, Canada

      Thank you very much, mehwish.

    • Vellur profile image

      Nithya Venkat 5 months ago from Dubai

      Blood clotting is an amazing process and you have explained it so clearly. Interesting and informative article, voted up.

    • AliciaC profile image
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      Linda Crampton 5 months ago from British Columbia, Canada

      Thank you very much for the comment, Vellur. I appreciate your visit and vote.

    • hania 2 months ago

      your explanation is fantastic thanks alot

    • AliciaC profile image
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      Linda Crampton 2 months ago from British Columbia, Canada

      Thanks, hania. I appreciate your comment.

    • mariya 2 months ago

      most helpful article ever! thankyou so much!!

    • AliciaC profile image
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      Linda Crampton 2 months ago from British Columbia, Canada

      Thank you very much for the kind comment, mariya.

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