Blood clots and coagulation

Clotting factors are proteins found in blood that work together to make a blood clot. They are designated by Roman numerals I through XIII.1

Blood vessels shrink so that less blood will leak out.1

Tiny cells in the blood called platelets stick together around the wound to patch the leak. Blood proteins and platelets come together and form what is known as a fibrin clot. The clot acts like a mesh to stop the bleeding.1

The clotting process1

Tile circled number one and a diagram of an injured or damaged blood vessel.

Injury or damage

Tile circled number two and a red circled tile colored exclamation point.

Factors are activated

Tile circled number three and a diagram of a blood vessel contracting.

Vessel contracts

Tile circled number four and a diagram of a blood vessel with platelet plug.

Platelet plug

Tile circled number five and a diagram of a blood vessel with fibrin clot.

Fibrin clot

Bleeding causes a biological "domino effect" in which a series of steps are set in motion. When your body detects a bleed, the clotting factors are switched on in a particular order, one after the other.1

Each factor activates the next until they form a clot. This is known as the coagulation cascade.1 Intrinsic and extrinsic pathways are 2 separate pathways that lead to the formation of a clot. The intrinsic pathway is activated early in the coagulation cascade, known as the initiation phase. The extrinsic pathway activates during the amplification phase of the coagulation cascade—increasing the number of platelets at the site of a bleed.2

Normal clotting1

Diagram of a blood vessel with three blood drops showing the activation of a factor that begins to clot.

Factors are activated and begin the clotting process

Diagram of a blood vessel with the factors activated and a clot forming to stop the bleeding.

All factors are activated and a clot is formed to stop bleeding

Clotting in hemophilia4

Diagram of a blood vessel with three red drops showing  all activated factor with 1 weak factor in the chain.

Factors are also activated, but 1 factor in the chain is weak or missing

Diagram of a blood vessel with three red drops showing not all activated factors in the chain and the clot being unable to form properly.

Not all factors in the chain are activated and the clot cannot form properly

You can imagine the process like a row of dominoes. Each factor signals the next, all working together like a row of dominoes falling into each other.1 If a single domino is missing in a row, dominoes will stop falling. In the same way, if a single factor is missing from the clotting process (like factor VIII or factor IX), the coagulation cascade gets interrupted and a proper clot will not form when you have a bleed.4

When certain blood-clotting factors are deficient or missing in a person with a bleeding disorder, the blood does not clot as it should, and it takes longer for a clot to form and for bleeding to stop.3 many factors

Bleeding disorders go beyond hemophilia A and B and include other factor deficiencies.

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  1. The clotting process. World Federation of Hemophilia website. Updated January 2014. August 21, 2019.
  2. Mackman N, Tilley RE, Key NS. Role of the extrinsic pathway of blood coagulation in hemostasis and thrombosis. Arteriosclerosis, Thrombosis, and Vascular Biology. 2007;27:1687-1693. Published June 7, 2007. Accessed August 26, 2019.
  3. What are rare clotting factor deficiencies? World Federation of Hemophilia website. Updated May 2014. Accessed August 21, 2019.
  4. Hemophilia. NIH Genetics Home Reference website. Accessed September 3, 2019.