Hemostasis and clotting factors: what are they and how are they impacted by hemophilia?

When you get injured and start bleeding, your body uses a process called hemostasis to help the blood clot, a complex chain reaction where each step triggers the next. The term "hemostasis" comes from "hem-," which means "blood," and "-stasis," which means "to stop."

There are 3 main stages of blood clotting: blood vessel response, platelet plug formation, and the clotting cascade, which lead to the formation of the final clot.

When you experience a bleed, your blood vessels respond by narrowing (a process called vasoconstriction). Like squeezing a garden hose to reduce water flow, your blood vessels tighten to slow down blood loss.

This response can begin within seconds and is controlled by signals from damaged tissue and your nervous system.

The next step of hemostasis involves platelets. Platelets are tiny cells in your blood that act like first responders when you get a cut or injury. As soon as the platelets receive a signal that a blood vessel has been damaged, they become activated and rush to the site, change shape, and become sticky.

Platelets clump together to form a temporary soft plug that helps stop the bleeding while your body works on a more permanent repair. They also release chemical signals that attract more platelets to the area and strengthen the plug further. This process is also known as "primary hemostasis."

The final step is the clotting cascade, in which your body creates a strong, stable clot from the soft platelet plug.

Your blood contains proteins called clotting factors. These work together in a chain reaction, helping to form strong fibers that weave through the platelet plug to reinforce it.

This transforms the loose platelet plug into a solid, durable clot that can effectively seal the wound until healing is complete. The clotting cascade is also known as "secondary hemostasis."

The clotting cascade consists of 3 pathways—the extrinsic, intrinsic, and common pathways—that interact to form a stable blood clot.

The extrinsic pathway is your body's response system at the site of injury. When you get a cut, cells in the damaged tissue release a protein called tissue factor, which combines with blood proteins to begin forming a clot and stop the bleeding.

The intrinsic pathway is your body's slower but longer-lasting clotting system that works inside your blood vessels. When blood vessels are damaged, a series of proteins work together like a chain reaction to help form and maintain stable clots.

The common pathway begins where the pathways come together, which leads to formation of a mesh-like structure, the beginning of a clot, which is then strengthened to form the final solid clot that stops the bleeding.

Clotting factors are special proteins in your blood, numbered from 1 to 13, or commonly written in Roman numerals as I to XIII. As mentioned, clotting factors interact with each other along with other proteins to drive the clotting cascade.

If one factor is missing or doesn't work properly, it can affect the entire process, leading to clotting disorders such as hemophilia A or B (in which your blood struggles to clot properly) or thrombosis (in which your blood clots too easily, even when it shouldn’t).

Hemophilia is a lifelong inherited bleeding disorder where blood doesn’t clot normally. Hemophilia occurs because the body has low levels of specific clotting factors, which are essential proteins that help the blood form clots and stop bleeding.

People living with hemophilia may experience prolonged bleeding and, in some cases, spontaneous bleeding into joints and muscles, even without an injury. To help manage bleeding and support clotting, treatment options may include clotting factor concentrates (CFCs) or nonfactor treatments.