Chapter 22: Blood

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22.21:

Coagulation

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Anatomy and Physiology

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JoVE Core Anatomy and Physiology

Coagulation

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22.9: Factors Affecting Erythropoiesis

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22.22: Extrinsic and Intrinsic Pathways of Hemostasis

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Coagulation or clotting is the last hemostasis phase. It involves a series of chemical reactions to form a blood clot, reinforcing the initial platelet plug. The clotting process is a cascade of extrinsic, intrinsic, and common pathway reactions involving procoagulants or clotting factors. The extrinsic and intrinsic pathway reactions result in the formation of an activated complex called the prothrombin activator. The prothrombin activator facilitates the conversion of prothrombin into thrombin in the blood for the downstream reactions of the common pathway. Once activated, thrombin protein acts as an enzyme, converting fibrinogen in the blood into insoluble fibrin. These fibers polymerize to form a mesh-like structure that captures blood cells, platelets, and plasma. Also, the fibrin fibers stick to damaged surfaces of blood vessels, creating a blood clot. Typically, under 3 to 6 minutes, the clot can fill the vascular wound, permanently sealing the injury.

22.21:

Coagulation

The coagulation phase is a critical part of the body's process to prevent blood loss following injury to blood vessels. It involves chemical reactions that form a clot to seal the injured area. The clotting process begins shortly after injury, within 15-20 seconds for severe damage and 1-2 minutes for minor injuries.

During the coagulation phase, clotting factors, or procoagulants, play a vital role in initiating and progressing the coagulation cascade. This cascade is a series of reactions that ultimately lead to the formation of a blood clot. Key factors involved include fibrinogen, prothrombin, tissue factor, calcium, and thrombin, each playing a unique role in the clotting process.

Platelets also play a crucial role. These small cell fragments respond to vascular damage by becoming sticky and adhering to the damaged vessel wall. Once activated, they release thromboxane A2 and adenosine diphosphate (ADP), substances that activate nearby platelets, contributing to the formation of a platelet plug. This plug acts as a temporary seal while the coagulation cascade is initiated.

A significant step during the coagulation phase is the transformation of prothrombin, a protein in the blood, into thrombin. The prothrombin activator complex facilitates this transformation. Thrombin is essential in the clotting process, acting as an enzyme to convert fibrinogen into fibrin fibers.

Thrombin then converts fibrinogen into fibrin fibers, creating a mesh-like structure that captures blood cells and other components. This structure forms the blood clot as a permanent seal for the injured vessel, preventing further blood loss. Typically, the clot takes 3 to 6 minutes to completely seal the injury, assuming the vessel opening is not excessively large.