What is the Difference Between Primary and Secondary Hemostasis

The main difference between primary and secondary hemostasis is that primary hemostasis is defined by the formation of the primary platelet plug whereas the secondary hemostasis or coagulation is defined by the formation of insoluble, cross-linked fibrin. Furthermore, activated platelets are responsible for primary hemostasis while activated coagulation factors are responsible for secondary hemostasis.

Primary and secondary hemostasis are two stages of hemostasis, a complex physiological process responsible for stopping the bleeding that occurs as a result of injury to blood vessels.

Key Areas Covered

1. What is Primary Hemostasis
     – Definition, Components, Process, Importance
2. What is Secondary Hemostasis
     – Definition, Components, Process, Importance
3. What are the Similarities Between Primary and Secondary Hemostasis
     – Outline of Common Features
4. What is the Difference Between Primary and Secondary Hemostasis
     – Comparison of Key Differences

Key Terms

Activated Platelets, Fibrin, Platelet Plug, Primary Hemostasis, Secondary Hemostasis, Subendothelial Matrix

What is Primary Hemostasis

Primary hemostasis is the initial stage of hemostasis defined by the formation of a platelet plug at the site of injured blood vessels, preventing the leakage of blood. The three main components of primary hemostasis are activated platelets, von Willebrand factor (vWf), and the vessel wall. Generally, the intact endothelium serves as a physical barrier, separating the circulating platelets from thrombogenic substances including extracellular matrix proteins in the extravascular space. However, as a result of an injury, the procoagulant subendothelial matrix which contains proteins including collagen, laminin, and fibronectin is exposed, immediately initiating primary hemostasis.

Figure 1: Aggregation of the Activated Platelets

The three sequential events in primary hemostasis are platelet adhesion, platelet activation, and platelet plug formation. However, as hemostasis occurs in microvessels, these vessels immediately undergo vascular spasm prior to primary hemostasis. Here, the smooth muscles of blood vessels responsible for the constriction of blood vessels near the injury point reduce the blood loss. Then, the adhesion of platelets to the exposed subendothelial matrix occurs via vWf. Once adhered, platelets become activated, recruiting more platelets to the site of injury. Also, the incoming platelets become activated and bind to the initial platelets forming the platelet plug.

What is Secondary Hemostasis

Secondary hemostasis is the second stage of hemostasis that results in the formation of a stable, fibrin clot by aggregated platelets and fibrin. Secondary hemostasis involves a number of components including cells such as platelets, fibroblasts, endothelial cells, and lymphocytes, enzymatic coagulation factors including factor XI, X, IX, VII, and II, cofactors including tissue factor, Factors V, and VIII, fibrinogen, phosphatidylserine, and calcium.

Figure 2: Hemostasis

Furthermore, coagulation is another name for the secondary hemostasis. Moreover, the formation of fibrin from fibrinogen or secondary hemostasis initiates simultaneously through the exposure of the subendothelial matrix. It proceeds by the activation of clotting factors through sequential events called the coagulation cascade, including the intrinsic pathway, extrinsic pathway, and the common pathway. Both intrinsic pathway and extrinsic pathway give rise to the common pathway, which includes the activation of thrombin, which in turn activates fibrinogen into fibrin. This fibrin produces a mesh all around the platelet plug, holding it in the place. Also, secondary hemostasis completely stops the blood loss from the injury and with time, the injured endothelium can heal. Then, the plug will be dissolved by fibrinolysis.

Similarities Between Primary and Secondary Hemostasis

• Primary and secondary hemostasis are the two sequential stages of hemostasis.
• Both are responsible for sealing the ruptured blood vessels by injury.
• Besides, the activation of different components dissolved in the blood plasma is responsible for both.

Difference Between Primary and Secondary Hemostasis

Definition

Primary hemostasis refers to the interaction between the blood vessel, von Willebrand factor, and platelets in order to form the initial platelet plug while secondary hemostasis refers to the cascade of enzymatic reactions that ultimately results in the conversion of fibrinogen to fibrin monomers. Thus, this is the main difference between primary and secondary hemostasis.

Components

Components that participate in these processes are another difference between primary and secondary hemostasis. Endothelial cells, platelets, adhesive proteins including vWf and collagen, and facilitators including thrombin are the components of primary hemostasis while cells, enzymatic and non-enzymatic coagulation factors, phosphatidylserine, and calcium are the components of the secondary hemostasis.

Sequence of Events

Platelet adhesion, platelet activation, and platelet plug formation are the sequential events of primary hemostasis while initiation of thrombin generation, amplification of thrombin generation, propagation of thrombin generation, and fibrin formation are the sequential events of secondary hemostasis.

Final Result

Primary hemostasis is responsible for the formation of a platelet plug, adhering the endothelial wall while secondary hemostasis is responsible for the formation of a stable clot. Hence, this is another major difference between primary and secondary hemostasis.

Importance

Also, one other difference between primary and secondary hemostasis is that primary hemostasis serves as an immediate response to the vascular injury, limiting bleeding while secondary hemostasis produces a reinforcement to the platelet plug from fibrin, stabilizing it.

Conclusion

Primary hemostasis is the initial stage of hemostasis characterized by the formation of the platelet plug. Activated platelets are mainly responsible for it. The main function of primary hemostasis is to seal an injured blood vessel immediately. In comparison, secondary hemostasis is the subsequent stage of hemostasis characterized by the formation of a stable, fibrin clot, reinforcing the platelet plug by fibrin. Therefore, the main difference between primary and secondary hemostasis is their mechanism, components, final result and importance of the process.

References:

1. Gale, Andrew J. “Continuing education course #2: current understanding of hemostasis” Toxicologic pathology vol. 39,1 (2010): 273-80. Available Here

Image Courtesy:

1. “Thrombocyte aggregation” By Dietzel65, Steffen Dietzel – Own work (CC BY-SA 3.0) via Commons Wikimedia 
2. “1909 Blood Clotting” By OpenStax College – Anatomy & Physiology, Connexions Web site. Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia