Coagulation (secondary hemostasis)

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Coagulation (secondary hemostasis)

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Preguntas

Preguntas del estilo USMLE Step 1

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A lab study is simulating secondary hemostasis. In the study, both the intrinsic and extrinsic pathways of coagulation lead to the formation of the active form of factor X (Xa). Which of the following is the correct order of events to occur afterward?  

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External References

First Aid

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2023

2022

2021

Anticoagulation

coagulation cascade and p. 416

Antithrombin

coagulation cascade and p. 416

Blood

coagulation and kinin pathways p. 418

Clotting factors p. 69

Coagulation p. 69

Coagulation cascade components p. 416

Coagulation disorders p. 431

mixed platelet/coagulation p. 433

Coagulation pathways p. 418

Combined pathway for coagulation p. 418

Contact activation pathway for coagulation p. 418

Extrinsic pathway p. 204

coagulation p. 418

warfarin and p. 441

Granzyme B

extrinsic pathway and p. 204

Hemostasis p. 413

coagulation p. 417

Heparin p. 440

in coagulation cascade p. 418

Intrinsic pathway p. 204

for coagulation p. 418

coagulation defects of p. 431

Neonates

coagulation cascade in p. 416

Perforin

extrinsic pathway and p. 204

TNF- α p. 106

extrinsic pathway and p. 204

Vitamin K

coagulation cascade p. 418

Warfarin p. 441

coagulation cascade p. 418

Enlaces externos

Transcript

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Hemostasis can be broken down into hemo referring to blood or bleeding and stasis meaning to stop - so together it means stopping bleeding. Hemostasis has two phases: primary and secondary hemostasis.

In primary hemostasis, platelets aggregate to form a plug at the site of an injured blood vessel. While these platelets are aggregating, coagulation, or secondary hemostasis starts.

This is where numerous enzymes that are always floating around in the blood called clotting factors get proteolytically activated, meaning that activation happens when a small piece is chopped off - a bit like pulling the pin out of a grenade.

These factors activate one another, eventually leading to the activation of fibrin or factor Ia. That results in a fibrin mesh which forms around the platelet plug to reinforce it and hold it together.

Without primary and secondary hemostasis, our body would suffer massive blood loss from even the most minor injuries--imagine losing all of your blood volume from something as simple as a pinprick!

So let’s get into the details of secondary hemostasis. The process of forming the fibrin mesh begins via two pathways --the extrinsic and intrinsic pathways.

The intrinsic pathway is called intrinsic because all of the factors required to activate it are intrinsic, or found within the blood.

Conversely, the extrinsic pathway is called extrinsic because it’s activated by tissue factor found extrinsically, or outside of the blood.

Both pathways can become activated independently and ultimately culminate in the activation of factor X, which then proceeds to activate the rest of the coagulation cascade via the common pathway.

Let's start with the extrinsic pathway. It starts when trauma damages the blood vessel, and exposes the cells under the endothelial layer, like smooth muscle cells, which have tissue factor or factor III in their membrane.

Now, it turns out that there’s an enzyme called factor VII floating around in the blood, and some of it is active, meaning that it’s already set to proteolytically cleave other proteins.

When it’s in that state, it’s called active factor VII or VIIa. “a '' for active. Factor VIIa binds to a tissue factor and a calcium ion -which get released by nearby activated platelets--and it forms a VIIa-TF complex on the surface of the smooth muscle cell.

Resumen

Coagulation is the process of formation of a blood clot. It is secondary hemostasis that occurs after vascular damage has occurred and platelets have been activated. Its purpose is to form the fibrin mesh necessary to stabilize the platelet plug to stop bleeding. Coagulation happens in a series of steps called the coagulation cascade, which is a series of enzymatic reactions that leads to the conversion of fibrinogen to fibrin. The final product is a stable plug that stops the bleeding. Factors XII, XI, IX, VIII, VII, V, and IV are essential for the initiation of coagulation. Proteins C and S inhibit the coagulation cascade by inactivating factors Va and VIIIa, respectively.

Fuentes

  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "Contact system revisited: an interface between inflammation, coagulation, and innate immunity" J Thromb Haemost (2016)
  6. "Thrombus formation in vivo" Journal of Clinical Investigation (2005)
  7. "Remodeling the Blood Coagulation Cascade" Journal of Thrombosis and Thrombolysis (2003)
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