Haemostasis Part II: Secondary Haemostasis

Haemostasis - Part II: Secondary Haemostasis

• Secondary haemostasis is a crucial part of the overall haemostasis process, following primary haemostasis. It strengthens the initial platelet plug.
• Secondary haemostasis involves a cascade of coagulation factors.
• Coagulation factors are primarily proenzymes (zymogens), requiring activation to function.
• Several coagulation factors (Factors II, VII, IX, and X) are vitamin K-dependent and synthesised in the liver.
• The active form of a coagulation factor is denoted with an 'a' suffix (e.g., FVIIa).

Preparation for the Lecture

• Students need to have completed the previous lecture on Haemostasis Part I.
• Students should prepare a written out summary/diagram of the pathways. The method of presentation should help the student to best understand the pathways.
• Students need to understand the importance of the previous lecture to the current topic, which is Secondary Haemostasis.
• Caffeine is shown as a visual aid in the presentation.

Aims and Objectives

• The lecture aims to outline the processes in normal haemostasis.
• The learner will be able to explain the significance of effective haemostasis.
• Learners will describe the key components of haemostatic mechanisms.
• Learners will illustrate how secondary haemostasis works.

Primary and Secondary Haemostasis

• Primary haemostasis involves platelet activation, adhering to the vessel wall and forming a plug.
• Secondary haemostasis:
  • Involves activation of intrinsic and extrinsic coagulation pathways.
  • Activates the common pathway.
  • Forms a fibrin strand to strengthen the platelet plug.
• All phases of haemostasis are interconnected and often occur simultaneously.

Secondary Haemostasis Details

• A weak platelet plug needs strengthening by the coagulation factors of secondary haemostasis.
• Most coagulation factors are proenzymes (zymogens) synthesised in the liver.
• Vitamin K dependent factors (II, VII, IX, and X) are vital to the process.
• Proenzymes need activation to do their job.

Secondary Haemostasis - Four Stage Process

• Secondary haemostasis occurs in a four-stage process:
  • Tissue factor expression or release.
  • Activated platelets form phospholipid complexes, where coagulation factors bind and become activated.
  • Prothrombin converts to thrombin.
  • Fibrinogen converts to fibrin.

The Coagulation Cascade

• The coagulation cascade includes three pathways:
  • Extrinsic pathway.
  • Intrinsic pathway.
  • Common pathway.
• These pathways are essential in understanding how medicines that prevent blood clots function. Also vital to understand in-vitro diagnostic tests.

Extrinsic Pathway

• The extrinsic pathway is initiated by vascular injury, for example; physical injury, inflammation, and atherosclerotic plaques.
• Tissue factor (TF) is exposed following the initiation.
• TF binds to FVII, activating it to FVIIa.
• TF-FVIIa complex then activates factor X (FX), initiating the common pathway.

Intrinsic Pathway

• The intrinsic pathway is initiated by contact with negatively charged surfaces. This includes subendothelial matrix, collagen and negatively charged phospholipids on platelets.
• Contact with these surfaces activate factor XII (FXII).
• A series of activations occurs through factors XI, IX, VIII, and eventually activate factor X.

Common Pathway

• FXa, along with factor Va and calcium ions, activate prothrombin (Factor II).
• Prothrombin converts into thrombin (Factor IIa).
• Thrombin catalyzes the conversion of fibrinogen to fibrin.
• Fibrin and activated platelets create a thrombus (blood clot).
• The clot is stable and strengthened.

What Happens to the Clot Afterwards?

• When the injury heals, the clot is broken down by fibrinolysis.
• Tissue plasminogen activator (tPA) is released by endothelial cells.
• tPA converts plasminogen to plasmin.
• Plasmin breaks down fibrin into small fragments (d-dimers)

Why Don't We Get Clots All The Time?

• Haemostasis is tightly regulated.
• The intrinsic and extrinsic pathways lead to thrombin generation, activating platelets.
• Endogenous inhibitors prevent excessive clotting.
  • Nitric oxide and prostacyclin are produced by endothelial cells, inhibiting platelet activity.
• Endogenous inhibitors of secondary haemostasis include:
  • Tissue factor pathway inhibitor (TFPI).
  • Antithrombin III.
  • Protein C.

Summary

• Secondary haemostasis is divided into extrinsic, intrinsic, and common pathways.
• Clotting factors work in a cascade to generate a fibrin clot.
• Fibrin and activated platelets create a thrombus.
• The thrombus is broken down by plasmin, creating d-dimers.

This quiz covers the essential concepts of secondary haemostasis, building on the foundational knowledge from Haemostasis Part I. It focuses on the cascade of coagulation factors, their activation, and the role of vitamin K-dependent factors. Prepare your summary diagrams to better understand the pathways involved.