Haemostasis - Part I: Primary Haemostasis

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Questions and Answers

What is haemostasis primarily concerned with?

Enhancing platelet production

Stopping blood loss (correct)

Increasing blood flow

Promoting blood vessel dilation

Which of the following is NOT a major component of the haemostatic system?

Blood vessels

Platelets

Coagulation inhibitors

Neutrophils (correct)

What is the primary function of platelets during primary haemostasis?

Inhibiting blood flow

Producing coagulation factors

Adhering to the vessel wall (correct)

Forming fibrin strands

Which phase of haemostasis involves the activation of coagulation pathways?

Secondary haemostasis Signup and view all the answers

How does primary haemostasis strengthen the platelet plug?

Via adhesion of platelets to collagen Signup and view all the answers

What is the role of intact endothelial cells in the haemostatic process?

Acting as anti-thrombotic agents Signup and view all the answers

What happens when there is a failure in the haemostatic system?

Thrombosis or bleeding Signup and view all the answers

Which best describes the relationship between primary and secondary haemostasis?

Both phases occur simultaneously and integrate with each other Signup and view all the answers

What role does von Willebrand Factor (vWF) play during the injury to blood vessels?

It mediates platelet adhesion to the subendothelium. Signup and view all the answers

Where is von Willebrand Factor stored in endothelial cells before it is secreted?

In Weibel-Palade bodies Signup and view all the answers

How is thromboxane A2 produced in activated platelets?

By synthesizing from arachidonic acid. Signup and view all the answers

What happens to platelets after their normal lifespan of 6-9 days in circulation?

They are removed by the spleen. Signup and view all the answers

What occurs immediately after injury to a blood vessel?

Vasospasm occurs to slow blood flow. Signup and view all the answers

What type of granules do platelets release to promote hemostasis?

Storage granules Signup and view all the answers

What is the normal range for platelet count in healthy adults?

150 to 400 x 10^9/L Signup and view all the answers

What is the fate of large multimers of von Willebrand Factor upon digestion by ADAMTS-13?

They are broken down into smaller units. Signup and view all the answers

Study Notes

Haemostasis - Part I: Primary Haemostasis

Haemostasis is the process of stopping bleeding. It's a tightly regulated system.
The five major components of the haemostatic system are: blood vessels, platelets, coagulation factors, coagulation inhibitors and fibrinolysis inhibitors.
The aim of this lecture is to outline the processes involved in normal haemostasis.
Learning objectives: explain the importance of effective haemostasis, describe major components of haemostatic mechanisms, describe primary haemostasis mechanisms.

What is Haemostasis?

Haemostasis is the process of stopping the flow of blood.
It's a biological process that controls bleeding from damaged vessels. It involves a complex interplay between several components.
It's critically important in controlling bleeding while maintaining blood flow and preventing excessive clotting (thrombosis).

Primary and Secondary Haemostasis

Primary haemostasis: The initial response to vessel injury.
- Involves platelets adhering to the vessel wall.
- Adherence of platelets to the vessel wall, forming a platelet plug.
- Platelets respond to the vessel wall injury.
- Platelets adhere to the vessel wall.
- A primary platelet plug is formed.
Secondary haemostasis: The reinforcing steps that stabilise the platelet plug.
- Activation of the coagulation pathways (intrinsic and extrinsic).
- Activation of the common pathway.
- Formation of a fibrin strand to strengthen the platelet plug.
Both phases are integrated and often occur concurrently.

Blood Vessels in Haemostasis

The walls of arteries and veins have different layers (intima, media, adventitia).
Intact endothelium (inner lining) releases anti-thrombotic substances like prostacyclin and nitric oxide.
Damage to the endothelium exposes underlying prothrombotic substances (like collagen, elastin) which triggers the haemostatic response.
Vasospasm (vessel constriction) occurs during injury to slow blood flow.

Von Willebrand Factor (vWF)

vWF is a large multimeric glycoprotein.
It is stored in Weibel-Palade bodies in endothelial cells.
It's secreted into the plasma when endothelial cells are activated (e.g., by thrombin, inflammation, or vascular damage).
vWF mediates platelet adhesion to the subendothelium.
Large vWF multimers can be broken down into smaller units by ADAMTS-13.
vWF can be stored in platelet α-granules.

Platelets

Platelets are formed from megakaryocytes in bone marrow and lungs.
Normal platelet count is 150-400 x 10⁹/L.
10 billion platelets are produced daily.
Platelets circulate for 6-9 days before being removed by the spleen.
Platelets have a characteristic purple appearance on blood films (2-3 µm diameter).
They require activation to play a role in haemostasis.
Platelets contain storage granules (α-granules, dense granules, lysosomes).
Activation releases substances that contribute to haemostasis and other functions. For example, granules contain factors like Von Willebrand Factor (vWF), coagulation factors, and growth factors. Thromboxane A2 (TXA2) is a potent vasoconstrictor released from platelets to further constrict the damaged vessel.

Primary Haemostasis: Part I

Injury to a blood vessel triggers vasoconstriction (narrowing of the vessel).
vWF in the plasma is activated and binds to exposed collagen (a component of blood vessel walls).
Platelets at the edge of the circulation are captured and attached to vWF by their GPIb receptors.

Primary Haemostasis: Part II

Platelets roll and form additional vWF-GPIb interactions.
Stable platelet adhesion occurs.
GPVI activation occurs by collagen.
Activated integrin α₂β₃ binds to fibrinogen. This initiates platelet-platelet interactions.
Platelets undergo an irreversible change (shape change).

Primary Haemostasis

Primary haemostasis forms a temporary platelet plug.
The plug is unstable and requires strengthening (secondary haemostasis).

Summary

Primary haemostasis involves vasoconstriction of the injured blood vessel followed by platelet activation.
Platelets adhere to the site of injury, bind to exposed matrix via vWF and GPVI, and release their granule contents.
Platelets form an unstable plug that needs strengthening through subsequent coagulation mechanisms.

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Description

This quiz covers the essential concepts of haemostasis, focusing on primary haemostasis. It explores the components involved in stopping bleeding and the mechanisms that ensure effective blood flow while preventing excessive clotting. Test your understanding of this critical biological process and its importance in vascular injury.