Blood Clotting Pathways

Question 1

What is haemostasis?

Answer 1

The arrest/halt of blood flow

Question 2

How can this occur clinically?

Answer 2

After everyday trauma

e.g. injections, surgical incisions, tooth extractions etc.

Question 3

What are the 3 major steps of haemostasis?

Answer 3

1. Vascular spasm
2. Formation of a platelet plug
3. Blood coagulation (clotting)

Question 4

Why must haemostasis be appropriately controlled?

Answer 4

To avoid inappropriate clot formation. Clots must be eventually degraded as they are only a temporary solution.

Question 5

What happens during a vascular spasm?

Answer 5

VASOCONSTRICTION (contraction) of blood vessels occurs after injury:

• Muscle cells contract
• Constriction reduces blood flow, limits blood loss
• Opposing endothelial surfaces are pressed together and adhere on contact.

Question 6

What is the process of a vascular spasm mediated by?

Answer 6

This process is mediated by the platelet-derived products serotonin and thromboxane A2 (TXA2).

Question 7

What does normal endothelium produce?

Answer 7

Normal endothelium produces vasodilators and inhibitors of platelet aggregation to keep vessels open.

Question 8

What are platelets also known as?

Answer 8

Thrombocytes

Question 9

What are platelets?

Answer 9

Platelets are small fragments derived from megakaryocytes (bone marrow cells)

Question 10

What do platelets contain?

Answer 10

They contain:

• megakaryocyte cytoplasm
• intracellular granules
• other organelles for generating energy
• high concentrations of actin & myosin

Question 11

What do the high concentrations of actin & myosin allow the platelets to do?

What else can platelets do?

Answer 11

These high concentrations allow platelets to contract

Platelets can also release secretory products

Question 12

Under normal conditions, what do platelets not stick to?

Answer 12

Blood vessel endothelium.

Question 13

What 4 chemical factors can activate platelets?

Answer 13

1. ADP
2. Collagen
3. Thrombin
4. PAF

Question 14

What is PAF?

Answer 14

A phospholipid (not a protein)

Question 15

What happens to the shape of platelets when they become activated?

What does this change in shape facilitate?

Answer 15

Disc shape to sphere shape with extended pseudopodia.

This change in shape facilitates aggregation and coagulant activity.

Question 16

As well as changing shape, what 4 other effects does activation have on platelets?

Answer 16

1. They release compounds involved in homeostasis (e.g. ADP, serotonin, fibronectin and von Willebrand Factor (vWF))
2. They aggregate
3. They adhere/stick to the vessel wall
4. They synthesise thromboxane A2 (TXA2)

Question 17

What is thromboxane A2 (TXA2)?

Answer 17

A mediator of platelet activation and vasoconstriction.

Question 18

What is platelet activation followed by?

Answer 18

Their ADHESION to the vessel wall at the site of injury.

Question 19

What do the platelets do at the site of injury?

What does the product form?

Answer 19

The platelets AGGREGATE and build up an occlusive PLATELET MASS.

This mass forms the PRIMARY HAEMOSTATIC PLUG.

Question 20

When vasoconstriction eases, what may happen to the plug?

Answer 20

Plug may be washed away by local blood flow.

Question 21

What must the plug be stabilised by?

Answer 21

The plug must be stabilised by FIBRIN

Question 22

What are the two main steps of the formation of the Platelet Plug?

Answer 22

1. Platelets aggregate to form a primary plug

2. Platelets release chemicals which enhance blood coagulation.

Question 23

When looking at platelet disorders and platelet-related bleeding disorders, what is the most common defect?

Answer 23

The most common defect is von Willebrand disease.

Question 24

What is von Willebrand disease?

Answer 24

A group of autosomal diseases affecting vWF

Question 25

What can some less common disorders affect?

Answer 25

Less common disorders can affect the receptors found in the platelet membrane, that are involved in platelet adhesion and aggregation.

Question 26

Normally bleeding stops because platelets stick together and form a clot. With vWD what happens to the platelets?

Answer 26

With vWD, platelets don’t stick together as they should.

Question 27

What are antiplatelet drugs used to treat?

Answer 27

Arterial thrombosis

blood clot within a vessel

Question 28

How do antiplatelet drugs work?

Answer 28

They decrease platelet aggregation and inhibit thrombus formation.

Question 29

Name 3 examples of anti platelet drugs.

Answer 29

1. Aspirin
2. Platelet receptor antagonists
3. Platelet ADP receptor antagonists

Question 30

What does aspirin do?

Answer 30

Reduces thromboxane A2.

Question 31

What do platelet receptor antagonists do?

Answer 31

Targets GpIIb which binds vWF.

Question 32

What is the role of collagen in small blood vessels?

Answer 32

Collagen has an important role in the structure and function of small blood vessels.

Question 33

Identify vascular causes of excessive bleeding.

Answer 33

Congenital or acquired deficiencies in collagen synthesis

Question 34

Give an example of a congenital disorder.

Answer 34

Ehlers-Danlos Syndrome

Question 35

Give an example of an acquired disorder.

Answer 35

Vitamin C Deficiency
OR
excessive exogenous or endogenous Glucacorticoids (steroid hormones)

Question 36

What do blood coagulation factors interact to form?

Answer 36

• The secondary, fibrin-rich, HAEMOSTATIC PLUG in small vessels
• and the secondary fibrin THROMBUS in arteries and veins