Haemostasis

Question 1

What are the steps involved in platelet plug formation?

Answer 1

• Vessel constriction occurs as a first-response to vessel injury
• Formation of the unstable platelet plug, which involves the processes of platelet adhesion, and platelet aggregation
• Stabilisation of the plug with fibrin, which involves the blood coagulation system
• Dissolution of the clot and vessel repair, which involves fibrinolysis

Question 2

What happens if the platelet plug is not completely stabilised?

Answer 2

platelet masses embolises as the fibrin didn’t spread all the way through

Question 3

What do endothelial cells stop from coming into contact?

Answer 3

collagen and tissue factor

Question 4

Where is tissue factor found and what does it do?

Answer 4

expressed on the surface of a number of cells, and triggers coagulation

Question 5

What are the functions of endothelium?

Answer 5

• Maintain barrier between blood and procoagulant subendothelial structures
• It is anticoagulant so allows smooth blood flow
• Synthesis of PGI2, thrombomodulin, vWF, plasminogen activators

Question 6

What happens when the endothelial lining is damaged?

Answer 6

• The basement membrane is exposed
• Von Willebrand Factor binds to this collagen that is exposed
• Glycoprotein-1-b receptors on platelets can bind to vWF, to form a bridge between exposed collagen in the vessel and the platelet indirectly
• Alternatively, glycoprotein-1-a receptors on platelets bind directly to exposed collagen

Question 7

What happens when the platelet binds?

Answer 7

It becomes activated leading to the release of ADP and thromboxane. This further activates the platelets.
Glycoprotein IIb/IIIa activates and binds to circulating fibrinogen to form a platelet aggregate.

Question 8

What is the main outcome of the clotting cascade?

Answer 8

Fibrinogen is ultimately converted to fibrin, to fully stabilise the clot.

Question 9

What can thrombin do?

Answer 9

Thrombin can also activate platelets, and activate the IIb/IIIa complex

Question 10

What must happen in order for thromboxane to be generated within the platelet?

Answer 10

• Phospholipases are activated during the platelet activation
• Phospholipases act on membrane phospholipids to generate arachidonic acid
• Cyclo-oxygenase then converts arachidonic acid to endoperoxides, which then generate thromboxane A2 (via thromboxane synthetase)

Question 11

What is the importance of thromboxane?

Answer 11

Thromboxane is a potent activator of platelets.

Question 12

What do anti-platelet drugs like aspirin target?

Answer 12

Cyclo-oxygenase to irreversibly inactivate the enzyme

Question 13

Give examples of anti-thrombotic drugs and what they do and what they act on

Answer 13

1. COX 1 antagonists (aspirin)
2. ADP receptor antagonists – stop activation (clopidogrel, prasugrel)
3. Gp IIb/IIIa – stop aggregation (abciximab, tirofiban, eptifibatide)

Question 14

Name some tests that can be done to monitor platelets and their functions

Answer 14

1. Platelet count to monitor thrombocytopenia
2. Bleeding time
3. Platelet aggregation

Question 15

Go over coagulation cascade

Answer 15

during revision

Question 16

What is the importance of doing a platelet count?

Answer 16

• If the platelet count is progressively reduced, vascular accidents can progressively increase
• If it drops below 100 109/l, there is no spontaneous bleeding, but bleeding occurs with trauma
• As the platelet count is progressively reduced, we see more and more spontaneous bleeding

Question 17

What happens in autoimmune thrombocytopenia?

Answer 17

Extensive bruising of the limbs. This is caused by autoantibodies against the GpIIb/IIIa complex -> lowers platelet count

Question 18

What is a bleeding time test and why is it done?

Answer 18

• Standard incision is made
• Cuff with 40mm pressure on arm above cut
• Done when platelet count is normal but bleeding is still occurring
• Tests the platelet and vessel wall interaction
• The time for the bleeding to stop is monitored
• If the time is prolonged, it indicates that the platelets are acting abnormally with the vessel wall
• E.g. in renal disease

Question 19

How are platelets formed?

Answer 19

stem cell precursors -> megakaryocytes -> megakaryocytes mature and granulate -> megakaryocytes produce ~4000platelets from finger-like structures

Question 20

What is the lifespan of a platelet and where are the majority of them stored?

Answer 20

10 days

1/3 stored in the spleen

Question 21

What is the definition of hameostasis?

Answer 21

The cellular and biochemical processes that enables both the specific and regulated cessation of bleeding in response to vascular insult.

Question 22

What is haemostasis for?

Answer 22

To stop blood loss from intact and injured vessels and enable tissue repair

Question 23

What does haemostasis maintain a balance between?

Answer 23

bleeding and thrombosis - many regulators are involved in this

Question 24

What can tip the haemostatic balance towards bleeding?

Answer 24

• high number of fibrinolytic factors or anticoagulant proteins
• low number of coagulants or platelets

Question 25

What can tip the haemostatic balance towards thrombosis?

Answer 25

- low number of fibrinolytic factors or anticoagulants | - high number of coagulants and platelets

Question 26

Why is studying haemostasis important?

Answer 26

- bleeding disorders - thrombotic disorders - most people die with a haemostatic end point

Question 27

What is the size of a platelets and does it have a nucleus?

Answer 27

- 2-4 micrometeres | - no

Question 28

What is normal platelets count?

Answer 28

150 – 400 x 109/l

Question 29

FDFD

Answer 29

Platelets – highly coordinated, no nucleus, many receptors on the surface which can interact to help dictate platelet function Granules in the platelets with growth factors for example Phospholipid membrane – on a activated platelet it turns inside out, becomes negatively charged which allows it to attract the coagulation factors Actin and myosin – part of the cytoskeleton allowing shape change

Question 30

What is the importance of the platelet cytoskeleton?

Answer 30

- important for platelet morphology | - needed for changing shape and blocking the bleeding effectively

Question 31

What are the multiple roles of platelets?

Answer 31

- important in cancer metastasis - haemostasis and thrombosis - atherosclerosis - infection - inflammation

Question 32

FDF

Answer 32

Under normal physiological conditions, platelets circulate in close contact to the endothelial cell lining of the blood vessel wall & VWF circulate in a globular conformation with its plt binding site cryptic. VWF circulating in the plasma – molecular ball of string which hides platelet binding sites normally in its ball. (golubular) When the vessel is damaged for example, collagen, fibronectin and laminin for example gets exposed and can bind to collagen in the subendothelial layer, it changes and become linear polypeptide due to dhear forces of blood and platelet binding sites are exposed. Platelets can bind to it VWF can bind to collagen via its A3 domain Platelet binds via the platelet's GP1ba to VWF

Question 33

DFD

Answer 33

Interaction of platelets with collagen activates the platelets, causes shape change, granules to be releasd e.g. ADP,thrombanxe membrane to change. Released like a cloud so nearby platelets are primed too Platelets will bind to each other via fibrinogen on activated aIIbb3 integrin. Platelet aggregation

Question 34

What is the importance of integrins on the platelets?

Answer 34

When activated can bind to fibrinogen and also bind platelets and cross link them - integrin aIIbb3

Question 35

How does the shape of the platelet change once it activates and why is this important?

Answer 35

Change in the platelet shape – philipodia stick out making platelets sticky. They bind to the vessel wall and flatten out and spread to effectively prevent blood loss

Question 36

Where are coagulation factors made?

Answer 36

1. The liver – most plasma haemostatic proteins 2. Endothelial cells – VWF, TM, TFPI 3. Megakaryocytes – VWF, FV

Question 37

What are clotting factors?

Answer 37

All of them in the plasma are serine protein zymogens

Question 38

DED

Answer 38

MK looses its ability to divide, however continue to replicate its DNA becomes polyploid, cytoplasm enlarge. MK matures, becomes granular and form platelets that will be released in the circulation. model MK migrate from endosteal niche to the vascular niche where they do not pass the EC of sinusoidal blood vessels and remain in vascular niche. They form pseudopodia-like extensions (proplatelets) that extend in the lumen plt are released from tip of these long extensions by shear forces. Other model is following migration from endosteal niche to vascular niche matuer MK pass the ec barrier and enter the circulation. Due to large size of MK they get trapped in the microvasculature of lungs mechanical force induce fragmentation of MK