Platelets And Haemostasis

Question 1

Define primary haemostasis, secondary haemostasis and fibrinolysis

Answer 1

1. Primary haemostasis is the reduction of blood flow formed when a plug is formed in a damaged blood vessel by the interaction of platelets + blood vessel
2. Secondary haemostasis is the conversion of soluble fibrinogen to insoluble fibrin which strengthens the haemostatic plug
3. Fibrinolysis is the degradation of the plug when wound is repaired

Question 2

Give the term and explain platelet production and basic platelet structure

Answer 2

1.thrombopoiesis
2. Platelets formed via the shedding of bone marrow megakaryocytes
3. Platelets contain mitochondria and smooth ER

Question 3

Describe the platelet plasma membrane

Answer 3

1. Membrane has a glycocalyx surface coat
2. Glycocalyx coat contains glycoproteins for adhesion and aggregation, also receptors for coagulation factors
3. The plasma membrane contains ADP receptors and platelet factor VIII

Question 4

Describe the contents of platelet granules

Answer 4

1. Contain calcium and magnesium ions
2. ADP and ATP required for platelet aggregation
3. Serotonin and Thromboxane leads to vessel vasoconstriction
4. PDGF (platelet derived growth factor) is a mitogen for fibroblasts (stimulates mitosis) and a chemokine for neutrophils (stimulates leukocyte movement)

Question 5

Explain the platelet phase of hemostasis
Explain the vascular phase of hemostasis

Answer 5

1. Exposed collagen binds and activates platelets
2. Platelets and damaged area release platelet factors
3. Attracting more platelets
4. Platelets adhere to the exposed basement membrane
5. Adhesion requires vWF and factor VIII
6. Receptors on platelets for ADP, collagen and thrombin increase once localised on BM
7. Activation of these receptors leads to granule release
8. ADP and thromboxane A2 attract platelets to aggregate and form the plug
9. Injury to blood vessel leads to vasoconstriction
10. The injury breaks endothelial lining exposing collagen = platelet factors
11. Contraction of vessel exposes more collagen = more platelet factors
12. Platelet activation leads to more vasoconstriction via thromboxane and serotonin

Question 6

Role of the endothelium

Answer 6

1. Produces vWF (von Willebrand Factor)
2. Synthesises basement membrane to repair damage
3. Synthesises prostacyclin which inhibits platelet aggregation and causes vasodilation - reduced when endothelium is damaged
4. Binds and extravasates immune cells
5. Synthesises tissue factor important in secondary haemostasis