Platelets I

Question 1

1. What is the diameter of platelets

Answer 1

1-4 µm in diameter

Question 2

4. What is the blood concentration of platelets

Answer 2

Blood concentration = 150,000-400,000 per µl of blood

Question 3

5. How long do platelets circulate in the blood

Answer 3

Circulate for 7-10 days

Question 4

6. How are platelets removed from the blood

Answer 4

Removed by macrophages in the liver and spleen

Question 5

8. What is the ultrastructure of platelets enclosed by

Answer 5

Enclosed by plasma membrane with open canalicular system

Question 6

9. What is expressed on the surface of platelets

Answer 6

Surface expression of glycoprotein receptors

Question 7

10. What type of ring of cytoskeleton does a platelet have

Answer 7

Ring of microtubule cytoskeleton

Question 8

12. What organelles can be found in platelets

Answer 8

Mitochondria
Golgi apparatus
Endoplasmic reticulum

Question 9

13. What does the endoplasmic reticulum store in platelets

Answer 9

stores Ca2+ (calcium)

Question 10

15. What do platelet granules store

Answer 10

stores LOTS of mediators important for haemostasis

Question 11

16. What 8 mediators are found in platelet granules

Answer 11

Clotting factors (see later)
ADP/ATP
Calcium
Serotonin
Prostaglandins
Thromboxanes
Fibrin-stabilising factor
Platelet-derived growth factor (PDGF)

Question 12

18. Define coagulation

Answer 12

the process through which blood changes from a liquid to a clot (thicker)

Question 13

20. Define fibrinolysis

Answer 13

removal of a clot (thrombus) after it has formed

Question 14

21. Outline what happens during the vascular spasm (phase 1) phase of primary haemostasis

Answer 14

1. Endothelial cell contraction
2. Release of chemical mediators/hormones

Question 15

22. What does endothelial cell contraction allow to happen

Answer 15

allows exposure of underlying collagen matrix to bloodstream

Question 16

23. What happens after the release of chemical mediators/hormones

Answer 16

1. smooth muscle contraction
2. proliferation of endothelial, smooth muscle and fibroblast cells

Question 17

24. What does phase 1 result in and what does this do for the injured vessel

Answer 17

• Results in vasoconstriction.
• It reduces blood flow through the injured vessel

Question 18

25. Outline what happens during platelet phase (phase 2) of primary haemostasis

Answer 18

Platelet adhesion
Platelet activation
Platelet aggregation

Question 19

26. What happens during platelet adhesion

Answer 19

Platelets adhere to exposed collagen and themselves via platelet glycoprotein receptors

Question 20

27. What does von Willebrand factor do

Answer 20

Encourages adherence to endothelium

Question 21

28. What happens during platelet activation

Answer 21

1. Platelets become activated and:
   
   • develop finger-like projections
   • release granule contents
2. ADP, thromboxane A2 (TxA2) and thrombin activate more platelets
3. Serotonin and TxA2 maintain vasoconstriction

Question 22

29. What happens during platelet aggregation

Answer 22

1. ADP makes platelets become sticky
2. clotting factors are released
3. Conformational change in the platelet receptor IIb/IIIa allows binding of fibrinogen which forms bridges between platelets
4. The formed mass forms the platelet plug

Question 23

30. What does the coagulation of blood depend on

Answer 23

depends on the balance between procoagulants and anticoagulants

Question 24

31. When does coagulation occur

Answer 24

occurs when vessel injury initiates a chain reaction resulting in activation of clotting factors

Question 25

32. What are clotting factors

Answer 25

Clotting factors are serum proteases

Question 26

34. Explain how the common pathway works

Answer 26

1. Activated Factor X combines with prothrombinase, Factor Va and calcium to convert prothrombin to thrombin 2. Thrombin converts soluble fibrinogen into insoluble fibrin 3. Factor XIIIa cross links the fibrin fibres and fibrin combines with calcium too to form a stable fibrin clot

Question 27

36. Give features of the extrinsic pathway

Answer 27

- Cascade of reactions involving factors that originate OUTSIDE of the blood - Triggered with tissue damage - FAST (~15 sec!)

Question 28

37. Give features of the intrinsic pathway

Answer 28

- Cascade of reactions involving factors that originate WITHIN the blood - Triggered when blood contacts with a negatively charged surface (e.g. glass, activated platelets) - Slow (1-6 mins)

Question 29

38. Explain how the extrinsic pathway activates Factor X

Answer 29

1. Peripheral tissues have membrane glycoprotein called Tissue Factor (thromboplastin) 2. When the endothelium is damaged, Tissue Factor comes in contact with Factor VII, activating it. 3. Factor VIIa, Tissue Factor and calcium combine to activate Factor X

Question 30

39. Explain how the intrinsic pathway activates Factor X

Answer 30

1. Factor XII is activated when blood comes in contact with negative charges (e.g glass, activated platelets, collagen) 2.activated platelets also release Platelet Factor 3 3. Factor XIIa initiates a series of proteolytic steps that are calcium dependent which lead to the formation of Tenase complex 4. This Tenase complex along with Platelet Factor 3 activates Factor X

Question 31

40. What 4 clotting factors undergo post-translational modification when in the presence of the reduced form of Vitamin K

Answer 31

Factor: - II (prothrombin) - VII - IX - X

Question 32

41. What does vitamin K reductase do

Answer 32

catalyses the conversion of vitamin K to its reduced form

Question 33

42. How does warfarin lead to reduced clotting

Answer 33

inhibits vitamin K reductase which reduces the action of Factors II, VII, IX and X

Question 34

43. Explain how clot retraction works to contribute to haemostasis

Answer 34

1. Platelets attached to fibrin fibres contract 2. This compresses the fibrin meshwork into a smaller mass 3. As the clot retracts, it pulls the torn edges of the vessel closer together