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1. What is the first stage in hemostasis? Vascular Spasm

2. How does this stage contribute to hemostasis? Restricts blood flow to injured vessel

3. What is the initial cause of this stage? Stimulation of nociceptors

4. What causes it to be maintained for a longer time period? Damage to smooth muscle in tunica media & serotonin & thromboxane released by platelets

5. What is the second stage in the process of hemostasis? Formation of Platelet Plug

6. What is the initial cause of this stage? Exposure of platelets to subendothelial collagen

7. How is the stage accelerated by a positive feedback mechanism? (answer should include degranulation) As platelets attach to injured site, they degranulate, releasing ADP & thromboxane which increases adhesion of platelets to each other which causes more degranulation, and so on.

8. How does this stage contribute to hemostasis? Plugs up wound

9. What is the third stage of hemostasis? Coagulation

10. What is meant by reaction cascade and enzyme amplification that is key to this stage? Reaction cascade: The product of one reaction either becomes the reactant or enzyme in the next reaction and a whole series of reactions occurs. Enzyme amplification: if an enzyme is produced, it can lead to the production of much more product. With each step more and more product is produced, so a little reactant at the first step can result in large amounts of product in the last step.

11. What are the differences between the two pathways that can initiate this stage?

Intrinsic – starts when platelets release Factor XII (Hageman Factor), has more steps and takes longer (3-6 min for fibrin to occur)

Extrinsic – starts when damaged perivascular tissue releases Factor III (Thromboplastin), has fewer steps and fibrin is produced in <1min

12. What happens when prothrombin activator is produced?

Thrombin acts as an enzyme to convert soluble fibrinogen into insoluble fibrin. In the presence of Ca⁺⁺ and Factor XIII, fibrin polymerizes into a fibrin network around the platelet plug

13. How does this stage contribute to hemostasis? The clot is tighter and less likely to come apart. Fibrin can also bring the edges of the wound closer together.

14. What is the last stage of hemostasis? Clot retraction

15. What happens during this stage? The pseudopods of the platelets contract making the clot more compact and extruding serum.

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Last revised: Wednesday, 11 April 2007