Haemostasis

1. Haemostasis

2. Hemostasis
• Definition?
• Prevention of blood loss (in vessel wall injury)
• and maintenance of blood in a fluid state within the vascular system (no injury).

3. Mechanisms that act to prevent blood loss:
❑ Primary hemostatic:
1.Vascular spasm.
2. Formation of platelet plug (role of platelet).
❑ Secondary hemostatic:
3.Formation of blood clot (role of coagulation Fs).

4. 1-Vascular spasm
❑ The first and immediate response. Aim?
❑ The blood vessel diameter decreases 🡪 reduction in blood loss
❑ Is it enough? Why?

5. Mechanisms of vasospasm
•1. Myogenic: rebound contraction of the vascular smooth muscles as a
result of traumatic stretch.
•2. Neural: pain stimulates sympathetic nervous system
• 3. Humoral: vasoconstrictors released by platelets (5HT , Thromboxane A2)
– most important mechanism, platelets def ?

6. Mechanism of platelet plug formation
• Collagen under broken endothelial cells attract platelets 🡪adhesion?
• Adhesion activates platelets:
• Swell & become irregular in shape ?
• Release their granules, how?
Serotonin causes vasoconstriction
ADP recruits more platelets🡪 aggregation (also Thromb.A2) – more sticky
a mass of platelets temporarily plugs the damage, is it sufficient?

7. Mechanism of platelet plug formation

8. Formation of the definitive clot
•1. Formation of prothrombin activator.
•2. The prothrombin activator catalyzes conversion of prothrombin into thrombin.
•3. Thrombin acts as enzyme 🡪 conversion of fibrinogen (a soluble plasma protein)
into the insoluble protein fibrin.
What the role of fibrin?

9. Coagulation pathways (In vivo)
❑ We have two pathways that all will lead to activate factor X but in different
step and activated by different factors
i. Intrinsic pathway: activated by exposure to Collagen
ii. Extrinsic pathway: activated by exposure to Tissue thromboplastin

11. Fibrinolytic System
❑ Mechanism:
▪ Fibrinolysis begins with the activation of the proenzyme plasminogen into
plasmin by tissue plasminogen activator (t-PA).
▪ Plasmin digests the fibrin strands and erodes the foundation of the clot 🡪
fibrin degradation products (FDP). Significance?
▪ Following lysis of the blood clot: plasmin, t-PA and FDP removal?
▪ inhibition?.