HEMOSTASIS

Due to damaged blood vessels Events that stop bleeding

Sequence of events

1. Vascular spasm1. Circular smooth muscle of walls

contracts

2. Reduces blood loss during repair

2. Platelet plug formationa. platelet adhesion: platelets stick to blood vesselb. platelet release reaction: platelets release contents – ADP and thromboxane A2 activate other platelets, serotonin and thromboxane A2 vasocontrictc. platelet aggregation: ADP makes other platelets sticky, forms platelet plug

3. Blood clotting def: a series of chemical

reactions that results in fibrin threads background: too easy –

thrombosis; too long - hemorrhage

a. Formation of prothrombinase1. Extrinsic pathway: (seconds) tissue factor

(protein) leaked into blood from surface of hurt cells. TF + calcium activates factor X. Factor X + factor V + calcium to form prothrombinase

2. Intrinsic pathway: (minutes) damaged endothelial cells (lining) exposes collagen fibers. Contact with collagen fibers activates clotting factor XII. Activates factor X. Factor X + factor V + calcium prothrombinase

b. Prothrombinase + Calcium convert prothrombin (protein) to thrombin (an enzyme)

c. Thrombin (+ calcium) converts fibrinogen (protein) to fibrin threads.