HEMOSTASIS Stages of Blood Clotting 1 Vascular Spasms

HEMOSTASIS (Stages of Blood Clotting)

1. Vascular Spasms • In response to damage of an artery or arteriole the rings of smooth muscle in the walls of the vessel begin to spasm immediately • This reduces blood loss and gives time for other mechanisms to begin • The spasms are caused by chemicals that are released by the smooth muscle, platelets, and pain receptors in the area

2. Platelet Plug Formation • Platelets come into contact with the damaged vessel and stick to the parts of it that are torn (called platelet adhesion) • The adhesion activates the platelets and they change • Each platelet extends projections that allow them to touch one anther and they begin to empty the contents of their vesicles (called platelet release reaction)

2. Cont. • ADP and thromboxane A 2 are released to activate nearby platelets • Serotonin and thromboxane A 2 cause vasoconstriction of the blood vessel which in turn reduces blood flow from the damaged vessel • The ADP makes the platelets nearby sticky and they migrate and join the first responder platelets (platelet aggregation) • This will lead to the formation of a solid mass called the platelet plug.

Platelet Plug Formation Process Diagrams Step-by-Step Copyright © 2007 by John Wiley & Sons, Inc.

Red blood cell Platelet Collagen fibers and damaged endothelium 1 1 Platelet adhesion

Red blood cell Platelet Collagen fibers and damaged endothelium 1 1 Platelet adhesion Liberated ADP, serotonin, and thromboxane A 2 2 2 Platelet release reaction

Red blood cell Platelet Collagen fibers and damaged endothelium 1 1 Platelet adhesion Liberated ADP, serotonin, and thromboxane A 2 2 2 Platelet release reaction Platelet plug 3 3 Platelet aggregation

3. Blood Clotting (Coagulation Events) • Takes advantage of the fact that blood only remains a liquid when contained in vessels • Dependent on injured tissues releasing tissue factor (TF) which interacts with PF 3 on platelets • The first reactions form the molecule prothrombin (all controlled by thromboplastin) • Thromboplastin converts prothrombin into an enzyme called thrombin via reaction with Ca 2+ ions • Thrombin converts soluble fibrinogen into insoluble fibrin which forms the threads of the clot

After Affects • Once the clot is formed it will slowly begin to retract and pull the damaged ends of the vessel together • During this phase some blood serum can escape but the actual blood cells cannot • Eventually fibroblasts will begin to form new tissues in the area and new endothelial cells will repair the damaged lining • In the end the vessel will return back to normal. • Calcium ions (Ca 2+) are the main ion involved in the clotting process

Blood Clotting Process Diagrams Step-by-Step Copyright © 2007 by John Wiley & Sons, Inc.

(a) Extrinsic pathway Tissue trauma (b) Intrinsic pathway Blood trauma Damaged endothelial cells expose collagen fibers Tissue factor (TF) Damaged platelets Activated XII Activated platelets Ca 2+ Platelet phospholipids Activated X V 1 V Ca 2+ PROTHROMBINASE

(a) Extrinsic pathway Tissue trauma (b) Intrinsic pathway Blood trauma Damaged endothelial cells expose collagen fibers Tissue factor (TF) Damaged platelets Activated XII Activated platelets Ca 2+ + Platelet phospholipids Activated X V 1 V Ca 2+ + PROTHROMBINASE (c) Common pathway Ca 2+ Prothrombin (II) THROMBIN 2

(a) Extrinsic pathway Tissue trauma (b) Intrinsic pathway Blood trauma Damaged endothelial cells expose collagen fibers Tissue factor (TF) Damaged platelets Activated XII Activated platelets Ca 2+ + Platelet phospholipids Activated X V 1 V Ca 2+ + PROTHROMBINASE (c) Common pathway Ca 2+ Prothrombin (II) THROMBIN Ca 2+ Fibrinogen (I) Loose fibrin threads 2 XIII Activated XIII STRENGTHENED FIBRIN THREADS 3