Hemostasis (Hemo-blood Stasis-standing still) Stoppage of bleeding resulting from a break in a blood vessel Hemostasis involves three phases 1. Vascular spasms 2. Platelet plug formation 3. Coagulation (blood clotting)

Step One: Vascular spasms Vasoconstriction causes blood vessel to spasm Spasms narrow the blood vessel, decreasing blood loss

Step 2: Platelet plug formation Collagen fibers are exposed (because of the break in a blood vessel

Step 2: Platelet plug formation Chemical signals cause the platelets to become activated (sticky) and cling to collagen fibers Anchored platelets release chemicals to attract more platelets Platelets pile up to form a platelet plug

Step 3: Coagulation Injured tissues release tissue factor (TF) protein clotting factors and calcium ions trigger a clotting cascade

prothrombin is activated prothrombin to thrombin (an enzyme) thrombin builds fibrinogen proteins into hair-like molecules of fibrin (insoluable) fibrin forms a meshwork (the basis for a clot)

Hemostasis Blood usually clots within 3 to 6 minutes The clot remains as endothelium regenerates The clot is broken down after tissue repair