Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings 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)

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings Step One: Vascular spasms  Vasoconstriction causes blood vessel to spasm  Spasms narrow the blood vessel, decreasing blood loss

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings Step 2: Platelet plug formation  Collagen fibers are exposed (because of the break in a blood vessel

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings 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

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings Step 3: Coagulation  Injured tissues release tissue factor (TF)  protein clotting factors and calcium ions trigger a clotting cascade

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings  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)

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings Summary of steps

Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings Hemostasis  Blood usually clots within 3 to 6 minutes  The clot remains as endothelium regenerates  The clot is broken down after tissue repair