1. Hemostasis Events

• What does the term hemostasis mean?

  Prevention of blood loss
• What are the mechanisms of hemostasis?

  1. Vascular constriction
  2. Formation of a platelet plug
  3. Blood coagulation
  4. Growth of fibrous tissue into the clot
• What happens immediately after a blood vessel is cut?

  Smooth muscle contracts, reducing blood flow
• What causes the contraction of smooth muscle in a ruptured vessel?

  Local myogenic spasm, autacoid factors, nervous reflexes
• What initiates nervous reflexes during vascular trauma?

  Pain nerve impulses from the traumatized area
• What substance do platelets release to cause vasoconstriction?

  Thromboxane A2
• How long can vascular spasm last after vessel trauma?

  Minutes to hours
• What are platelets also known as?

  Thrombocytes
• What is the normal concentration of platelets in blood?

  150,000 to 300,000 per microliter
• What do platelets lack that prevents them from reproducing?

  Nuclei
• What do actin and myosin in platelets do?

  Contract and facilitate platelet function
• What is the role of mitochondria in platelets?

  Form ATP and ADP
• What do platelets secrete that activates nearby platelets?

  ADP and thromboxane A2
• What happens to platelets when they contact damaged vascular surfaces?

  They swell and become sticky
• How does a platelet plug form?

  Activated platelets attract more platelets
• What is the initial state of a platelet plug?

  Loose
• What reinforces the platelet plug during blood coagulation?

  Fibrin threads
• How quickly does a blood clot begin to form after vessel rupture?

  15 to 20 seconds for severe trauma
• What initiates the clotting process?

  Activator substances from the vascular wall
• How long does it take for a clot to fill a vessel opening?

  3 to 6 minutes
• What happens to a clot after 20 minutes to an hour?

  It retracts, closing the vessel further
• What are the two possible courses for a formed blood clot?

  Invaded by fibroblasts or dissolved
• What promotes the invasion of fibroblasts into a clot?

  Growth factor secreted by platelets
• How long does it take for a clot to organize into fibrous tissue?

  1 to 2 weeks
• What happens when excess blood leaks into tissues?

  Substances activate to dissolve the clot
• What are the physical events in blood coagulation?

  1. Platelet activation
  2. Fibrin formation
  3. Clot retraction
  4. Tissue repair
• Where do thrombocytes form?

  Megakaryocytes fragmentation in bone marrow or soon after entering blood
• Components of Thrombocytes


  1. Actin and myosin
  2. Residuals of ER and GA
  3. mitochondria and enzyme systems forming ATP and ADP
  4. Enzymes for prostaglandin
  5. Fibrin-stabilizing factor
  6. Growth factor
• What are on the membrane of platelets?

  Glycoproteins, that allow platelets to stick to injured endothelium. Phospholipids, activate clotting
• Half life of platelets

  8 to 12 days. They will then be eliminated by tissue macrophage system.
• Mechanism of platelet plug

  1. In damaged vascular surface , especially with collagen, platelets change characteristics
  2. Platelets become irregular and have pseudopods.
  3. Contractile proteins release granules making them more sticky
  4. Platelets start to adhere to collagen and von Willebrand factor leaking from damaged tissue.
  5. ADP and thromboxane released activates more platelet