L3.4: Hemostasis

• HEMOSTASIS

  ● Hem = blood, Stasis = standing still

  ● A fast and localized process of stopping the bleeding that results from a break in a blood vessel

  ● Blood usually clots within 3-6 minutes

  ○ The clot remains as endothelium regenerates

  ○ The clot is broken down after tissue repair
• THREE PHASES OF HOMEOSTASIS

  1. Vascular spasms occur
  2. Platelet plug forms
  3. Coagulation events occur
• THREE PHASES OF HOMEOSTASIS

  1. Vascular spasms occur

  ● Immediate response to blood vessel injury

  ● Vasoconstriction (narrowing of blood vessels) causes blood vessel spasms (“involuntary contraction”)

  ● Spasms narrow the blood vessel, decreasing blood loss
• THREE PHASES OF HOMEOSTASIS

  2. Platelet plug forms

  ● Collagen fibers are exposed by a break in a blood vessel

  ● Platelets become sticky and cling to fibers

  ● Anchored platelets release chemicals to attract more platelets

  ● Platelets pile up to form a platelet plug
• THREE PHASES OF HOMEOSTASIS

  3. Coagulation events occur

  ● Injured tissues release tissue factor (TF)

  ● PF3 (a phospholipid) interacts with TF, blood protein clotting factors, and calcium ions to trigger a clotting cascade

  ● Prothrombin activator converts prothrombin to thrombin (an enzyme)
• THREE PHASES OF HOMEOSTASIS 3. Coagulation events occur

  ● Thrombin joins fibrinogen proteins into hairlike molecules of insoluble fibrin

  ● Fibrin forms a meshwork (the basis for a clot)

  ● Within the hour, serum is squeezed from the clot as it retracts to pull edges of the blood vessel together

  ● Serum is plasma minus clotting proteins
• DISORDERS OF HEMOSTASIS

  Undesirable Clotting

  • Thrombus
  • Embolus

  Bleeding Disorders

  • Thrombocytopenia
  • Hemophilia
• Thrombus

  ● A clot that develops and persists in an unbroken blood vessel

  ● Can be deadly in areas such as the lungs
• Embolus

  ● A thrombus that breaks away and floats freely in the bloodstream

  ● Can later clog vessels in critical areas such as the brain
• Thrombocytopenia

  ● Insufficient number of circulating platelets

  ● Arises from any condition that suppresses the bone marrow

  ● Even normal movements can cause bleeding from small blood vessels that require platelets for clotting

  ● Evidenced by petechiae (small purplish blotches on the skin)
• Hemophilia

  ● Hereditary bleeding disorder

  ● Normal clotting factors are missing

  ● Minor tissue damage can cause life-threatening prolonged bleeding