CHAPTER 4 Flashcards
Define Primary hmostasis and secondary hemostasis
Primary hemostasis forma a weak platelet plug and is mediated by interaction between platelets and the vessel wall
Secondary hemostasis stabilizes the platelet plu and i mediated by the coagulation Cascadia
what are the steps involved in primary hemostasis
Step 1 - Transient vascoconstriction of damaged vessel
1. mediated by reflex neural stimulation and endothelin release from endothelial cells
Step 2 - Platelet adhesion to the surface of disrupted vessel
1. Von Willebrand Factor (vWF) binds exposed subendothelial collagen
2.Platelets bind vWf using GPIb receptor
3. vWF is derived weibel palade bodies of endothelial cells and a-granules of platelets
Step 3 - Platelet degranulation
1 adhesion induces shape change in platelets and degranultion with the release of multiple mediators
i. ADP is released from platelet dense granules; promotes exposure of GPIIb/IIIa receptor on platelets
ii. TXA2 is synthesized by platelet cyclooxygenase(COX) and released promotes platelet aggregation
Step 4: Platelet aggregation
1 platelet aggregate at the site of injury via GPIIb/IIIa using fibrinogen (from plasma) as a linking molecule; results in formation of platelet plug
2. Platelet plug is week coagulation cascade (secondary hemostasis) stabilizes it.
What is Immune Thrombocytopenic Purpura
A. Autoimmune production of antibodies against platelet antigens (eg. GPIIb/IIIa)
most common cause of thrombocytopenia in both children and adults
Autoantibodies are produced by plasma cells in the spleen
Antibody bound platelets are consumed by splenic macrophages , resulting in thrombocytopenia
Laboratory Findings
Normal Pt/PTT
decreased platelet count, often <50K/muL
Increased megakaryocyte
Explain chronic and acute forms of ITP
Acute form usually arises in children after a viral infection or immunization ; self-limited, usually resolves within weeks of presentation
Chronic form arises in adults , usually women of child-bearing age . may be primary or seconday (eg. SLE). May cause short lived thrombocytopenia in offspring since antiplatelet IgG can cross the placenta
Microangiopathic Hemolytic Anemia
Pathologic formation of microthrombi in small vessels
Platelets are consumed in the formation of microthrombi
RBCs are “sheared” as they cross microthrombi, resulting in hemolytic anemia with schistocytes
Seen in Thrombotic thrombocytopenic Purpura and hemolytic Uremic Syndrome (HUS)
Due to decreased ADAMTS13, an enzyme that normally cleaves vWF multimers into smaller monomers for eventual degradation .
Large, uncleaved multimers lead to abnormal platelet adhesion, resulting in microthrombi
Decreased ADAMTS13 is usually due to an acquired autoantibodymost commonly seen in adult females
What is the cause of Hemolytic Uremic Syndrome
HUS is due to endothelial damage by drugs or infection
Clasically seen in children with EColi O157H7 dysentery,which results from exposure to undercooked beef
Ecoli verotoxin damages endothelial cells resulting in platelet microthrombi
