Hemostasis Flashcards
Tissue factor
TF is a membrane protein on fibroblasts and other cells in the vessel wall; it is exposed to the blood with endothelial injury
TF binds activated, circulating VIIa in order to begin the extrinsic coagulation cascade
Tenase
Cofactor VIII supports IXa and X; X becomes activated to Xa
Prothrombinase
Cofactor V supports Xa and II; II becomes activated to IIa
Vitamin K-dependent proteins
II, VII, IX, X
Protein C, Protein S
Role of Vitamin K in coagulation
Factors II, VII, IX, and X undergo vitamin K-dependent gamma carboxylation of glutamic acid in the liver; this reaction oxidizes vitamin K, which must be reduced by Vitamin K oxidoreductase in order to continue synthesizing functional clotting factors
Warfarin - Mechanism
Factors II, VII, IX, and X undergo carboxylation of N-terminal glutamates by enzymes that are Vitamin K-dependent; without this carboxylation they are unable to bind Ca2+ and are non-functional
Vitamin K undergoes redox during this reaction and the recycling of Vitamin K to its reduced, active form by Vitamin K Reductase is inhibited by Warfarin, resulting in depletion of Vitamin K and active coagulation factors
Tissue Factor Pathway Inhibitor (TFPI)
Binds and inhibits Tenase and free, activated Xa
Antithrombin (AT3)
Binds and inhibits thrombin, as well as factors Xa and XIa
This binding interaction is enhanced by heparin
Protein C System
Thrombomodulin (TM) activates PC to APC; APC binds PS as a co-factor to cleave and inactivate Va and VIIIa
Tissue Plasminogen Activator (TPA)
Converts plasminogen to plasmin; plasmin cleaves fibrin to fibrin split products (FSPs) and Dimers
Plasminogen activator inhibitor-1 (PAI-1)
Binds and inhibits tissue plasminogen activator (TPA), preventing the formation of plasmin and inhibiting fibrinolysis
TF-VIIa
Converts X to Xa and IX to IXa
Results in the production of a small amount of thrombin
Propagation Phase
Production of Xa (by Tenase) and Thrombin (by Prothrombinase)
Role of Thrombin in Amplification
Thrombin activates co-factors VIII and V
Thrombin also activates XI
Mechanism of fibrin cross-linking
Factor XIIIa is activated by thrombin to covalently cross-link fibrin, leading to formation of a “hard clot”
Contents of Platelet Dense Granules
ATP ADP Ca2+ Histamine Serotonin
Contents of Platelet Alpha Granules
Procoagulant proteins - fibrinogen, factor V, vWF
Platelet activation factors
Platelet-derived growth factor
Contents of Platelet Lysosomal Granules
Acid hydrolases
GPIa/IIa
Binds Collagen in the exposed sub-endothelium at the site of vascular injury
GPIIa-IIIb
Binds vWF in the sub-endothelium at the site of vascular injury
Also binds fibrinogen to cross-link platelets
GPIb
Binds vWF in the exposed sub-endothelium at the site of vascular injury
Platelet Adhesion
Platelets adhere to the injured surface and build at temporary clot by virtue of binding interactions:
GPIba binds vWF
GPIIb/IIIa binds Fibrinogen
GPIa/IIa binds collagen
Platelet Activation
Interaction of the extracellular, soluble agonists thrombin, thromboxane A2, and ADP with their respective GCPRs in the platelet membrane signal calcium influx that results in secretion of alpha and dense granule contents; calcium also activates the conversion of arachidonic acid to TXA2 via COX1
End result of agonist binding is a conformational change in the platelet to expose GPIIa/IIIb binding sites for fibrinogen
Platelet aggregation
Platelet adhesion & binding of soluble agonists converts GPIIa-IIIb to a high-affinity state where it can bind fibrinogen and vWF; GPIIa-IIIb bound to fibrinogen acts as a bridge to lace platelets together into a mesh; locally, thrombin acts to convert fibrinogen to fibrin, stabilizing the platelet mesh
Mechanisms by which endothelial cells inhibit coagulation (3)
- Secreting a heparin-like molecule to suppress coagulation cascade activity
- Expressing thrombomodulin which, when bound to thrombin, activates Proteins C and S
- Secretion of NO and prostacyclin, inhibitors of platelet activation
Immune thrombocytopenic purpura ITP)
Caused by the production of auto-antibodies directed against platelet antigens, leading to their removal by macrophages of the RE system
May be acute or chronic
Acute onset is often preceded by viral infection and presents as petechiae and nosebleeds
Chronic is often associated with other auto-immune disorders (SLE, RA, HIV)
Treatment for ITP (4)
Corticosteroids - suppress inappropriate immune response to platelets
IVIG - Blocks Fc receptors on splenic macrophages
Splenectomy - prevents sequestration and destruction of opsonized platelets
Rituximab - anti CD20, depletes B cells
Alloimmune thrombocytopenia
Production of antibodies against platelet antigens not present on the patient’s own platelets
Occurs in the setting of a patient receiving platelet transfusions, or in a neonate through passive transfer of maternal IgG alloantibodies across the placenta to attack fetal platelets
Thrombotic thrombocytopenic purpura (TTP)
Due to mutation in or antibodies against the ADAMTS13 protein that normally functions to digest vWF; as a result, large vWF multimers are released from endothelial cells in the setting of damage; these large vWF multimers mediate platelet adhesion and aggregation forming diffuse platelet plugs in small arterioles
Presents as renal insufficiency, mental status changes, thrombocytopenia
Von Willebrand Disease - Types
The most common congenital bleeding disorder; 3 types:
Type I - Partial quantitative deficiency of vWF
Type II - Partial qualitative deficiency of vWF
Type III - Total absence of vWF
Pathophysiology of Von Willebrand Disease
vWF facilitates platelet adhesion to injured vascular endothelium and also serves as a carrier protein for Factor VIII
Deficiency of vWF leads to defects in platelet aggregation and plug formation (primary hemostasis) as well as in fibrin deposition (secondary hemostasis) due to decreased Factor VIII levels
Diagnosis of Von Willebrand Disease
vWF antigen measures quantity of vWF present; decreased in Type I and absent in Type III vWD disease
vWF Ristocetin Cofactor Activity measures activity of vWF present; decreased in Type I and II, absent in type III vWF disease
PFA elevated (ADP & Epi)
Decreased VIII levels
Prolonged PTT
Treatment of von Willebrand Disease
DDAVP - enhances release of vWF from endothelial stores; effective for treatment of Type I disease only
Factor VIII replacement
Bernard-Soulier Syndrome
Autosomal recessive disorder caused by decreased expression of GPIb on the platelet surface, leading to defective adhesion and bleeding
Afibrogenemia
Inherited mutation in fibrinogen; causes defects in primary hemostasis due to inability of fibrinogen to cross-link GPIIb-IIIa in platelet aggregation; also causes defects in secondary hemostasis due to lack of fibrinogen for formation of cross-linked fibrin clots
Patients present with platelet-type mucosal and cutaneous bleeding as well as deep muscle hematomas more characteristic of coagulation defects