123 Normal Hemostasis: Coagulation / Fibrinolysis

Question 1

Primary vs Secondary Hemostasis

Answer 1

• •Primary hemostasis: vasoconstriction & aggregated platelets to temporarily plug the vascular wound
  
  • 1. Vessel injury provokes vasoconstriction (neural)
  • 2. Weibel-Palade organelles of endothelial cells release the vasoconstrictor (endothelin), Von Willebrand factor (VWF) and selectins (cell adhesion molecules)
• Secondary hemostasis: fibrin formation to achieve a permanent clot & initiate healing

Question 2

Von Willebrand Factor (VWF)

Answer 2

• •Polymerizes in Weibel-Palade body of endothelial cell
• •Injury provokes release of multmeric VWF to cell surface where blood flow promotes its elongation
• •Cleaved by an endothelial cell protease, ADAMTS13
• •Uncleaved VWF binds platelets, initiating hemostasis

Question 3

P selectin

Answer 3

P-selectin induces rolling of platelets & leukocytes on endothelium; leukocytes release microparticles

Question 4

Clotting factors & Fibrin formation

Answer 4

• •Coagulation is initiated when blood is exposed to the transmembrane protein, tissue factor, or comes in contact with a foreign surface.
• •Clotting factors were designated by Roman Numerals in the order in which they were discovered, not in the order in which they participate in clotting.
• •Each is a latent serine protease sequentially activated during coagulation by the factor immediately preceding it in the sequence.

Question 5

Overview Scheme Tissue Factor Clotting Pathway

Answer 5

Limitaiton of TF Pathway

• •TF-VIIa & Xa generate only trace amounts of thrombin because of rapid inactivation by tissue factor pathway inhibitor (TFPI)
• •The intrinsic pathway (FVIII,FIX,FXI) is required to generate enough thrombin to form fibrin
• •Hemophilia is a disease of the intrinsic pathway; TFPI accounts for bleeding despite normal TF-VIIa

Question 6

Overview Scheme Principle Coagulation Clotting Pathway

Answer 6

Question 7

Factor X binding

Answer 7

Question 8

Role of Calcium in Coagulation

Answer 8

• •During platelet activation, an enzyme (flippase) transports phosphatidylserine from the inner leaflet to the outer leaflet of the platelet membrane.
• •Negatively-charged gamma carboxyglutamic acid residues are present on the amino terminal portion of clotting factors
• •The bivalent, positively-charged calcium ions are essential for the assembly of activated coagulation factors on the negatively-charged phosphate groups of membrane phospholipid micelles.

Question 9

Microparticles in clotting

Answer 9

• •Tissue injury releases microparticles (vesicles (≤ 1 µm)), from membranes of endothelial, mononuclear, etc cells
• •Microparticles bear tissue factor, a transmembrane glycoprotein, de-encrypted from the membrane bilayer

Question 10

Activation of Contact System of Coagulation

Answer 10

Ex: introduce a bad thing with your heroin

Question 11

Coagulation Cascade

Answer 11

Question 12

What are physiologic roles of FXII, prekallikrein, & high mol wt kininogen?

Answer 12

• •Pre-kallikrein circulates bound to HMWK & is converted to kallikrein by FXIIa
• •Kallikrein
  
  • -Activates FXII
  • -Converts HMWK to bradykinin
  • -Activates plasminogen to plasmin
  • -Contributes to formation of complement C3 & C5
  • -Converts prorenin to renin in the angiotensin system

Question 13

• Activated partial thromboplastin time (aPTT)
• Prothrombin time (PT)

Answer 13

• •Blood is collected in citrate; this binds calcium & prevents clotting; centrifugation provides plasma for testing
• •aPTT: plasma is mixed with silicate to activate FXII; the “partial thromboplastin” is a phospholipid that provides a binding site for clotting factors; adding calcium results in conversion of fibrinogen to fibrin in 30 to 40 seconds.
• •PT: plasma is mixed with a high concentration of tissue factor to activate FVII & overwhelm TFPI*; calcium is added and fibrin is formed in 10 to 15 seconds.

Question 14

Disorders detected by the PTT/PT

Answer 14

Question 15

Intrinsic vs Extrinsinc Coagulation Cascade flow chart

Answer 15

Question 16

Coagulation INhibitors

Answer 16

•Intact endothelium generates a variety of substances that inhibit coagulation:

• -Nitric Oxide (NO)
• -Prostacyclin (PGI2)
• -CD66 (ectoADPase)
• -ADAMTS13 cleaves von Willebrand factor
• -Tissue factor pathway inhibitor(TFPI) -Thrombomodulin(TM): binds thrombin
• -Proteoglycans: activate antithrombin(AT)

Physiologic Anticoagulants

• •Antithrombin
• •Protein C
• •Protein S
• •Protein Z
• •Tissue Factor Pathway Inhibitor
• •Thrombomodulin

Question 17

Antithrombin (III)

Answer 17

Question 18

Proteins C & S

Answer 18

• •Proteins C&S are synthesized in the liver, & have homology with II,VII, IX, X
• •Protein C activated by thrombin-thrombomodulin
• •Activated Protein C inactivates clotting factors Va and VIIIa; Protein S is the cofactor
• •Protein S circulates free(1/3) and bound(2/3) to the C4B-binding protein(C4BP); only the free fraction is cofactor for Protein C

Question 19

Fibrin Formation

Answer 19

Changes plasma from liquid to gel

Question 20

Fibrinolysis: Plasminogen to Plasmin

Answer 20

• Plasmin lyses fibrin strands, liberating cross-linked D-Dimers
• A rise in D-Dimers suggests ongoing thrombosis

Summary:

• •Endothelium is the principal anti-thrombotic mechanism, supplemented by circulating anticoagulants
• •Platelets and tissue factor initiate thrombosis, and thrombin is critical for thrombus augmentation
• •Clot lysis is the first step in vessel repair; impaired lysis leads to thrombus propagation, vessel occlusion, and organ ischemia