Hemostasis

Question 1

Name the basic steps of hemostasis.

Answer 1

platelet adhesion
platelet secretion, vasoconstriction and aggregation
thrombin generation
(clot lysis with wound healing)

Question 2

What components of the hemostatic system are important for formation of primary hemostatic plug and coagulation cascade?

Answer 2

platelets and vWF form primary hemostatic plug

coagulation cascade is important in formation of fibrin got (stabilizes and strengthens fibrin clot)

Question 3

When is reduction in platelet count reach a clinically significant threshold?

Answer 3

typically below 20-30K, note about 30% are sequestered in the spleen (more if the spleen is enlarged)

megakaryocyte development under the influence of thrombopoietin

Question 4

What structural features of platelets are important for the activation of clotting activity

Answer 4

granules: electron dense(ADP, Ca++, serotonin: recruit), a granules (proteins that participate in coagulation)

membrane glycoproteins: vWF and fibrinogen receptors

membrane phospholipids kept in check by “flippase”

Question 5

Name the 3 most potent platelet activators.

Answer 5

thrombin
ADP
thromboxane
…each with corresponding platelet membrane receptor

additionally: prostacyclin, TPO, Epi, adenosine, sertonin, collagen

Question 6

What are the function of platelets in intact vessels?

Answer 6

maintain vessel integrity by preventing RBC from leaking from uninjured vessels (petechiae)

effect is mediated by VEGF, angiopoietin, EGF

Question 7

List common platelet inhibitors.

Answer 7

NO
prostacyclin
aspirin
clopidogrel

Question 8

What is the function of vWF and where is it produced?

Answer 8

mediates platelet adhesion via GP Ib receptor released into the blood and subendothelium (adheres to sub endothelial matrix)

immobilization and unfolding of vWF increase adhesion, larger multimers are most effective; unfolding occurs under shear stress

Question 9

What is the activity of ADAMTS13?

Answer 9

can regulate vWF multimer size by cleaving unfolded multimers; modulates the “stickiness” of vWF

autoimmune destruction of ADAMTS13 can cause TTP

Question 10

How is thrombin clot formation initiated?

Answer 10

tissue factor triggers enzymatic cascade that results in formation of thrombin
thrombin converts fibrinogen to fibrin, fibrin polymerizes and polymers are cross linked to form stable clot

Question 11

What types of cells express tissue factor?

Answer 11

TF is a ubiquitous membrane-associated lipoprotein that is expressed by most cells, not normally expressed by endothelial cells or leukocytes although expression can be induced in theses cells by inflammatory cytokines

there is high expression in the adventitia of blood vessels, brain, skin, an mucosal tissue

Question 12

What factors are required for the generation of thrombin?

Answer 12

enzyme Xa, prothrombin (substrate), helper protein Va, phospholipid surface and calcium

both enzyme and substrate contain calcium binding regions that are vitamin K-dependent

Question 13

What clotting and anticoagulant proteins require vitamin K?

Answer 13

II, VII, IX, X and protein C and S : vitamin K is needed to create calcium-binding sites (Warfarin is an antagonist of vitamin K)

Question 14

What factor is initiated by tissue factor and how does the clotting cascade continue (just the initiation stage)?

Answer 14

tissue factor binds VIIa (7)

TF and 7 activate X and IX (9 and 10)
9 and 8 complex activate 10

10 and 5 activate II (2-prothrombin) to thrombin

TF and 7 complex quickly inhibited by tissue factor pathway inhibitor

Question 15

Describe the amplification of of the clotting cascade.

Answer 15

prothombin sends positive feedback to XI, VIII and V (5,8 and 11)

Xa and prothrombin activate more VII (10 begets more 7)

IX/VIIIa generate more X (8/9 complex begets more 10)

5/10 complex makes more prothrombin that makes fibrin from fibrinogen, fibrin is cross linked by 8

Question 16

How do deficits in one or several coagulation proteins effect the risk of bleeding?

Answer 16

a single factor doesn’t typically cause bleeding but trouble with more than one factor leads to additive effects

Question 17

Clotting factor deficiencies are most impactful in which tissues?

Answer 17

articular cartilage and skeletal muscle have relatively little tissue factor, low tissue factor expression can be really important (esp in hemophiliacs)

Question 18

Name the 3 components in the “contact system” and what is this system’s significance?

Answer 18

high molecular weight kininogen, prekallikrein, factor XII

pathways is only activated when plasma is exposed to a negatively charged surface, with end product of XII activation; this pathway has little if any role in physiologic coagulation (deficiency of contact factors does not cause bleeding)

Question 19

What is a D dimer made up of?

Answer 19

and E region that connects to two D regions from different fibrin molecules

Question 20

What anticoagulant proteins antagonize (slow the cascade) the clotting cascade and where do they act?

Answer 20

TFPI (tissue factor pathway inhibitor)- at tissue factor VIIa (7)
APC (activated protein C)- at IXa-VIIIa (8/9 complex) and X-V complex (5/10 complex)
AT3 (antithrombin 3) at X-V complex and thrombin

Question 21

Discuss properties/molecules that have an antithrombic effect on the endothelium.

Answer 21

sequesters tissue factor from the blood
prostaglandin I- inhibitis platelet aggregation
heparin sulfate catalyzes antithrombin
thrombomodulin catalyzes activation of protein C
NO inhibitis platelet aggregation
ADPas inhibits platelet aggregation

Question 22

What is the mechanism of anti-thrombin?

Answer 22

AT3 is a protease inhibitor that inhibits multiple coagulation enzymes, most significantly factors IXa, Xa and thrombin (IXa and Xa are protected from inhibition while incorporated in membrane-bound complexes, AT3 works largely as a scavenger of free proteases- localizing clotting)

AT3 activity is markedly accelerated by heparin

Question 23

Prothrombin bound to thrombomodulin can activate protein C, what is the down stream effect in this activation?

Answer 23

activation of protein C is a negative feedback loop that goes on to degrade Va and VIIIa, to slow/stop the clotting cascade

Question 24

What is the mechanism of TFPI, where does it bind and what does it inhibit?

Answer 24

TFPI bind activated Xa and together they bind to 7a/TF complex, stopping propagation of the clotting cascade

Question 25

T/F Partial deficicency of antithombin, protein C or protein S can contribute to the a thrombotic tendency.

Answer 25

True

Question 26

What factors are critical in the fibrinolysis pathway?

Answer 26

activation of plasminogen by plasminogen activator (also urokinase) and catalyzed by fibrin
(alpha 2 antiplasmin and plasminogen activator inihibitors regulate the process as excessive fibrinolysis can lead to bleeding- DIC)