Hemostasis - Coagulation/Fibrinolysis

Question 1

hemostasis

Answer 1

prevention of hemorrhage following vascular injury

Question 2

primary hemostasis

Answer 2

vasoconstriction and platelet plug formation

Question 3

secondary hemostasis

Answer 3

fibrin formation

Question 4

activation of coagulation

Answer 4

a. Vessel injury: exposure of subendothelial collagen. Collagen activates FXII and also binds glycoprotein VI, activating platelets. Activated platelets contribute to thrombin generation; this does not require FXII, explaining lack of bleeding with XII deficiency.
b. Endothelium releases endothelin, a vasoconstrictor, as well as P-selectin and von Willebrand factor from Weibel-Palade bodies (organelles in endothelial cells)
c. von Willebrand factor is necessary for platelet adherence to the subendothelium in high-shear vessels such as arteries and arterioles. It binds to collagen and platelet glycoprotein 1b, and is degraded by ADAMTS13.
d. P-selectin promotes rolling of leukocytes and platelets on activated endothelium
e. Blebs form on leukocytes and are shed as microparticles bearing tissue factor; these dock and fuse with activated platelets
f. In addition, glycoprotein Ib-V-IX on platelet surface binds vWF/FVIII, FXI, & thrombin, glycoprotein IIb/IIIa binds prothrombin and fibrinogen, and FV and fibrinogen are released from platelet α-granules.

Question 5

describe the coagulation cascade

Answer 5

at each step of the cascade, an enzyme cleaves (by proteolysis) the next factor, activating it. The cleaved factor (a zymogen) becomes an enzyme (or cofactor for an enzyme) and works on the next step of the process

Question 6

Prothrombin Time (PT)

Answer 6

addition of a high concentration of tissue factor results in formation of TF-VIIa complex which activates FX; FXa with FV & calcium, converts prothrombin to thrombin, & thrombin converts fibrinogen to fibrin (insoluble clot). Note that the clotting time becomes truly prolonged when the concentration of one or more clotting factors (FV, VII, X, prothrombin) is less than 30% of normal

Question 7

Activated Partial Thromboplastin Time (aPTT)

Answer 7

addition of a silicate activates FXII; provision of phospholipid & calcium accelerates the clotting time. As with the PT, the aPTT becomes clearly prolonged when the concentration of any of the clotting factors, except FVII, is less than 30% of normal

Question 8

FXII, FXI, PreKallikrein, & High Molecular Weight Kininogen

Answer 8

discovered because patients deficient in these factors had a prolonged aPTT. However, only FXI deficiency is associated with a mild/moderate bleeding disorder. FXII, Prekallikrein,& HMWKininogen involved in complement cascade and bradykinin formation. FXI binds to activated platelets, is activated by thrombin, and catalyzes the conversion of FIX to FIXa (but tissue factor-FVIIa complex also does this)

Question 9

factors involved in initiation of coagulation

Answer 9

Tissue factor and FVII

Question 10

factors involved in coagulation amplification

Answer 10

factors V, VIII, and XI

Question 11

factors involved in coagulation propagation

Answer 11

factor X and prothrombin

Question 12

Vitamin K and coagulation

Answer 12

Vitamin K is the coenzyme for the carboxylase that attaches carboxyl groups to glutamic acid residues of clotting factors II,VII, IX, X and inhibitors proteins C, S, and Z. These negatively charged groups enable the clotting or inhibitor proteins to form Ca2+ bridges with negatively charged membrane phospholipids, an essential step for their activity. During the carboxylase reaction, vitamin K is oxidized but returned to its reduced form by the liver enzyme, VKORC1

Question 13

thrombin

Answer 13

binds to and cleaves Protease Activated Receptor 1 (PAR1) on the platelet surface; PAR1 promotes platelet degranulation, changes in phospholipid composition, and activation of glycoprotein IIb/IIIa. Thrombin releases FVIII from von Willebrand factor and activates it, FV, FXI, & FXIII. In the absence of FVIII or FIX (hemophilia), FVIIa, FXa and thrombin are rapidly inactivated by tissue factor pathway inhibitor & antithrombin (see below), and fibrin formation is greatly attenuated

Question 14

fibrinogen

Answer 14

dimer of 3 interwoven peptide chains. When 2 small fibrinopeptides are cleaved from the Aα & Bß-chains by thrombin, the dimers aggregate by forming non-covalent bonds between terminal D-domains on one molecule and the central E-domain of a second molecule giving a pattern that looks like a bricks in a wall. Thrombin-activated FXIII then catalyzes bond formation between glutamic acid of one D-domain and lysine of an adjacent D-domain. This accounts for the incredible strength of the fibrin strand

Question 15

antithrombotic agents

Answer 15

nitric oxide, prostacyclin, and ecto-ADPase

tissue factor pathway inhibitor (TFPI)

thrombomodulin

proteoglycans

ADAMTS13

Question 16

tissue factor pathway inhibitor (TFPI)

Answer 16

blocks the FVIIa-tissue factor complex

Question 17

thrombomodulin

Answer 17

binds thrombin and activates protein C

Question 18

proteoglycans

Answer 18

activate antithrombin

Question 19

ADAMTS13

Answer 19

is present on the endothelial cell membrane and cleaves the highest molecular weight multimers of von Willebrand factor as they are released by the Weibel-Palade bodies

Question 20

antithrombin

Answer 20

a serine protease inhibitor (serpin) that blocks thrombin and activated factors X, IX, XI, XII, and the tissue factor-VIIa complex

Question 21

proteins C and S

Answer 21

synthesized in the liver and have homology with factors II, VII, IX, and X

protein C activated by thrombin-thrombomodulin

activated protein C inactivates clotting factors Va and VIIa, protein S is the cofactor

protien S circulates free (1/3) and bound (2/3) to the C4B-binding protein (C4BP)

only free fraction is cofactor for protine C

Question 22

protein Z

Answer 22

binds to Z protease inhibitor and the complex inhibits factor Xa

Question 23

phospholipid-binding proteins

Answer 23

annexin V and beta2-glycoprotein-1, bind to phosphatidyl serine, preventing binding of the coagulation complexes

Question 24

fibrinolytic system

Answer 24

consists of plasminogen, which forms plasmin, the enzyme that lyses fibrin clots.

Tissue plasminogen activator (t-PA) and urokinase convert plasminogen to plasmin, but this process is most effective when plasminogen is bound to lysine-binding sites on fibrin.

Question 25

inhibitors of the fibrinolytic system

Answer 25

Plasminogen activator inhibitor-1 (PAI-1), lipoprotein(a), and thrombin activatable fibrinolysis inhibitor (TAFI) either bind to or degrade lysine binding sites on fibrin and thereby inhibit fibrinolysis. Epsilon aminocaproic acid and tranexamic acid are pharmacologic lysine-mimetics. Antiplasmin directly inhibits plasmin