Robbins Ch. 4 - Hemostasis and the Clotting Cascade

Question 1

What is hemostasis?

Answer 1

Process by which blood clots form at injury sites

Precisely orchestrated cascade involving platelets, clotting factors, and endothelium that occurs at the site of a vascular injury and culminates in the formation of a blood clot

clot stops bleeding and loss of fluids

Question 2

A 27 yo male prisoner is shanked in the arm with a long screwdriver. What is the first step in hemostasis/formation of clot/plugging hole?

Answer 2

Arteriolar vasoconstriction

Question 3

What 2 things mediates the first step in the clotting process?

Answer 3

First step is vasoconstriction

Transient, mediated by reflex neurogenic mechanisms

augmented by localized secretion of things like endothelin, an endothelium-derived vasoconstrictor

Question 4

After vasoconstriction, what happens next in hemostasis to control bleeding?

Answer 4

Formation of platelet plug

Question 5

How is the platelet plug created?

Answer 5

Endothelium is breached/shanked

Endothelium will now show the previously hidden von Willebrand Factor (vWF) and collagen

vWF and collagen adhere and activate platelets

activated platelets go from cute, clumsy blebs to spiky tinker toys to increase surface area.

Platelet granules released.

Additional platelets are recruited, aggregate to form primary hemostatic plug

Question 6

Congratulations, a primary hemostatic plug has been formed. Snitches will indeed only get stitches today. What is the next step in hemostasis?

Answer 6

Secondary hemostasis: deposition of fibrin

Question 7

What is the process for secondary hemostasis?

Answer 7

Tissue factor is also revealed when endothelium is breached

• procoagulant glycoprotein, expressed in subendothelial vessel wall (in smooth mm cells, fibroblasts)

Tissue factor binds and activates Factor VII

Clotting cascade happens

Thrombin formed

Thrombin cleaves fibrinogen into fibrin

Question 8

Fibrin has been created in secondary hemostasis. How does this help with hemostasis?

Answer 8

Fibrin will form an insoluble meshwork

• also a potent activator of platelets, creating more platelet aggregation at injury site

Secondary hemostasis will consolidate plug created in primary hemostasis

Question 9

Primary and secondary hemostasis has occurred, and a robust clot/plug has been formed. What happens now?

Answer 9

Clot stabilization and resorption

Polymerized fibrin and platelet aggregates undergo contraction to form a solid, permanent plug that prevents further hemorrhage

t-PA works here

Question 10

What is t-PA?

Answer 10

tissue plasminogen activator

A blood protein that limits clotting to site of injury

Question 11

What general role do platelets play in hemostasis?

Answer 11

Forms initial primary plug

• seals vascular defects/injury/site of shanking
• makes surface that binds and concentrates activated coagulation factors

Question 12

What is the cellular origin and rough morphology of platelets?

Answer 12

Platelets are disc-shaped bags of proteins, with no nucleus

Shed from megakaryocytes in bone marrow, released into blood stream

Question 13

What general components do platelets need in order to function?

Answer 13

1. several glycoprotein receptors
2. contractile cytoskeleton
3. 2 types of cytoplasmic granules

Question 14

Platelets have 2 types of granules - alpha and gamma. What is in alpha granules? (7)

Answer 14

Alpha granules

adhesion molecule P-selectin on membranes

also contain coagulation proteins - fibrinogen, Factor V, vWF

wound healing proteins - fibronectin, platelet factor 4, PDGF, TGF-þ

Question 15

What are the contents of gamma granules in platelets? (5)

Answer 15

Gamma/dense granules:

ADP

ATP

Ca++

5HT - serotonin

epinephrine

Question 16

After a traumatic injury, platelets come into contact with vWF and collagen. What series of reactions happen next to form a platelet plug?

Answer 16

Platelet adhesion

Platelets changing shape

Secretion/release of granule contents

Platelet activation

Platelet aggregation

Question 17

How is platelet adhesion mediated?

Answer 17

Interactions with vWF

vWF will act as a ‘bridge’ between platelet surface glycoprotein receptor Ib (GpIb) and exposed collagen

Question 18

Platelets need to change shape in order to form a plug and create hemostasis. What other changes accompany this process?

Answer 18

Platelets get spiky in order to increase surface area

With this you get:

• alterations in glycoprotein IIb/IIIa, increases affinity for fibrinogen
• translocation of negatively charged phospholipids to platelet surface

Question 19

Platelets need to put negatively charged phospholipids on its surface during adhesion. Why? What phospholipids?

Answer 19

Often will phosphatidylserine on platelet surface

Negative phospholipids will bind Ca++ and serve as nucleation sites for assembly of coagulation factor complexes

Question 20

Platelet activation often includes what 2 events?

Answer 20

Release of granules

Change in shape

Question 21

What triggers platelet activation?

Answer 21

Thrombin cleaving PAR (protease-activated receptor, a GPCR)

Release of ADP

Question 22

What is recruitment?

Answer 22

Platelet activation and release of ADP, which induces more platelets to activate and join the plug.

Question 23

After platelet activation has happened, what occurs?

Answer 23

Platelet aggregation

Conformational change in GIIb/IIIa allows for fibrinogen to bind

• fibrinogen will form bridges between adjacent platelets, which leads to aggregation

Question 24

Platelet aggregation also occurs with thrombin activation. What does thrombin activation contribute to?

Answer 24

Converts fibrinogen into insoluble fibrin

• cements platelets in place and creates definitive secondary hemostasic plug

Question 25

In addition to fibrin and platelets, what else should you expect to see in a hemostatic plug?

Answer 25

Trapped RBCs and leukocytes - leukocytes get stuck on p-selectin on activated platelets

Question 26

Each step of the clotting cascade involves what 3 general actors?

Answer 26

Enzyme - activated coagulation factor Substrate - inactive proenzyme form of a coagulation factor Cofactor - reaction accelerator

Question 27

What coagulation factors need calcium as a cofactor? What other cofactor do these need?

Answer 27

Factors II, VII, IX and X Also will need Vitamin K Antagonized by coumadin

Question 28

What are the steps of the intrinsic pathway?

Answer 28

Negative charged surface XII → XIIa XIIa, XI → XIa XIa, IX → IXa + VIIIa IXa + VIIIa → X X → Xa + Va prothrombin → thrombin fibrinogen → fibrin

Question 29

What is the quick and dirty run down of the steps of the intrinsic pathway?

Answer 29

XII XI IX VIII X and V thrombin and fibrin

Question 30

What are the steps of the extrinsic pathway?

Answer 30

TF and VII → TF + VIIa X → Xa + Va Prothrombin to thrombin fibrinogen to fibrin

Question 31

You have learned about the intrinsic and extrinsic pathway for clotting *in vitro*, what is the cascade for clotting *in vivo*?

Answer 31

VII + TF → VIIaTF, works on IX → IXaVIIIa, works on X → XaVa, works on prothrombin → thrombin fibrinogen → fibrin

Question 32

What are the inhibitory elements of the clotting cascade *in vivo*?

Answer 32

Thrombin will inhibit severel places upstream: XaVa IxaVIIIa conversion of Xi to XIa XIa will inhibit activation of IX

Question 33

What are the platelet inhibitory effects of endothelium?

Answer 33

Serves as a barrier between platelets and vWF Releases: Prostacyclin - PGI2 NO Adenosine disphospatase - breaks down ADP Binds and alters activity of thrombin (potent platelet activator)

Question 34

What are the anticoagulant effects of endothelium?

Answer 34

Normal epithelium shields coagulation factors from tissue factors Expresses factors that actively oppose coagulation: thrombomodulin endothelial protein C receptor heparin-like molecules tissue-factor pathway inhibitor

Question 35

What does thrombomodulin and endothelial protein C receptor do?

Answer 35

Bind thrombin and protein C in a complex on the endothelial surface - thrombin will lose ability to activate coagulation factors and platelets - thrombin will activate protein C

Question 36

What is protein C? How does it do, and how is it activated?

Answer 36

It's a vitamin-K dependent protease that requires a cofactor - protein S Protein C/Protein S is a potent inhibitor of Factors Va and VIIIa

Question 37

What do heparin-like molecules present on the surface of endothelium do?

Answer 37

Bind and activates thrombin III

Question 38

What does heparin III do?

Answer 38

Heparin III inhibits thrombin and factors IXa, Xa, XIa, and XIIa - most of the intrinsic pathway

Question 39

How does tissue factor pathway inhibitor work?

Answer 39

Requires protein S as a cofactor Binds and inhibits tissue factor and factor VIIa complexes

Question 40

Endothelium also has some fibrinolytic effects. How are these mediated?

Answer 40

Normal epithelials make t-PA, a key component of the fibrinolytic pathway