Platelets and Coagulation

Question 1

List the sequence of events following blood vessel injury.

Answer 1

Two simultaneous processes:

1 - Platelet adhesion to subendothelium of a damaged vessel.

2 - Platelet activation.

3 - Platelet aggregation.

1 - Activation of the coagulation cascade.

2 - Fibrin formation.

• Fibrin and activated aggregated platelets form a thrombus.

Question 2

List the molecular components of the haemostatic response.

Answer 2

1 - Platelets.

2 - von Willebrand factor.

3 - Clotting factors.

4 - Cofactors.

5 - Fibriongen.

Question 3

List the clotting factors that are vitamin K dependent.

Answer 3

Factors 2 (prothrombin), 7, 9, 10 (AKA thromboplastin or thrombokinase) and 11.

Question 4

What is clotting factor 2 known as?

Answer 4

Prothrombin.

Question 5

List 6 inhibitors of the haemostatic response.

Which of these are inhibitors of platelets?

Answer 5

1 - Tissue Factor Pathway Inhibitor (TFPI).

2 - Antithrombin.

3 - Protein C.

4 - Protein S.

Inhibitors of platelets:

5 - Prostacyclin.

6 - Nitric oxide.

Question 6

List the 3 components of the coagulation cascade.

Answer 6

1 - Intrinsic pathway.

2 - Extrinsic pathway.

3 - Common pathway.

Question 7

Describe the process of platelet adhesion to the subendothelium of a damaged vessel.

Answer 7

1 - When the integrity of the endothelium is breached, subendothelial proteins such as collagen and von Willebrand factor come into contact with the blood.

2 - These proteins interact with a family of platelet-surface glycoprotein receptors such as GPIb (vWF receptor) and GPVI (collagen receptor).

3 - This interaction results in platelets adhering to the site of injury, forming a platelet plug.

Question 8

Describe the stages of platelet activation.

Answer 8

*The ultimate objective here is to produce TXA2 and ADP.

1 - Initiated by exposure to soluble agonists such as exposed collagen and thrombin.

2 - These activators lead to an increase in intracellular Ca2+ and the activation of myosin light-chain kinase in the platelet.

3 - Myosin light-chain kinase phosphorylates myosin light chains in the platelet.

4 - Phosphorylated myosin light chains interact with actin, disrupting the cytoskeleton and changing the shape of the platelet.

5 - The shape change initiates a release reaction that expels mediators such as ADP via release of granules.

6 - The increase in intracellular Ca2+ also activates phospholipase A2, which liberates arachidonic acid from membrane phospholipids.

7 - Arachidonic acid is converted by COX-1 to thromboxane A2, which diffuses from the platelet.

Question 9

Describe the process by which thromboxane A2, ADP, collagen and thrombin lead to platelet aggregation.

Answer 9

1 - ADP, thromboxane A2, collagen and thrombin interact with platelet surface receptors to trigger intracellular pathways that express and activate GPIIa and GPIIb collagen receptors on the surface of platelets.

2 - The GPIIa and GPIIb receptors on different platelets are crosslinked to each other by fibrinogen in the plasma, producing irreversible platelet aggregation.

Question 10

List the types of granules that are secreted by activated platelets.

Answer 10

1 - Dense granules.

2 - Alpha granules.

3 - Lysosomes.

Question 11

List 3 important molecules that are contained within dense granules.

Answer 11

1 - ADP.

2 - 5HT.

3 - Ca2+.

Question 12

List 5 important molecules that are contained within alpha granules.

Answer 12

1 - FGN.

2 - FVIII.

3 - vWF.

4 - FV.

5 - PDGF.

Question 13

How is a platelet plug organised?

List the differences between the layers of organisation.

Answer 13

Into an inner and outer core:

1 - The inner core has fully activated platelets, whereas the outer shell has partial platelet activation.

2 - Platelets of the inner core secrete alpha granules, whereas platelets of the outer shell does not.

3 - The inner core has densely packed platelets, whereas the outer shell has looseley packed platelets.

4 - The inner core has stable adhesion of platelets, whereas the outer shell does not.

5 - The inner core has high thrombin activity, whereas the outer shell does not.

6 - The inner core has fibrin formation, whereas the outer shell does not.

Question 14

What lines the base of a platelet plug?

Answer 14

A procoagulant membrane, which is composed of damaged / activated endothelial cells, platelets and prothrombinase complexes.

Question 15

List the membrane channels that are involved in platelet membrane changes on activation.

Answer 15

1 - Flippase.

2 - Floppase.

3 - Scramblase.

Question 16

List the functions of flippase, floppase and scramblase.

Answer 16

• Flippases are transporters that moves lipids from the exoplasmic face to the cytosolic face (exo → cyto).
• Floppases transport in the reverse direction (cyto → exo).
• Scramblase is a transporter that moves lipids between the exoplasmic face and cytosolic face in both directions.

Question 17

List the initiation steps of the coagulation cascade.

Which molecule brings about these steps?

Answer 17

1 - Tissue factor converts F7 into F7a.

2 - F7a catalyses the activation of F9 and F10 into F9a and F10a.

Question 18

List the amplification steps of the coagulation cascade.

Answer 18

1 - F10a catalyses the activation of prothrombin (F2) into thrombin (F2a).

2 - Thrombin (F2a) catalyses the activation of fibrinogen (F1), F5, F8 and F11.

Question 19

List the propagation steps of the coagulation cascade.

Answer 19

1 - Factor 11a (activated by thrombin) catalyses the activation of factor 9 into 9a.

2 - F9a and F8a together catalyse the activation of F10 into F10a.

3 - F10a and F5a together catalyse prothrombin (F2) into thrombin (F2a).

Question 20

List 6 laboratory tests used to investigate the haemostatic system.

Answer 20

1 - Platelet count.

2 - Prothrombin time.

3 - Activated partial thromboplastin (factor 10, AKA thrombokinase) time.

4 - Fibrinogen level.

5 - Coagulation factor assays.

6 - Platelet aggregation studies.

Question 21

Define prothrombin time.

Define activated partial thromboplastin time (APTT).

Answer 21

• Prothrombin time is the time required for fibrin clot formation after addition of Ca2+ and thromboplastin to a blood sample (from tissue factor activation of F7 to fibrinogen activation).
• APTT is the time taken for fibrin clot formation after addition of Ca2+ to a blood sample (from stimulation of factor 11 to fibrinogen activation).

Question 22

How are the clotting factors divided into the intrinsic, extrinsic and common pathways?

Answer 22

Intrinsic:

1 - F8.

2 - F9.

3 - F11.

Extrinsic:

1 - Tissue factor.

2 - F7.

Common:

1 - Fibrinogen (F1).

2 - Prothrombin (F2).

3 - F5.

4 - F10.

Question 23

List 3 causes of F7 deficiency.

Answer 23

1 - Hereditary F7 deficiency.

2 - Mild liver disease.

3 - Mild vitamin K deficiency.

Question 24

What does a slow prothrombin time indicate?

Answer 24

Poor F7 function.

Question 25

What does a slow activated partial thromboplastin time indicate when:

A mixing test of 50:50 patient:normal plasma corrects the APTT to normal?

A mixing test of 50:50 patient:normal plasma does not correct the APTT to normal?

Answer 25

• Correction to normal indicates a deficiency of F8, 9, 11 and 12.
• Failure to correct indicates that there is an inhibitor present, such as lupus anticoagulant or F8 antibody.

Question 26

What does prolongation of both the prothrombin time and the activated partial thromboplastin time indicate?

Answer 26

• Single factor deficiencies in the common pathway (F1 (fibrinogen), F2 (prothrombin), F5 or F10.
• Multifactorial deficiencies:

1 - Liver disease.

2 - Vitamin K deficiency.

3 - Warfarinisation.

4 - Disseminated intravascular coagulation.

5 - Dilutional coagulopathy.

Question 27

List 3 uses of laboratory tests used to investigate the haemostatic system.

Answer 27

1 - Finding faults in the system.

2 - Monitoring the response to replacement of clotting factors.

3 - Monitoring the effect of anticoagulants.

Question 28

List the clotting factors that may be deficient with a prolonged APTT only.

Answer 28

1 - F8.

2 - F9.

3 - F11.

4 - F12.

Question 29

List the clotting factors that may be deficient with a prolonged PT only.

Answer 29

F7.

Question 30

List the clotting factors that may be deficient with both a prolonged PT and APTT?

Answer 30

1 - F1.

2 - F2.

3 - F5.

4 - F10.

Question 31

List 5 hereditary clotting conditions that cause an increased tendency to bleed.

For each, give the clotting factor that is deficient if possible.

Answer 31

1 - Haemophilia A (F8).

2 - Haemophilia B (F9).

3 - von Willebrand disorder (vWF).

4 - Glanzmann thrombasthenia (a platelet disorder).

5 - Bernard-Soulier syndrome (a platelet disorder).

Question 32

List 4 acquired clotting conditions that cause an increased tendency to bleed.

Answer 32

1 - Liver disease.

2 - Vitamin K deficiency.

3 - Warfarinisation.

4 - Disseminated intravascular coagulation (but also causes thrombosis - see later card).

Question 33

List 3 mechanisms which prevent intravascular clot formation.

Answer 33

1 - Unobstructed, non-turbulent flow.

2 - Intact vascular endothelium.

3 - Circulating anticoagulant proteins such as antithrombin, protein C and protein S.

Question 34

What is thrombomodulin?

Answer 34

A membrane protein that binds to thrombin (F2a) that serves an anticoagulant function.

Question 35

List the stages of protein C activation.

Answer 35

1 - Thrombin (F2a) binds to thrombomodulin.

2 - The thrombin-thrombomodulin complex catalyses the activation of protein C into activated protein C.

Question 36

List the stages by which protein S and protein C prevent coagulation.

What is the function of C4B binding protein?

Answer 36

1 - Free protein S binds to activated protein C.

2 - The protein C / S complex lyses factor 5 and 8, preventing their role in the coagulation cascade.

• C4B binding protein can bind to anticoagulant protein S, resulting in a decreased cofactor function of protein S for activated protein C.

Question 37

Describe the role of PF3 in fibrin clot formation.

Answer 37

• PF3 is secreted by platelets upon platelet shape change.

- It is involved in the activation of factor 10 and 2.

Question 38

How do fibroblasts and monocytes contribute to the coagulation cascade?

Answer 38

They can secrete tissue factor at the site of injury.

Question 39

List 5 hereditary disorders that predispose to venous thrombosis.

Answer 39

1 - Factor 5 Leiden mutation.

2 - Prothrombin gene mutation.

3 - Antithrombin deficiency.

4 - Protein C deficiency.

5 - Protein S deficiency.

Question 40

Give an example of an acquired disorder that predisposes to venous thrombosis.

Answer 40

Lupus anticoagulant.

Question 41

Define fibrinolysis.

Answer 41

The breakdown of fibrin in a clot.

Question 42

List the steps involved in fibrinolysis.

Answer 42

1 - Thrombin binds to undamaged endothelial cells near to the site of injury.

2 - This releases tissue plasminogen activator (tPA).

3 - tPA and plasminogen (present in the blood) bind to fibrin on the surface of the clot.

4 - Plasmin is cleaved from the plasminogen associated with the fibrin.

5 - Plasmin lyses fibrin molecules and the clot dissolves, releasing fibrin degradation products (FDPs).

Question 43

Describe the pathophysiology of disseminated intravascular coagulopathy (DIC).

Answer 43

1 - Systemic activation of coagulation leads to intravascular deposition of fibrin, depletion of platelets and depletion of coagulation factors.

2 - Intravascular deposition of fibrin causes thrombosis of small vessels and organ failure.

3 - Depletion of platelets and coagulation factors causes bleeding.