Haemostasis

Question 1

What is haemostasis

Answer 1

the cellular and biochemical processes that enables both the cessation of bleeding in response to vascular insult

Question 2

What is the function of haemostasis

Answer 2

to prevent blood loss from intact and injured vessels, enable tissue repair

Question 3

What are the 4 stages of haemostasis

Answer 3

1. Vessel constriction
2. Formation of an unstable platelet plug
3. Stabilisation of the plug with fibrin
4. Vessel repair and dissolution of clot

Question 4

Describe the vessel constriction response in haemostasis

Answer 4

Mainly important in small blood vessels
Local contractile response to injury
Limits blood flow to the injured vessel

Question 5

What is involved in unstable platelet plug formation and what is the purpose

Answer 5

Platelet adhesion and aggregation

Limits blood loss + provides surface for coagulation

Question 6

What is involved in stabilisation of a platelet plug and what is the purpose

Answer 6

Blood coagulation

Stops blood loss

Question 7

What does vessel repair involve and what is the function

Answer 7

Cell migration/proliferation and fibrinolysis

Restores vessel integrity

Question 8

Describe the normal vessel wall

Answer 8

Endothelial cell - anticoagulant barrier (TM, EPCR, TFPI, GAG)
Subendothelium - procoagulant
Basement membrane - elastin, collagen
VSMC  - TF
Fibroblasts - TF

Question 9

Describe platelets (size, organelles, lifespan, count)

Answer 9

Small (2-4µm)
Anuclear
Lifespan - 10 days
Platelet count = 150-350 
10^11 produced every day

Question 10

What is the role of the platelet cytoskeleton

Answer 10

Important for morphology, shape change, pseudopods, contraction and clot retraction

Question 11

What are the ultrastructural features of the platelet

Answer 11

On membrane:
Alpha granules 
Dense granules 
Thrombin receptor 
Phospholipid membrane
Microtubules and actomyosin
Glycoproteins (GbIb/V/IX)

Question 12

What is contained in the alpha granules of platelets

Answer 12

Growth factors,
Fibrinogen
FV
VWF

Question 13

What is contained in the dense granules of platelets

Answer 13

ADP
ATP
Serotonin
Ca2+

Question 14

Describe platelets and VWF in a normal blood vessel

Answer 14

Platelets circulate in close contact with the endothelial cell lining of the blood vessel wall
VWF circulates in a globular conformation. Binding sites are hidden from GpIb

Question 15

What is vWF

Answer 15

vWF is a blood-borne glycoprotein which also adheres to endothelial collagen, and binds Gp1b especially well under high shear stress.

Question 16

What occurs in the adhesion stage of platelet plug formation

Answer 16

1. Platelets bind to exposed endothelial collagen with their Gp1a.
2. Platelets bind to von Willebrand factor (vWF) with their Gp1b.
3. vWF unravel under high shear stress
4. Binding of VWF to to platelets recruits other platelets (also Bia GPVI and a2b1 on collagen at low shear)
5. Activation of platelets

Question 17

What occurs in the aggregation stage of platelet plug formation

Answer 17

Release of ADP and thromboxane by platelets causes them to stick together via fibrinogen using their GpIIb/IIIa. Ca2+ has to be present.

Question 18

What are prostaglandins important metabolites of and which structures are involved in their formation

Answer 18

Arachidonic acid

Platelets and endothelial cells

Question 19

What do platelets convert arachidonic acid to

Answer 19

Thromboxane A2 via COX-1

Thromboxane A2 is a potent inducer of platelet aggregation.

Question 20

What do endothelial cells covert arachidonic acid to

Answer 20

Prostacyclin (PGI2) via cyclooxygenase-2 (COX-2).

Prostacyclin inhibits platelet aggregation.

Question 21

What are the ways to monitor platelet function

Answer 21

platelet count
Bleeding time (40m pressure + incision, normal = 3-8)
Platelet aggregation （vWF)

Question 22

At what platelet counts will spontaneous bleeding and bleeding occur

Answer 22

Severe spontaneous - <10x10^9
spontaneous - <40x10^9
With trauma - 100x10^9
Normal - >150

Question 23

Describe vessels disorders and give examples

Answer 23

Bleeding due to impaired interaction of platelets with the compromised vessel
e.g. scurvy, allergic vasculitis

Question 24

Give examples of platelet disorders

Answer 24

Low platelet count (thrombocytopenia)

Drug-induced

Question 25

What are the 3 mechanisms from which thrombocytopenia can arise

Answer 25

Failure to produce platelets Shortened platelet half life Increased pooling of platelets in an enlarged spleen

Question 26

What are the cutaneous features of immune thrombocytopenia

Answer 26

Purpura Multiple bruises Ecchymoses

Question 27

Where are clotting factors produced

Answer 27

Liver - most Endothelial cells - VWF, TM, TFPI Megakaryocytes - VWF, FV

Question 28

Describe the coagulation cascade

Answer 28

1. Tissue Factor, exposed by vessel damage, forms a complex with FVII 2. Formation of an active TF-VII complex 3. Activation of the extrinsic pathway via FX -> FXa 4. Activation of the intrinsic pathway via FIX -> FIXa 5. Xa leads to conversion of prothrombin -> thrombin (IIa) 6. Activation of FV and FVIII by thrombin and platelets 7. Enhanced thrombin formation 8. Thrombin then converts fibrinogen to fibrin, which is cross-linked by the XIIIa enzyme (activated by thrombin) to form cross-linked fibrin

Question 29

Where are tissue factors highly expressed

Answer 29

``` Lungs Brain Heart Testis Uterus Placenta ```

Question 30

Draw a diagram that represents the initiation of coagulation

Answer 30

-

Question 31

Describe how coagulation is regulated (and which each mechanism inhibits)

Answer 31

Antithrombin = FXa, FIXa and thrombin Tissue factor pathway inhibitor = TF-VIIa + FXa Protein C + protein S = FVa, FVIIIa

Question 32

Describe the protein C pathway

Answer 32

1. Thrombin binds to TM with high affinity 2. Serine protease activates protein C 3. Protein C becomes localised to endothelial surface 4. Thrombin cleaves protein C -> activation peptide released 5. Activates protein C zymogen to APC 6. release of APC so PC can bind to EPCR 7. APC down-regulates thrombin generation

Question 33

What is warfarin and how does it work

Answer 33

Anti-coagulant - long-term anticoagulation after venous thrombosis and for atrial fibrillation patients Inhibits thrombin production by inhibiting vitamin K epoxide reductase

Question 34

Explain how inhibition of vitamin K epoxide reductase (warfarin) causes an anticoagulant effect

Answer 34

Before factors II, VII, IX, and X can bind to platelet membrane phospholipids, their glutamate residue needs to be gamma-carboxylated in the liver, using vitamin K.

Question 35

What is heparin and how does it work

Answer 35

an anticoagulant Accelerates the action of antithrombin Heparin binds to antithrombin and makes it easier to bind thrombin/Xa. Heparin is used for immediate anticoagulation in venous thrombosis or pulmonary embolism.

Question 36

Describe fibrinolysis

Answer 36

1. Plasminogen is converted to plasmin via tissue plasminogen activator (tPa) on the fibrin clot surface 2. Plasmin breaks down fibrin clot 3. Fibrin degradation products are produced

Question 37

What are the therapeutic uses of tPA

Answer 37

Thrombolysis for: Myocardial infarction Ischaemic stroke

Question 38

Give examples of anti platelet agents

Answer 38

Aspirin (COX-1) | P2Y12 blockers

Question 39

What are the 3 ways of monitoring blood coagulation

Answer 39

``` Activated partial thromboplastin time (APTT) Prothrombin time (PT) Thrombin time (TT) ```

Question 40

What is the Activated partial thromboplastin time (APTT)

Answer 40

This tests the intrinsic and common pathways. Coagulation is initiated via factor XII and any abnormalities can be detected. Used to monitor heparin treatment.

Question 41

What is prothrombin time

Answer 41

This tests the extrinsic and common pathways. Coagulation is initiated via tissue factor and any abnormalities can be detected. Used to monitor warfarin treatment.

Question 42

What is thrombin time

Answer 42

This tests the common pathway only. Thrombin is added and the conversion of fibrinogen to fibrin can be checked for abnormalities. Also known as thrombin clotting time (TCT).