Haemostasis

Question 1

What is haemostasis?

Answer 1

process whereby haemorrhage (bleeding) following vascular injury is arrested.

Question 2

What does haemostasis depend on?

Answer 2

 blood vessel wall,
 circulating platelets
 coagulation factors

Question 3

How does an intact cell wall prevent haemostasis through endothelial cells?(4)

Answer 3

 Prostacyclin: - this causes vasodilation and inhibits platelet aggregation

 Antithrombin & Protein C activator: - both inhibit coagulation

 Tissue plasminogen activator: activates fibrinolysis

 Von Willebrand
factor (vWF), which can bind platelets

Question 4

What are the membrane receptors found in the membrane of platelets?

Answer 4

 GPIa/IIb complex & GPVI which are receptors for collagen

 Glycoproteins (GPIb & IIb/IIIa allows attachment of platelets to vWF, then to endothelium.

Question 5

What is vasoconstriction?

Answer 5

 Immediate vasoconstriction of the injured vessel and reflex constriction of adjacent small arteries and arterioles - responsible for an initial slowing of blood flow to the area of injury

 reduced blood flow allows contact activation of platelets and coagulation factors

Question 6

Describe platelet plug formation

Answer 6

1. subendothelial collagen is exposed.
2. von Willebrand Factor (vWF) is released causing the platelets to change form and adhere to the subendothelial collagen via GPIa/IIb.
3. which activates it.
4. platelets change their shape from a disc to a more rounded form and they release the contents of their granules (platelet release reaction),
5. the most important content of which is ADP. The platelets are also stimulated to produce the prostaglandin, thromboxane A 2 , from arachidonic acid derived from the cell membrane.
6. The release of ADP and thromboxane A 2 causes an interaction of other platelets with the adherent platelets and with each other (secondary platelet aggregation), thus leading to the formation of a platelet plug

Question 7

Describe the coagulation cascade(5)

Extrinsic pathway

Answer 7

1. Coagulation begins when tissue factor activated on the surface of injured cells binds and activate factor VII
2. Recruitment of other clotting factors and activation of platelets (amplification). Factor 7 activates Factor 10 which turns prothrombin to thrombin
3. Huge burst of thrombin from activated platelets change fibrinogen into fibrin (propagation).
4. Intrinsic pathway activated by tissue damage and extrinsic pathway activated by surface contact.
5. Each reaction requires phospholipid and specific co-factors, and calcium to bind them.

Question 8

How is coagulation regulated? (3)

Answer 8

 Anti-thrombin - an important inhibitor of terminal proteins of cascade especially FXa and thrombin (Heparin potentiates its action markedly)

2. Protein C and S.- Protein C is vit K-dependent and inactivates the cofactors of Va and VIIIa, and stimulates fibrinolysis. Protein S acts as cofactor for C.
3. Tissue Factor pathway inhibitor (TFPI) - inactivates factor Xa and then the TFPI/FXa complex inhibits FVIIa

Question 9

What is fibrinolysis?(3)

Answer 9

 Fibrinolysis (like coagulation) is a normal haemostatic response to vascular injury.
 is the enzymatic breakdown of fibrin in blood clots
 fibrin is degraded by plasmin

Question 10

Describe the steps of fibrinolysis

Answer 10

1. Plasminogen (precursor) is activated by tissue plasminogen activator (t-PA).
2. Produces plasmin, an enzyme which metabolises clots.
3. Fibrin degradation products (FDP), such as D dimers from the degradation of cross-linked fibrin, are produced.

Question 11

What is thrombosis?(3)

Answer 11

 the pathological process whereby platelets and fibrin interact with the vessel wall to form a haemostatic plug to cause vascular obstruction
 may be arterial, causing ischaemia
 or venous, leading to stasis

Question 12

What can thrombosis lead to?(4)

Answer 12

underlies ischaemic heart, cerebrovascular and peripheral vascular diseases, venous occlusion and pulmonary embolism.

Question 13

How can defective haemostasis arise?(3)

Answer 13

 A vascular disorder
 Thrombocytopenia or a disorder of platelet function
 Defective blood coagulation

Question 14

Summarise the screening tests

Answer 14

 Prothrombin Time (PT)
 Activated Partial Thromboplastin Time (APTT)
 Thrombin (clotting) time (TT)

Question 15

What is Prothrombin Time (PT) ?(2)

Answer 15

 measures factors VII, X, V, prothrombin (II) and fibrinogen
 Tissue thromboplastin (a brain extract) or [synthetic] tissue factor with lipids and calcium is added to citrated plasma

Question 16

What is Activated Partial Thromboplastin Time (APTT)?(2)

Answer 16

 Measures factors VIII, IX, XI and XII in addition to factors X, V, prothrombin (II) and fibrinogen.

 Three substances – phospholipid, a surface activator (e.g. kaolin) and calcium are added to citrated plasma

Question 17

What is Thrombin (clotting) time (TT)?(2)

Answer 17

 sensitive to a deficiency of fibrinogen or inhibition of thrombin
 adding thrombin to plasma and measuring the time taken to clot, which is prolonged when there is an inherited or acquired deficiency of fibrinogen, or an inherited or acquired abnormal fibrinogen molecule luted bovine thrombin is added to citrated plasma

Question 18

What us fibrinogen quantitation?

Answer 18

Measures fibrinogen deficiency

Question 19

What is DIC?

Answer 19

disseminated intravascular coagulation

Question 20

What is FDP?

Answer 20

• Fibrin degradation products (FDPs), also known as fibrin split products, are components of the blood produced by clot degeneration. Clotting, also called coagulation, at the wound site produces a mass of fibrin

Question 21

What are the principles of clotting?(5)

Answer 21

	Incubate plasma with reagents necessary for coagulation
	Phospholipid, co-factors
	Trigger or activator
	Calcium
	Measure time taken to form fibrin clot

Question 22

VWF synthesis and storage

Answer 22

Synthesis
→Endothelial cells contain Weibel Palade bodies

→Megakaryocytes
→Platelet a granules

→Plasma VWF entirely derived from endothelial cells

Question 23

Distribution of VWF

Answer 23

→Constitutive path (95%)
→Regulated path (5%)
→Weibel-Palade bodies (storage granules of endothelial cells)

Question 24

describe Initiation of coagulation

Answer 24

The TF leads to the production of a small local amount of thrombin, which is the initiation step of the coagulation process

Question 25

what do activated platelets present?

Answer 25

→negatively-charged phospholipid membrane at the site of the injury →on which the process of coagulation (secondary haemostasis) can occur, if needed

Question 26

how does a tissue factor drive coagulation?

Answer 26

→TF is outside the lumen →Formation of TF-FVIIa complex → Recruitment of FX and formation of thrombin

Question 27

what happens after FVII is activated?

Answer 27

→the complex binds small amounts of FX and FV to the exposed endothelial surface, → produce small quantities of thrombin

Question 28

what do the activated factors (among them FVIII and FIX) enable the binding of ?

Answer 28

enable the binding of activated FX and FV to the surface of platelets whose activation has produce conformational changes in their surface membranes to expose the ‘reaction sites’ necessary for continuation of the process

Question 29

what are the key players in fibrinolysis?

Answer 29

→Plasminogen → Tissue plasminogen activator (t-PA) & urokinase (u-PA) →Plasminogen activator inhibitor -1 and -2 α2-plasmin inhibitor

Question 30

what do the activated factors (among them FVIII and FIX) enable the binding of ?

Answer 30

→enable the binding of activated FX and FV to the surface of platelets whose activation has produce conformational changes in their surface membranes to expose the ‘reaction sites’ necessary for continuation of the process

Question 31

what are the simplified steps of haemostasis?

Answer 31

``` →Tissue injury →Vasoconstriction →Platelet activation →Haemostatic plug →Coagulation →Stable clot formation →Clot dissolution ```

Question 32

Primary haemostasis:

Answer 32

→Vasoconstriction (immediate) →Platelet adhesion (within seconds) →Platelet aggregation and contraction (within minutes)

Question 33

Secondary haemostasis:

Answer 33

→Activation of coagulation factors (within seconds) →Formation of fibrin (within minutes)

Question 34

Intrinsic pathway:

Answer 34

1. Factor 12 is activated by prekallikrein 2. 12a activates 11 3. 11a activates 9 4. Factor 10 activates converts Prothrombin to thrombin 5. Thrombin turns fibrinogen to fibrin