4.1 Normal Coagulation

Question 1

[Platelets]
Platelets form a plug by adherence (binding to the subendothelial matrix), activation, and aggregation (binding to each other):
• Anucleated fragments of the multi-nucleated megakaryocyte which resides in the __________ and extends pro-platelets to the __________
• Platelets then fragment into the circulation (each megakaryocyte producing 4000 platelets, each with a lifespan of _________)

Platelets are highly organised and rich in organelles with ________________________ (provides platelets with energy):
• _______________ under the cell surface helps to maintain the discoid shape of a resting platelet and facilitates a shape change during activation
• Contains Microtubule network (marginal band) with an dense granules and α granules (allows granule contents to be discharged into it and released)
• ______________ allow the platelets to stick to the damaged endothelium and also to each other

• Dense granules: Small non-protein molecules like _______________________ (needed for aggregation of platelets)
• α granules: Large number of proteins involved in haemostasis, inflammation, wound repair, including _____________________________

Answer 1

bone marrow;

endothelial wall;

10 days;

glycogen granules and mitochondria;

microtubule network (marginal band);

open canalicular system;

surface glycoproteins;

ADP, ATP, serotonin and Ca2+;

growth factors (e.g. TNF, PDGF, VEGF), fibrinogen, factor V and vWF

Question 2

[PRIMARY HAEMOSTASIS]

Adhesion
- Platelets bind to the subendothelial matrix of damaged blood vessels using the surface glycoproteins ___________
• ________________ is exposed upon vessel damage, allowing vWF to stick directly to it, then platelets can bind to vWF via __________
• Platelets can also stick directly to the collagen in the subendothelium via ________

Answer 2

GpIa or GpIb;

Subendothelial matrix;

GpIb;

GpIa;

Question 3

Activation
- After platelet adhesion to the subendothelial matrix, they are activated, causing a shape change, altered ______________ , and release of chemicals
- Shape: Platelets ______________, which increases the surface area of contact with the subendothelium (conversion from a passive to activated cell)
- Surface glycoprotein expression: New surface glycoproteins _____________ are activated on the cell surface (binds to fibrinogen and links platelets to each other)
- Release of chemicals (e.g. ADP, thromboxane A2): Production of thromboxane A2 occurs in the ___________________:
• ________________ is converted to cyclic endoperoxides by cyclo-oxygenase (COX)
• Cyclic endoperoxides are converted into thromboxane A2 (by thromboxane synthetase in platelets) or prostacyclin (by prostacyclin synthetase at vessel wall)
• ___________ induces platelet aggregation, while __________ inhibits platelet aggregation

Answer 3

surface glycoprotein expression;

spread out and flatten;

GpIIb and GpIIIa;

platelets from the membrane phospholipids;

Arachidonic acid;

Thromboxane A2;

prostacyclin

Question 4

Aggregation
- The release of _______________________, coupled with thrombin (factor II), stimulate platelets to aggregate and form an unstable primary haemostatic plug:
• Link to each other via fibrinogen (divalent molecule linking one platelet to another) linking to _____________ (links platelets to fibrinogen) and ________ (multivalent)

Answer 4

ADP and prostaglandins (thromboxane A2);

GpIIb/IIIa ;

vWF

Question 5

SECONDARY HAEMOSTASIS
Secondary haemostasis involves the coagulation cascade leading to the formation of thrombin and the conversion of fibrinogen to fibrin:
• _________ helps convert of fibrinogen to fibrin (forming the final clot), activate cofactors of coagulation, activate anticoagulants and promote platelet activation
• Both the intrinsic and extrinsic pathways lead to the activation of _________ and thus the common pathway and eventually crosslinking of fibrin:
o Small amount of protein leads to generation of large amounts of proteins at the end (cross-linked fibrin)

The intrinsic pathway starts with a contact activation system of factor XII to XIIa:
• Factor XIIa then activates Factor XI to XIa, then Factor IX to IXa
• The conversion of Factor X to Xa (common pathway) requires the tenase complex (_________________) apart from Factor IXa

The extrinsic pathway starts when damaged vessels release tissue factor (TF), which combines with Factor VIIa to activate ___________:
• All coagulation is initiated by TF released during vessel damage in the human body (only extrinsic pathway occurs → physiological cascade)

The common pathway involves the conversion of Factor X to Xa, which then activates prothrombin (factor II) to thrombin (IIa):
• Thrombin converts inactive fibrinogen (I) to insoluble fibrin (Ia), which is then crosslinked by ________
• Conversion of prothrombin to thrombin requires the prothrombinase complex (______________________)

Both the intrinsic and extrinsic pathways require phospholipids and calcium (Ca2+) for the coagulation cascade to work:
• Phospholipids are provided by the ______________ in vivo, and must be added for in vitro coagulation
• Contributes to the formation of the ________________

Answer 5

Thrombin;

factor X;

Factor VIIIa, phospholipids, Ca2+;

Factor X;

factor XIIIa;

Factor Xa, cofactor Va, phospholipid, Ca2+;

platelet surface;

tenase complex and prothrombinase complex

Question 6

PHYSIOLOGICAL CASCADE
The physiological cascade is divided into an initiation phase and a propagation phase:

Initiation: ________________ activates factor X to Xa directly, and also activates factor IX to IXa (which also activates factor X):
• Factor Xa then converts prothrombin to thrombin
• Small amounts of _________ are formed when TF is released

Propagation: Thrombin then activates cofactors factor V and VIII to Va and VIIIa respectively, and factor XI to Xia
• Produces a huge amount of thrombin (including those produced from factor Xia in the intrinsic pathway)

Inhibitors of the physiological cascade include tissue factor pathway inhibitor (TFPI), antithrombin, and protein C and S

• TFPI: Inhibits the ___________________
• Antithrombin: Directly inactivates ______________

Protein C/S
- Inactivates factor Va and VIIIa (blocks cofactors) and thus indirectly inhibits _____________:
• Protein C is activated by thrombin (increased when thrombin binds to cofactor ______________) → negative feedback
• Thrombomodulin is a glycoprotein on the surface of ___________ which modifies the substrate specificity of thrombin
• Acts with cofactor protein S
*Patients with antithrombin deficiency or protein C/S deficiency are more likely to have thrombosis.

Answer 6

Tissue factor and factor VIIa;

thrombin;

TF/factor VIIa complex and factor Xa;

thrombin and other clotting proteinases (e.g. factor Xa);

thrombin production;

thrombomodulin;

endothelial cells

Question 7

FIBRINOLYSIS As fibrin is made, it is dissolved by plasmin via fibrinolysis:
• Plasminogen (precursor of plasmin) exists in the general circulation, and is activated by tissue plasminogen activator (tPA) to form plasmin
• Plasmin breaks down the fibrin clot to form fibrin degradation products (FDPs)
• tPA and _________________are used in therapeutical thrombolysis for myocardial infarction (clot busters)

There are 2 main systems which limit the amount of plasmin in the circulation:
- Antipasmin: Directly inhibits plasmin to prevent clot breakdown
- tPA inhibitors (PAI-1, PAI-2)
Plasminogen activator inhibitors (PAIs) inhibit tPA to prevent the conversion of _______________
• PAI-2 is made in the ________________

Answer 7

streptokinase (bacterial activator) ;

plasminogen to plasmin;

placenta

Question 8

HAEMOSTATIC BALANCE
There must be a balance between fibrinolytic factors and anticoagulant proteins, and coagulation factors and platelets to prevent bleeding or thrombosis:
• Most coagulant, fibrinolytic, and anticoagulant factors are synthesised in the liver, so liver diseases can lead to coagulation problems (bleeding, clotting)
• Some coagulation factors (e.g. vWF) are made in ____________, and others are made in _________________ (e.g. vWF, factor V)

Bleeding tendency

• Excess fibrinolytic factors or anticoagulant proteins
• Decreased coagulation factors and platelets

Clotting tendency (thrombosis)

• Decreased fibrinolytic factors or anticoagulant proteins
• Excess coagulation factors and platelets

Answer 8

endothelial cells;

megakaryocytes and platelets;