Thrombosis

Question 1

Why does coagulation occur (as in, what is its purpose)?

Answer 1

Coagulation prevents blood loss.

Inflammation activates coagulation and coagulation promotes inflammation. Coagulation is an immunological response.

Question 2

Describe both arterial and venous thrombosis.

Answer 2

ARTERIAL THOMBOSIS:

• they mostly result from an atheroma rupture or damage to the endothelium (eg. MI, stroke)
• it is a platelet-rich, ‘white’ thrombosis; it’s mostly primary haemostasis
• it may block downstream arteries

VENOUS THROMBOSIS:

• it often results from stasis or a hyper-coagulant state (eg. DVT)
• it is a platelet-poor, ‘red’ thrombus; it is mostly secondary haemostasis
• it may move to the lungs

Question 3

List some factors/substances that affect the coagulation-fibrinolysis balance.

Answer 3

• tissue plasminogen activator: initiates fibrinolysis
• von Willebrand factor: activates platelets
• tissue factor: initiates clotting
• antithrombin: inhibits clotting
• prostaglandin I2: inhibits platelets
• nitric oxide: inhibits platelets

Question 4

What is Virchow’s Triad, and what are its three components?

Answer 4

It describes the three categories that are thought to contribute to thrombosis. These three categories are:

• STASIS: static blood lacks kinetic energy and tends to clot
• HYPER-COAGULANT STATE: eg. infection, hereditary, drugs (eg. HRT)
• ENDOTHELIAL DAMAGE: eg. surgery or cannula

Question 5

How are the valves involves in stasis?

Answer 5

Blood tends to eddy around the valves, increasing the risk of stasis.

Question 6

What are the four possible fates of a thrombus?

Answer 6

• RESOLUTION: where the fibrinolytic system destroys the whole clot over time
• EMBOLISM: where the thrombus dislodges and goes to the heart/lungs, causing death
• ORGANISED: where the endothelial cells just grow over the clot; this makes the person more prone to having another one due to the narrower vein
• RECALANISED AND ORGANISED: where the thrombus is so big you can’t grow over it, but you can grow through it

Question 7

What is the difference between a proximal DVT and a distal DVT?

Answer 7

With a proximal DVT, there is a higher risk of a pulmonary embolism and post-thrombotic syndrome (causing pain, swelling, and sometimes ulcers).

With a distal DVT, it rarely causes a pulmonary embolism or post-thrombotic syndrome.

Question 8

What does a platelet release when it has been activated?

Answer 8

The activated platelet releases thromboxane A2 (TxA2) and adenosine diphosphate (ADP); these induce receptors for fibrinogen.

Question 9

Describe the common pathway.

Answer 9

The common pathway pathway reactions run at a trickle and are easily overpowered by inihibitors.

1) Factor IXa activates Factor X by proteolysis to create Factor Xa.
2) Factor Xa (FXa) cleaves prothrombin to form thrombin (FIIa).
3) Thrombin (FIIa) is a protease that cleaves fibrinogen into fibrin. Fibrinogen is a large molecule that’s present in plasma - once cleaved, it becomes insoluble fibrin.
4) Thrombin cleaves Factors V and VIII to give FVa and FVIIIa. This is known as amplification, as FVIIIa and FVa amplify the existing reactions, making them harder to overpower.
5) FVa and FVIIIa together with the plasma Ca2+ form the:
- Tenase complex - FVIIIa + FIXa = FXa
- Prothrombinase complex - FVa + FXa = thrombin
These complexes assemble on the charged phospholipid surfaces in the activated platelets.

Fibrinogen also promotes blood clotting by forming bridges between, and activating, blood platelets through binding to their GpIIb/IIIa surface membrane fibrinogen receptor.

Question 10

Describe the prothrombinase complex.

Answer 10

The negative surface of the activated platelet causes calcium, prothrombin and Factor Xa and Va to bind. This makes the prothrombinase complex. These components all bind by a particular domain of glutamic acids (GLA); they stabilise the complex.
Forming these GLA domains is Vit K dependent and can be inhibited by warfarin (rat poison).