Platelets

Question 1

Define haemostasis.

Answer 1

A rapid, local prevention of bleeding in damaged blood vessels

Question 2

Describe the first three steps in haemostasis.

Answer 2

1. Vascular spasm/vasoconstriction
   (blood vessel is constricting)
   - Occurs in small blood vessels or arterioles
   - After small blood vessel damage the surrounding smooth muscle contracts and slows blood flow through the site of injury
   - Blood loss is reduced for 20-30 minutes to allow for the next 2 stage of haemostatics to occur. The greater the tissue damage the more effective the vascular spasm
   - Platelets increase vascular spasm by releasing chemicals
2. Platelet plug formation
   - Platelets stick to the exposed collagen fibres which then cause activation of the platelets
   - Chemicals released from platelets recruit more platelets to the area in the process called aggregation
3. Coagulation (blood clotting)
   - Reinforcement of the platelet plug with fibrin threads that form a mesh
   - Blood cells are trapped effectively sealing the hole until repair can occur
   This process transforms blood from a liquid to a gel

Question 3

Clot retraction

Answer 3

Tightens the clot pulling the damaged vessels edges together
Fibrinolysis - is the process of dissolving the clot following repair

Question 4

Describe the inactivation and activation of clotting factors in the process of coagulation.

Answer 4

Coagulation is a multi-step process involving clotting factors
- They remain inactive when circulating in blood
- Activation turns them into an enzyme

Question 5

Briefly describe the coagulation cascade to include the intrinsic, extrinsic, and common pathways.

Answer 5

Extrinsic - Triggered by factors following exposure of blood to damaged tissue outside blood vessels

Intrinsic - Triggered by factors following damage inside the lumen of blood vessel

Common - “X factor” Prothrombinase converts prothrombin to thrombin, which then converts fibrinogen into…Fibrin Clot

Both the extrinsic and intrinsic pathways require Ca2+

Question 6

Describe how blood clots are dissolved (fibrinolysis).

Answer 6

When plasminogen is activated (by t-PA), the plasmin can then dissolve the clot
The process of dissolving a clot - helps to prevent blood vessels from blocking inappropriately.

Question 7

State a common anticoagulant and how it affects coagulation.

Answer 7

Aspirin: works by stopping platelet plug formation
Warfarin: works by inhibiting vitamin K
Heparin: a natural anticoagulant that inhibits thrombin

Question 8

Identify the role of vitamin K and Ca2+ in coagulation.

Answer 8

Vitamin K - is needed for the synthesis of some of these clotting factors
Ca2+ - is a non-enzymatic clotting factor