Blood Clotting Pathways

Question 1

What are the 3 steps of Haemostasis?

Answer 1

• 1. VASCULAR SPASM
• 2. FORMATION OF A PLATELET PLUG
• 3. BLOOD COAGULATION (CLOTTING)

Question 2

VASOCONSTRICTION of blood vessels after injury? (Vascular spasm)

Answer 2

• Muscle cells contract
• Constriction reduces blood flow, limits blood loss
• Opposing endothelial surfaces are pressed together and adhere on contact
• This process is mediated by the platelet-derived products serotonin and thromboxane A2 (TXA2)

Question 3

Label Vessel wall structure…

Answer 3

Question 4

What are platelets (also called thrombocytes)?

Answer 4

• they are small fragments derived from bone marrow cells called megakaryocytes
• contain high concentrations of actin & myosin, therefore can contract
• under normal conditions, platelets do not stick to blood vessel endothelium

Question 5

What 4 things can platelets be activated by?

Answer 5

1. ADP - released by platelets, erythrocytes & endothelial cells
2. COLLAGEN – in the connective tissue, exposed by vessel damage
3. THROMBIN – made by enzymatic cleavage of prothrombin
4. PAF – platelet activating factor (phospholipid) from the vessel wall & other cells

Question 6

What happens in the formation of the platelet plug?

Answer 6

• Platelet activation is followed by their ADHESION to the vessel wall
• The platelets AGGREGATE and build up an occlusive PLATELET MASS
• This mass forms the PRIMARY HAEMOSTATIC PLUG
• Plug may be WASHED AWAY by local blood flow, when vasoconstriction eases
• The plug must be STABILISED by FIBRIN

Question 7

What happens in the 2 pathways of blood coagulation and what happens in the final common pathway?

Answer 7

INTRINSIC PATHWAY: activated when FACTOR XII contacts COLLAGEN, exposed by vessel damage

EXTRINSIC PATHWAY: activated by tissue damage which exposes flowing blood to a protein called TISSUE FACTOR (TF)

FINAL COMMON PATHWAY: Conversion FIBRINOGEN to FIBRIN

Question 8

What is Fibrin?

What catalyses fibrinogen to fibrin?

Answer 8

FIBRIN

• insoluble
• forms a loose, mesh that traps blood cells
• generated from FIBRINOGEN (a large, soluble plasma protein synthesised by the liver)

THROMBIN (KEY ROLE)

• catalyses conversion of fibrinogen to fibrin
• activates Factor XIII

Question 9

What is Fibrinolysis?

What stops blood from always clotting?

Answer 9

Fibrinolysis – Dissolving the Clot

• FIBRINOGEN is always present in blood but THROMBIN must not be present, otherwise blood would always clot

Question 10

What are the 4 overlapping phases of wound healing?

Answer 10

1. Rapid Haemostasis
2. Appropriate Inflammation
3. Proliferation
4. Tissue Remodelling