Haemostasis

Question 1

What is haemostasis?

Answer 1

Haemostasis is the prevention of blood loss - stopping of bleeding.

Mechanisms:
1 - Vasoconstriction
2 - Formation of the platelet plug
3 - Blood coagulation via formation of blood clot
4 - Growth of fibrous tissue into the blood clot to close the hole

Question 2

Explain vascular constriction

Answer 2

Local myogenic spasm.
Local autacoid factors from the tissue and platelets.
nervous reflexes initiated by pain.

Question 3

Explain how the platelet plug is formed.

Answer 3

The platelet does not attach to normal endothelium, only to damaged (rough) endothelium.
When the platelet comes in contact with a damaged vascular surface (especially collagen in the damaged wall) it:
- Swells and changes its form.
- Actin and myosin contracts which releases granules with active factors.
- They become sticky so that they attach to collagen and to von Willebrand factors form the plasma.
- They release ADP and Thromboxane that activates nearby platelets.
- Later, fibrin due to blood coagulation attaches to the platelet plug.

The formation of the platelet plug is very important for the stopping and repairing of the many small holes in the small vessels made daily.

Question 4

Explain blood coagulation.

Answer 4

After major damage, clot forms after 15-20 seconds.
After minor damage, clot forms after 1-2 minutes.

Activator substances from are released from the:

• Vascular wall
• Platelets
• Blood proteins adhering to the vascular wall

If rupture is not too large, it will be filled with a clot within 3-6 minutes.
After 20 minutes to an hour, the clot will retract.

Question 5

Explain the mechanisms of blood clotting.

Answer 5

The damage of the endothelium starts a cascade of chemical reactions involving the clotting factors.
The net result is a complex called prothrombin activator.
The prothrombin activator catalyses conversion of prothrombin into thrombin.
The thrombin acts as an enzyme to convert fibrinogen into fibrin fibres, that will form the clot.

Question 6

Explain the conversion of prothrombin into thrombin.

Answer 6

Prothrombin activator is formed as a result of the rupture of a blood vessel or damage to the blood.
If sufficient Ca2+ is present, prothrombin activator will catalyse conversion of prothrombin into thrombin.
The thrombin causes polymerisation of fibrinogen into fibrin fibres.
Platelets are important for the conversion of prothrombin into thrombin because much of the prothrombin attaches to prothrombin receptors on the platelets that are attached to the ruptured wall.

The reactions after the formation of prothrombin activator are very fast, so the factor that is the bottle neck is the formation of prothrombin activator.

Question 7

Explain the conversion of fibrinogen into fibrin.

Answer 7

Fibrinogen is a protein formed in the liver.
Thrombin converts fibrinogen into fibrin monomer that will polymerise into fibrin fibres.
Fibrin stabilising factor from the platelets will make the fibres stronger.
Thrombin activates this process as well.

Question 8

Explain the positive feedback system of coagulation.

Answer 8

Once clotting begins, it will within minutes spread to the surrounding blood, that will promote more clotting.
This is mostly due to thrombin.
When thrombin is activated, it will work backwards on prothrombin so that even more thrombin is formed.

Question 9

Explain the intrinsic pathway of the coagulation cascade.

Answer 9

Trauma to blood or exposure of blood to collagen from traumatised vessel wall activates the intrinsic pathway.
Intrinsic pathway is slower, and takes 1-6 minutes to cause clotting.
Blood trauma or contact with collagen activates Factor XII and release of phospholipids from platelets.
Factor XII activates Factor XI.
Factor XI activates Factor IX.
Factor IX together with Factor VIII and phospholipids activates Factor X.
Factor X combines with Factor V and phospholipids to form prothrombin activator.
This step is the same as in the extrinsic pathways and from here the process is the same.

Ca2+:
Except for the first two steps of the intrinsic pathway, Ca2+ is requires in all blood clotting reactions.
Low Ca2+ concentrations will inhibit clotting.
–> If one wants to inhibit clotting, citrate ion or oxalate ion can be added which will reaction and bind the Ca2+.

Question 10

Explain the role of Ca2+ in the coagulation cascade.

Answer 10

Ca2+:
Except for the first two steps of the intrinsic pathway, Ca2+ is requires in all blood clotting reactions.
Low Ca2+ concentrations will inhibit clotting.
–> If one wants to inhibit clotting, citrate ion or oxalate ion can be added which will reaction and bind the Ca2+.

Question 11

Explain anticoagulation

Answer 11

The endothelial surface:

• Most important factor
• The smoothness prevents activation of intrinsic pathway
• The glycocalyx repels clotting factors and platelets

Protein C:
- Inactivates activated Factos V and Factos VIII

Antithrombin III (antithrombin-heparin cofactor): 
- Binds (deactivates) thrombin 

Heparin:
- Normally in low concentrations in the blood
- It will combine with antithrombin III which will make the antithrombin 100-1000 times more effective
- Also removes Factor XIII, XI, X and IX
- Produced in mast cells and basophilic granulocytes
Many of these in the lung capillaries and also in liver

Blood flow:
- Reduces the chance of interaction of factors

Question 12

Explain the International Standard Ratio test

Answer 12

INR:
Tests the extrinsic pathway, with coumarin.
INR=time of patient/standardised time.

1) Plasma is decalcified (with oxalate or citrate)c
2) Tissue thromboplastin (tissue factor)
3) Sample is re-calcified (clotting initiated)
4) Clotting should take 15-20 seconds.
5) Results of 0.9-1.1 are normal.

Exact temperature is not vital for INR test, since ratio is used.

Question 13

Explain the APTT test

Answer 13

Activated PT time test. 
Test intrinsic pathway with heparin. 
Plama (decalcified with EDTA) 
Moniter hepatin 
PT (partial thromboplastin = fake thromboplastin) 
Calcify 
Should take 40-50 seconds to form clot. 

Exact temperature is important for APTT test.

Question 14

When are clotting tests (INR and APTT) appropriate?

Answer 14

Before surgery, after birth, during anticoagulation therapy (e.g. after thrombosis)

Question 15

What are some anticoagulants?

Answer 15

Blocking Ca2+ (EDTA, oxalate, citrate):
- Lethal

Anti-vitamin-K (coumarin):

• Vitamin K is a cofactor that carboxylates the clotting factors.
• Rat poison

Heparin:

• A GAG
• A cofactor that pairs with antithrombin III, which stops thrombin production
• Also stops other clotting factors