Haematology: Blood Coagulation, Haemostasis & Its Investigation Flashcards
(46 cards)
1
Q
What is haemostasis?
A
- Protective process designed to stop bleeding and maintain blood flow
2
Q
The circulatory system of the horseshoe crab contaisn haemolymph. What does haemolymph contain?
A
- Contains amebocytes which contain:
- Proteins of the coagulation system
- Proteins & peptides of the immune system
3
Q
How does the fact that the horseshoe crab’s circulatory system is made up of haemolymph affect the way it fights infection?
A
- It means that in order to fight infection coagulation of that haemolymph would also have to occur
4
Q
What are some of the functions of haemostasis?
A
- Respond to tissue injury
- Curtail blood loss
- Restore vascular integrity & promote healing
- Limit infection
5
Q
Name the 4 main components of haemostasis
A
- Endothelium
- Coagulation
- Platelets
- Fibrinolysis
6
Q
What are the components of a thrombus (blood clot)?
A
- Fibrin mesh
- Platelets
- Red blood cells
7
Q
Name the 3 different stages of haemostatsis and state what occurs during each stage
A
-
Primary haemostasis
- Vasoconstriction
- Platelet adhesion
- Platelet aggregation and contraction
-
Secondary haemostasis
- Activation of coagulation factors
- Formation of fibrin
-
Fibrinolysis
- Activation of fibrinolysis
- Lysis of the plug
8
Q
What are the different stages of blood clot formation?
A
- Initiation
- Amplification
- Stable clot
- Lysis
9
Q
Describe the process of primary haemostasis
A
- Damage to endothelium exposes sub-endothelium to flowing blood
- This results in sub-endothelial cells releasing Von willebrand factor (VWF)
- Von willebrand factor binds to platelets and causes them to adhere to exposed collagen
- Platelets also become activated as a result of this
- Activated platelets release various pro-thrombotic substances, e.g. thromboxane A2, which help to clump (aggregate) platelets together
- This forms a loose platelet plug
- Loose platelet plug contracts to form a dense, adherent plug
- Platelets in platelet plug form negatively charged phospholipid surface which is required for coagulation
- Allows coagulation factors to interact much better with each other
10
Q
Why is the phospholipid surface formed by the platelet plug required for secondary haemostasis (coagulation)?
A
- Allows coagulation factors to interact much better with each other
11
Q
When released by sub-endothelial cells von willebrand factor goes through a conformational change. Describe what this conformational change is and why it occurs
A
- As a result of increased shear stress due to endothelium damage when released, von willebrand factor goes from a globular form to a filamentous form
- This filamentous form von willebrand factor then begins to bind to platelets as well as exposed collagen
12
Q
Where are most of the coagulation factors produced?
A
- In the liver
13
Q
Name some of the coagulation factors and state their functions in coagulation (secondary haemostasis)
A
- Fibrinogen (I) - Forms clot (fibrin)
- Prothrombin (II) - Active form (IIa) activates factors I, V, VII, XIII, protein C and platelets
- Tissue factor (III) - Co-factor of VIIa
- Factor V - Co-factor of X with which it forms the prothrombinase complex
- Factor X - Activates prothrombin by forming prothrombinase complex with factor V
- Von willebrand factor - Binds to VIII, mediates platelet adhesion
14
Q
Explain how the original waterfall hypothesis describes secondary haemostasis (coagulation)
A
- States that there are 2 seperate pathways for coagulation - extrinsic and intrinsic pathway which convene to form common pathway
-
Extrinsic pathway
- Tissue damage causes factor VII to activate and become VIIa
- Activated factor VII (VIIa) binds to tissue factor (TF) to form a complex
- This complex then activates factor X to form activated factor X (Xa)
-
Intrinsic pathway
- Blood comes in contact with non-physiological surface
- This causes kallikrein to activate factor XII
- Activated factor XII (XIIa) forms complex with high-molecular weight kininogen (HMWK) which activates factor XI
- Activated XI (XIa) activates factor IX
- Activated factor IX (IXa) forms complex with activated factor VIII (VIIIa) which activates factor X
-
Common pathway
- Activated factor X (Xa) causes factor II (prothrombin) to activate and become IIa (thrombin)
- Thrombin then catalyses conversion of fibrinogen to Fibrin
15
Q
Deficiencies in particular clotting factors may or may not cause bleeding. Deficiencies in which factors cause bleeding?
A
- Factor VIII deficiency (Hemophilia A)
- Factor VII deficiency
16
Q
Deficiencies in which factors don’t cause bleeding?
A
- Factor XII deficiency
17
Q
The thrombin produced by the extrinsic pathway isn’t enough for it to catalyse the production of fibrin. How is the extra thrombin needed to produce fibrin produced?
A
- Extra thrombin produced via a thrombin burst
- This thrombin burst is caused as a result of the thrombin formed via the extrinsic pathway activating factors IX and XI
18
Q
How does the revised waterfall hypothesis differ in the way it describes the process of secondary haemostasis (coagulation) comapred with the original waterfall hypothesis?
A
- Revised waterfall hypothesis states that there aren’t seperate pathways that join together in clotting cascade but instead all the reactions are interlinked
- Also states that each reaction needs:
- Ca2+
- Phospholipid surface
- ± Specific co-factors
19
Q
Explain the cell-based model of coagulation
A
- Initiation of coagulation occurs when sub-endothelial tissue is exposed to the circulation at a site of injury.
- Sub-endothelial cells express tissue factor at their surface, which binds to endogenous activated Factor VII to form a complex
- This complex binds small amounts of Factor X and Factor V to the exposed endothelial surface, which produce small quantities of thrombin
- The thrombin activates platelets that are attracted to the site by the process, as well as other plasma-borne clotting factors
- The activated factors (among them Factor VIII and Factor IX) enable the binding of activated Factor X and Factor V to the surface of activated platelets
- The surface membranes of the activated platelets have gone through conformational changes which expose the ‘reaction sites’ necessary for continuation of the process
- Activated platelets with Va and Xa bound foes on to cause prothrombin to be converted to thrombin which leads to the ‘thrombin burst’
- This thrombin burst is necessary for the large-scale production of fibrin and so the development of an effective clot
20
Q
What are the main functions of fibrinolysis?
A
- Limiting blood clots
- Repair and healing
21
Q
Name the main components of fibrinolysis
A
- Plasminogen
- Tissue plasminogen activator (t-PA) & urokinase (u-PA)
- Plasminogen activator inhibitor -1 and -2
- α2-plasmin inhibitor
22
Q
Describe the process of fibrinolysis
A
- Tissue plasminogen activator (tPA) catalyses the reaction that converts plasminogen into plasmin
- Plasmin then degrades cross-linked fibrin of fibrin clot to produce D dimers
23
Q
Plasmin can also degrade non-cross linked fibrin or fibrinogen. What is formed as a result?
A
- Fibrin degradation products (FDP)
24
Q
How can tPA be used therapuetically?
A
- tPA and a bacterial activator, streptokinase, can be used in therapeutic thrombolysis for myocardial infarction
25
What role do anticoagulant proteins play in haemostasis?
* Anticoagulant proteins cause the inactivation of the coagulation factors
26
What is thrombosis?
* The formation of a blood clot inside a blood vessel resulting in the obstruction of blood flow
27
Imbalance of factors that contribute to haemostasis can lead to thrombosis. What
* Increased activation of coagulation factors/inability for coagulation factors to be deactivated
* Increased no. of platelets
* Decreased no. of fibrinolytic factors
* Decreased no. of anticoagulant proteins
28
What condition can thrombosis lead to?
* Deep vein thrombosis
29
What condition can deep vein thrombosis become if it worsens? What are the symptoms of this condition?
* Chronic venous insufficiency, symptoms include:
* **Atrophic changes**
* **Hyperpigmentation**
* **Ulceration**
* **Infection**
30
Imbalance of factors that contribute to haemostasis can also lead to bleeding. Describe the imbalances in these factors that lead to bleeding
* Absence of clotting factors or platelets
* Presence of too many fibrinolytic factors or anticoagulant factors
31
What are the principles of clotting tests?
* Incubate plasma with reagents necessary for coagulation
* Phospholipid, co-factors
* Trigger or activator
* Calcium
* Measure time taken to form fibrin clot
32
What is prothrombin time (PT)?
* It's a blood test that evaluates the extrinsic pathway and to a lesser extent common pathway
* It is Tissue Factor driven
33
What is activated partial thromboplastin time (APTT)?
* It's a blood test that evaluates the intrinsic pathway and to a lesser extent common pathway
* It is contact activated
34
What is thrombin time (TM)?
* It's a blood test that evaluates how quickly fibrinogen can be converted into fibrin
35
For each of the clotting tests state the average time it takes for them to be completed
* **Prothrombin time:** 11-13 seconds
* **Partial activated thrmoboplastin time:** 30-45 seconds
* **Thrombin time:** 12-14 seconds
36
What are the limitations of the clotting tests?
* They only test the initiation stage of the process of blood clot formation rather than the entire process
37
What anticoagulant is used when collecting blood for a clotting test?
* Blood is anticoagulated with 3.2 % (0.109 M) Sodium citrate
38
How much blood and how much anticoagulant is needed in order for the blood collected to be used in a clotting test?
* Most tubes contain 0.3 mL anticoagulant and require 2.7 mLs of blood.
39
Why is it important to fill the tube with the exact quantity of blood needed for a clotting test?
* Under filling the tube yields grossly inaccurate results.
40
What are some pre-analytical errors that can occur during a clotting test?
* **Problems with blue-top tube**
* Partial fill tubes
* Vacuum leak and citrate evaporation
* **Problems with phlebotomy**
* Heparin contamination
* Wrong label
* Slow fill
* Underfill
* Vigorous shaking
* **Biological effects**
* Heamatocrit ≥55 or ≤15
* Lipaemia, hyperbilirubinaemia, haemolysis
* **Laboratory errors**
* Delay in testing
* Prolonged incubation at 37°C
* Freeze/thaw deterioration
41
What are the 2 reasons why a clotting test time would be prolonged?
* Clotting factor deficiency
* Something else interferring with clot formation
42
How can you differentiate the reason for a prolonged clotting time?
* You perform a mixing study
* This involves mixing the prolonged clotting time plasma with control plasma at a 50/50 mix
43
What does it mean if you perform a mixing study and the time for a clotting test is no longer prolonged?
* It means the reason for the prolonged time of the clotting test was that there was a deficiency in a clotting factor/s
44
What does it mean if you perform a mixing study and the time for a clotting test is still prolonged?
* It means the reason for the prolonged time of the clotting test was the presence of an antibody or another thing interefering with clot formation
45
What is D-dimer testing?
* A test that measures levels of the D-dimer, a fibrin degradation product
46
What conditions are associated with increased D-dimer levels?
* Found elevated in the situation of enhanced fibrinolysis (Thrombosis, Disseminated intravascular coagulation)
