Haemostasis Flashcards
What is the difference between fibrinolytic factors and coagulation factors?
fibrinolytic - anticoagulant proteins stop clotting
Coagulation factors - platelets- cause clotting
What is haemostasis and what three ways is it achieved?
Halting of blood
- vasoconstriction
- Primary haemostasis - formation of unstable platelet plug
- Secondary haemostasis - formation of a stable fibrin clot (coagulation)
Fibrinolysis after
What is haemostasis important to know?
Diagnosing and treating bleeding disorders
identify risk factors for thrombosis
treat thrombotic disorders
Monitor the drugs used to treat bleeding and thrombotic disorders
control bleeding in individuals who don’t have an underlying bleeding disorder
What causes vasoconstriction?
Injury occurs
nerves attached to endothelial cells and thesmooth muscle cellsdetect the injury and trigger areflexivecontractionof thesmooth musclesnear site called vascularspasm >
vessel narrows, reduceblood flowand decrease blood loss through the damaged artery >
secretion ofnitric oxideandprostaglandinsdecreases
endothelial cells secrete a protein calledendothelin which causes thesmooth musclesto contract
What do endothelial cells release which causes smooth muscle cells to relax/contract?
endothelial cells normally secretenitric oxideandprostaglandinsinto the blood which cause nearbysmooth musclesto relax
Secrete endothelin to contract
State the 5 basic steps to primary haemostasis.
- Endothelial injury
- exposure
- adhesion
- activation
- aggregation
What is a platelet? How is it made
Discoid, non-nucleated, granule containing cells derived from myeloid stem cells
Formed in bone marrow by fragmentation of megakaryocyte cytoplasm
circulate for 10 days
Why are plasma membrane glycoproteins significant in homeostasis?
Important for platelet interactions
Once an endothelial injury occurs how do platelets bind to endothelium wall?
Collagen is exposed so platelets bind either…
- directly to collagen via the GP1a receptor
- Indirectly via VWF - released by endothelial cells and
platelets bind via their GP1b receptor
Both are glycoprotein receptors
What happens when platelets are activated?
their shape changes from disc to rounded with spicules to encourage platelet-platelet interaction
They release
- a-granules
- dense granules
the platelet membrane invaginates to form canalicular system through which the contents are released: ADP. Fibrinogen, VWF
(thromboxane A2 (derived from arachidonic acid) - first produced then released)
What is thromboxane A2? How is it made?
A vasoconstrictor and has a role in platelet aggregation
Platelets are stimulated to produce prostaglandin thromboxane A2 from arachidonic acid
What effect does release of ADP and thromboxane A2 have?
Positive feedback - lead to further platelet recruitment, activation and adhesion by binding to P2Y12 and thromboxane A2 receptors respectively
Platelet activation causes a conformational change in the GPIIa/IIIa receptor so fibrinogen can bind
this causes further activation and links the platelets together to form the platelet plug
(platelet aggregation)
What prevents inappropriate platelet aggregation?
Active flow of blood
Prostacyclin (PGI2) which suppresses platelet activation by causing vasodilation
What are the two antiplatelet drugs and what conditions are they used to treat?
Aspirin and clopidogrel
prevention and treatment of cardiovascular and cerebrovascular disease
How does aspirin act as an anti-platelet drug?
Binds irreversibly to COX enzyme which inhibits prostaglandin production and thus thromboxane A2 production by platelets preventing their aggregation
Effects last 7 days till renewed
How is thromboxane A2 made?
what two things does it do?
membrane phospholipid > Phospholipase A2 enzyme > arachidonic acid > COX enzyme > Prostaglandin PGH2 > thromboxane synthase > thromboxane A2
TP-b receptor - vasoconstrict
TP-a receptor - platelet adhesion and aggregation
How is prostacyclin (PGI2) made?
what does it do?
membrane phospholipid > Phospholipase A2 enzyme > arachidonic acid > COX enzyme > Prostaglandin PGH2 > prostacyclin synthase > prostacyclin PGI2
vasodilation - prevents platelet activation
How does clopidogrel stop haemostasis ?
Irreversibly blocks the ADP receptors (P2Y12) on platelets
Effects last 7 days until renewed
What is von willebrand factor?
- a glycoprotein that is synthesised by endothelial cells and megakaryocytes (incorporated into platelet granules) and circulates in plasma as multimers of different sizes.
mediates the adhesion of platelets to sites of injury and promotes platelet-platelet aggregation
is a specific carrier for factor VIII (FVIII)
Why is secondary haemostasis important?
Primary is only useful for small vessel injury, in large vessels it will fall apart
Fibrin will stabilise the platelet plug
what basic principle does the blood coagulation pathway focus on?
Generation of thrombin (factor IIa) which cleaves fibrinogen (factor I) to generate a fibrin clot that stabilises the platelet plug at sites of vascular injury
How many factors are there?
name as many
Factors 1-13 I - fibrinogen II- prothrombin - essential III - tissue factor IV- calcium ion V- co factor VI VII - essential VIII - anti-haemophilic factor IX - essential X - essential XIII fibrin stabilising 2+7=9 and 10
What are the three phases of secondary haemostasis?
Initiation
Amplification
Propagation
Explain the process of forming a fibrin clot
Initiation
Tissue Factor (III) exposed on surface of endothelial cells, leucocytes and extravascular surfaces
This activates factor 7 => 7a
This then activates
9 => 9a; 10 => 10a
10a then activates 2(prothrombin) => 2a(thrombin)
Amplification
thrombin mediates activation of co-factors 5 and 8 and the zymogen 11 and platelets
Propagation
11 activates more 9 => 9a
which along with 8a amplifies 10 => 10a
burst of 2a thrombin
Cleaves circulating fibrinogen to make fibrin
Where are clotting factors synthesised?
Mostly in the liver and except VIII and VWF (made in endothelial cells)
Why is vitamin K needed?
Factors II (prothrombin), VII, IX and X are dependent on Vitamin K for gamma carboxylation of their glutamic acid residues, which is essential for the function of these clotting factors.
What are the two ways natural anti-coagulation occurs?
-Protein C and protein S (co-factor)
thrombin binds to thrombomodulin on endothelial cell surface causing protein C to become APC (activated protein C)
in the presence of S inactivates 8a and 5a
-antithrombin inactivates thrombin and 10a
the action of antithrombin is made more potent by heparin released from macrophages
What are three antiplatelet drugs and how do they work?
-Heparin
Potentiates action of antithrombin => inactivates 10a and 2a (thrombin)
Administered IV or SC
-Warfarin
Vit K antagonist => interferes with protein carboxylation
Reduces synthesis of Factors 2, 7, 9, 10 in the liver
Administered orally, takes several days to take effect as it deals with synthesis of new factors
Needs to be monitored by regular blood testing
DOACs (Direct Oral Anticoagulants)
Directly inhibits thrombin or 10a (without the involvement of antithrombin)
What is the fibrinolytic system?
How does it work?
Mechanism to lyse clots-
primary fibrinolytic enzyme - Plasmin circulates as plasminogen
it is activated by tissue plasminogen activator (t-PA) however this only happens when it comes together on lysine residues on fibrin
What does the breakdown of fibrin lead to? What does plasmin also break down and what is it inhibited by?
Fibrin degradation products
Also breaks down other plasma proteins ie factors 5 and 8 and fibrinogen
inhibited by anti-plasmin (which circulates in blood) and alpha 2 macroglobulin
What is thrombolytic therapy and where is it used?
What are the risks of it?
Recombinant t-PA (using bacteria to make it)
administered intravenously to selected patients presenting with ischaemic stroke.
time-dependent and so t-PA needs to be given to eligible patients as quickly as possible
There is a high risk of bleeding associated with its use.
Thrombolytic therapy can also be given to patients with life threatening pulmonary emboli
What are antifibrinolytic drugs and how do they work?
Tranexamic Acid => synthetic derivative of lysine, binds plasminogen
Prevents activation of plasminogen
Used to treat bleeding in trauma, surgery and for inherited bleeding disorders
What is the prothrombin time test and what does it show?
Prothrombin Time (PT) Test
It measures primarily the clotting ability of factors 1, 2, 5, 7, 10
If any of these factors are too low, it takes longer than normal for the blood to clot.
The results of this test will be normal among most people with haemophilia A and B.
Extrinsic Pathway
What is the activated partial thromboplastin time test and what does it show?
It measures the clotting ability of factors 8, 9, 11, 12.
The results of this test will show a longer clotting time among people with haemophilia A (VIII) or B (IX) and XI deficiency.
Intrinsic pathway
What three things can cause bleeding?
- Reduction in platelet number or function Primary haemostasis –platelet plug
- Reduction in coagulation factors Secondary haemostasis – fibrin clot
- Increased fibrinolysis
What is thrombosis?
Reduced anticoagulant proteins
Reduced fibrinolytic actvity e.g. pregnancy
Increased clotting factors
Factor 8 in pregnancy
Factor V Leiden (thrombophilia)
Myeloproliferative disorders
Why is the balance between fibrinolytic factors and coagulation factors important?
- allow the stimulation of blood clotting processes following injury, where blood changes from its liquid state (coagulation)
- Limit the extent of the response to the area to prevent excessive or generalised clotting (thrombosis)
- Start the process leading to breakdown of the clot as part of the healing process (fibrinolysis)
Which factors are essential to the stable fibrin clot?
II- prothrombin - essential
VII - essential
IX - essential
X - essential
2+7=9 and 10
How is the COX enzyme replaced?
endothelial cells can synthesise more COX where non nuclear platelets cannot
What two receptors does thromboxane A2 bind to and what does this cause?
TP-α platelet adhesion and aggregation
TP-β vasoconstriction
What is the effect of calcium ions during secondary haemostasis?
Calcium ionsplay an important role in the binding of activated clotting factors to the phospholipid surfaces of platelets.
What is Virchows triad?
The three contributory factors to pathological clotting or thrombosis.
- Blood: dominant in venous thrombosis
- Vessel wall: dominant in arterial thrombosis
- Blood flow: complex, contributes to both arterial and venous thrombosis
What can caused reduced platelet number or function
Failure of production - drugs/viruses, bone marrow infiltration
Shortened survival - immune thrombocytopenia, increased splenic pooling
Reduction in function- inherited causes or aspirin (antiplatelet drugs
What can cause reduction in coagulation factors?
Congenital
VWD
haemophilia A - Factor VIII deficiency
haemophillia B - Factor XI deficiency
Acquired
liver
Anticoagulant drugs
DIC -disseminated intravascular coagulation - condition in which blood clots form throughout the body, blocking small blood vessels)
Tx: Replacement with recombinant factors
What can cause increased fibrinolysis?
Increased fibrinolysis
DIC, due to thrombolytic therapy tPA
What changes in blood can increase the risk of venous thrombosis ?
• Reduced levels of anticoagulant proteins
• Reduced fibrinolytic activity
- pregnancy where there is an inhibition of plasminogen
• Increased levels of clotting factors or platelets
- Levels of VIII rising in pregnancy
- The activity of factor V is increased by a mutation - Factor V Leiden makes factor V more resistant to inactivation by protein C - inherited thrombophilia
- Platelets are increased in number in some myeloproliferative disorders, where the bone marrow output is increased.
