haemostasis Flashcards
1
Q
describe the flow and function of blood under normal conditions.
A
flows within vascular system
transports oxygen, nutrients and hormonal information around the body
removes metabolic waste
2
Q
the confinement of circulating blood to the blood vessels and maintenance of a fluid state are dependent on which factors?
A
fibrinolytic factors and anticoagulant proteins vs. coagulation factors and platelets
3
Q
why is the balance of fibrinolytic factors and anticoagulant proteins vs. coagulation factors and platelets important?
A
allows stimulation of blood clotting processes to allow coagulation after injury
limits extent of response to injury area to prevent thrombosis
starts process that eventually leads to fibrinolysis
4
Q
what is coagulation?
A
blood changes from its liquid state as a result of stimulation of blood clotting processes following injury
5
Q
what is thrombosis?
A
excessive/generalised blood clotting
6
Q
what is fibrinolysis?
A
breakdown of a clot as part of the healing process
7
Q
haemostasis describes what exactly?
A
‘halting of blood’ following trauma to blood vessels
8
Q
haemostasis results from which 3 intertwined processes?
A
1 - vasoconstriction (contraction of blood vessels)
2- primary haemostasis (formation of an unstable platelet plug at the site of vessel wall damage)
3- secondary haemostasis/coagulation (formation of a stable fibrin clot)
9
Q
what is vasoconstriction?
A
contraction of blood vessels
10
Q
what is primary haemostasis?
A
formation of an unstable platelet plug at the site of vessel wall damage
11
Q
what is secondary haemostasis/coagulation?
A
formation of a stable fibrin clot
12
Q
what are the 5 reasons why is it important to understand haemostatic mechanisms?
A
1- diagnose and treat bleeding disorders
2- identify risk factors for thrombosis
3- treat thrombotic disorders
4- monitor drugs used to treat bleeding and thrombotic disorders
5- control bleeding in individuals without an underlying bleeding disorder
13
Q
briefly outline the 4 stages of haemostasis.
A
1- response to injury (vessel constriction)
2- primary haemostasis
- platelet adhesion
- platelet aggregation
3- secondary hamostasis
- blood coagulation
4- fibrinolysis (dissolution of clot and vessel repair)
14
Q
describe platelets.
A
discoid
non nucleated
granule containing cells
15
Q
where are platelets derived from?
A
myeloid stem cells
16
Q
where are platelets formed?
A
bone marrow
17
Q
how are platelets formed?
A
fragmentation of megakaryocyte cytoplasm
18
Q
what is the circulating lifespan of a platelet?
A
around 10 days
19
Q
what proteins are present on the platelet plasma membrane that are important for platelet interactions?
A
glycoproteins (GPs)
specifically, GPIa and GPIb
20
Q
describe direct platelet adhesion.
A
following injury to vessel wall, platelets stick directly to collagen in the damaged endothelium via the GPIa receptor on the platelet plasma membrane.
21
Q
describe indirect platelet adhesion.
A
following injury to vessel wall, platelets stick indirectly to collagen in the damaged endothelium via von Willebrand factor, which binds to the GPIb receptor on the platelet plasma membrane.
22
Q
what kind of shape change does platelet adhesion cause in platelets?
A
from discoid to more rounded form with spicules
23
Q
why is the change in shape of platelets from discoid to more rounded with spicules after platelet adhesion relevant?
A
encourages platelet-platelet interaction
24
Q
what does platelet adhesion cause?
A
platelet release action
adhesion initiates activation of platelets and release of contents of their storage granules
25
what are the 2 main types of ultrastructually identifiable storage granules in platelets?
α-granules
dense granules
26
where are most clotting factors synthesised and what are the 2 exceptions?
liver
all except VIII and VWF
27
where are factor VIII and VWF synthesised?
endothelial cells
28
where can VWF also be synthesised aside from the endothelial cells?
megakaryocytes
incorporated into platelet granules
29
what is factor II also known as?
prothrombin
30
factors II, VII, IX and X are dependent on what for their function and why is it essential to their function?
vitamin K
needed for carboxylation of glutamic acid residues
31
what are factors V and VIII?
co factors
32
where are many factors believed to work and why is this important?
exposed phospholipid surface of platelets
helps localise and accelerate these reactions
33
what role do Ca²⁺ ions play in reference to the binding of clotting factors?
helps bind activated clotting factors to phospholipid surfaces of platelets
34
what stabilises the initial platelet plug?
fibrin formation
35
in which cases is the primary platelet plug sufficient?
small vessel injury
will fall apart in larger vessels
36
what do blood coagulation pathways centre on?
generation of thrombin
37
what is the role of thrombin?
cleaves fibrinogen to generate fibrin clot that stabilises platelet plug at site of vascular injury
38
each step in blood coagulation is characterised by what? outline the process.
conversion of inactive zymogen (proenzyme) to an active clotting factor
achieved by splitting peptide bonds
this exposes active enzyme site
39
what is the trigger to initiate coagulation at injury sites?
tissue factor (TF)
exposed on surface of endothelial cells. leukocytes and most extravascular cells in areas of tissue damage
40
why is the presence of TF at injury sites significant?
TF present in sites not usually exposed to the blood under normal physiological conditions
therefore blood only encounters TF at sites of vascular injury
41
what are the 3 phases of secondary haemostasis/coagulation?
initiation
amplification
propagation
42
describe the initiation phase of secondary haemostasis/coagulation.
TF binds to factor VIIa
leads to activation of factors IX to IXa and X to Xa
leads to activation of prothrombin (factor II)
generates small amount of thrombin (factor IIa)
43
describe the amplification phase of secondary haemostasis/coagulation.
small amount of thrombin generated during initiation mediates activation of:
- co factors V and VIII
- zymogen factor XI
- platelets
44
describe the propagation phase of secondary haemostasis/coagulation.
factor XI converts more factor IX to IXa
factor IXa works with factor VIIIa to amplify conversion of factor X to Xa
causes rapid burst in thrombin generation
thrombin cleaves fibrinogen to produce insoluble fibrin clot
45
how are the contents of platelet granules released after platelet adhesion?
platelet membrane invaginates to form a surace-connected cannalicular system through which contents are released
46
what components (among others) are released by platelet granules after platelet adhesion?
ADP
fibrinogen
VWF
47
what is thromboxane A2?
a prostaglandin
48
how is thromboxane A2 produced?
platelets stimulated to produce it from arachidonic acid derived from the cell membrane
49
what are the roles of thromboxane A2?
vasoconstrictor (important during tissue injury and inflammation)
in platelet aggregation
50
describe the process of platelet aggregation.
granular release of ADP and thromboxane A2 generation after platelet adhesion
ADP binds to P2Y12 receptor, thromboxane A2 binds to thromboxane A2 receptor
binding has positive feedback effects - further platelet recruitment, activation and aggregation
platelet activation causes conformational changes in the GPIIb/IIIa receptor, providing binding sites for fibrinogen
fibrinogen has a key role in linking platelets together to form the plug
51
fibrinogen has a key role in linking platelets together to form a platelet plug - how are these effects normally counterbalanced?
counterbalanced by active flow of blood
counterbalanced by release of prostacyclin (PGI2) from endothelial cells
52
what is prostacyclin?
powerful vasodilator
suppresses platelet activation, therefore preventing inappropriate platelet aggregation
53
what are antiplatelet drugs usually used for? give the 2 most commonly used.
prevention and treatment of cardiovascular and cerebrovascular disease
e.g aspirin, clopidogrel
54
how does aspirin function as an antiplatelet drug?
irreversibly blocks action of cyclo-oxygenase (COX), inhibiting thromboxane A2 production
therefore reduced platelet aggregation
although prostacyclin production is also inhibited by COX, endothelial cells can synthesise more (platelets, being non-nuclear, cannot)
therefore the effects of a dose of aspirin persists for around 7 days until most of the platelets present at the time of ingestion are replaced by new platelets
55
how does clopidogrel function as an antiplatelet drug?
irreversibly blocks P2Y12 receptor (ADP) on platelet cell membrane
effects of clopidogrel last around 7 days (until new platelets replaced those present at time of ingestion)
56
what is VWF and how is it synthesised?
multimeric glycoprotein
synthesised by endothelial cells and megakaryocytes
circulates in plasma
57
what are the 3 roles of VWF?
mediates adhesion of platelets to injury sites
promotes platelet-platelet aggregation
specific carrier for factor VIII in addition to adhesive properties
58
what do inhibitory mechanisms do?
prevent blood from clotting completely when clotting is initiated by vessel injury
59
what does the action of inhibitory mechanisms ensure?
coagulation is confined to the site of injury
prevents spontaneous activation of coagulation
60
what 3 things does the protein C pathway inhibitory mechanism involve?
protein C
protein S
antithrombin
61
describe the protein C pathway inhibitory mechanism.
downregulates thrombin generation
1- thrombin binds to thrombomodulin on endothelial cell surface
2- this activates protein C to activated protein C (APC)
3- APC inactivates factors Va and VIIIa in the presence of a co-factor (protein S)
4- thrombin and factor Xa are inactivated by the circulating inhibitor antithrombin
the binding of antithrombin to endothelial cell-associated heparins potentiates the action of antithrombin
62
what are anticoagulant drugs used for?
prevention and treatment of thrombosis
63
what are the 3 main anticoagulant drugs?
heparin
warfarin
direct oral anticoagulants (DOACs)
64
how does heparin work as an anticoagulant drug?
administered intravenously or by subcutaneous injection
works indirectly
potentiates action of antithrombin
factor Xa and IIa (thrombin) inactivated
65
how does warfarin work as an anticoagulant drug?
given as an oral tablet, anticoagulant effect needs monitoring by regular blood testing
vitamin K antagonist
interferes with protein carboxylation
reduces synthesis of functional factors II, VII, IX and X by the liver (takes several days to take effect because it is reducing synthesis rather than inhibiting existing molecules)
66
how do DOACs (direct oral anticoagulants) work as anticoagulant drugs?
orally available drugs, do not usually require monitoring
directly inhibit either thrombin or factor Xa (i.e without the involvement of antithrombin?
67
describe the process of fibrinolysis.
plasminogen is activated by tissue plasminogen activator (tPA) to plasmin (principal fibrinolytic enzyme) when both tPA and plasminogen bind to lysine residues on fibrin
breakdown of fibrin by plasmin leads to generation of fibrin degradation products (FDPs)
68
how is plasmin activated?
inactive zymogen form plasminogen circulates in the blood
tissue plasminogen activator (tPA) activates plasminogen to plasmin when both tPA and plasminogen bind to lysine residues on fibrin
69
what can plasmin break down?
primarily fibrin, but is not fibrin-specific
also breaks down other protein components of plasma (including fibrinogen, factors Va and VIIIa)
70
how is plasmin inhibited?
antiplasmin
circulates in the blood
71
what is the aim of thrombolytic therapy?
generates plasmin to lyse clots
used for patients with ischaemic stroke, pulmonary emboli etc
(used to be used in myocardial infarctions but this has been generally replaced with angioplasty and stenting the diseased coronary vessels)
72
how is a thrombolytic agent such as recombinant tPA used in dealing with ischaemic stroke?
recombinant tPA generates administered intravenously
benefit is time dependent (needs to be given as quickly as possible, preferably within an hour)
generates plasmin to lyse clots
73
what is the primary risk with using a thrombolytic agent such as recombinant tPA (e.g in dealing with ischaemic stroke?)
high risk of bleeding
74
give an example of an antifibrinolytic drug.
tranexamic acid
75
what is tranexamic acid?
synthetic derivative of lysine
antifibrinolytic drug
76
how does tranexamic acid work as an antifibrinolytic drug?
binds to active site on plasminogen
prevents binding of plasminogen to lysine residues on fibrin (competitive inhibition)
plasminogen cannot be activated to plasmin, fibrinolysis cannot occur
77
where is tranexamic acid used?
antifibrinolytic drug
used to treat bleeding in trauma and surgical patients
treatment of inherited bleeding disorders
78
what does the intrinsic coagulation cascade model refer to?
system where all components are in the plasma (factors IX, X, XI, XII, co-factors V and VIII)
79
what does the extrinsic coagulation cascade model refer to?
comprises TF, co-factor V and factors VII and X
80
how does the prothrombin time (PT) work as a test of coagulation?
measures integrity of extrinsic pathway
blood combined with sodium citrate (chelates calcium, prevents clotting) and spun to produce platelet-poor plasma
source of TF and phospholipid (usually recombinant thromboplastin) as well as calcium is added to the plasma
time taken to clot is recorded
81
what does a prolonged PT indicated?
reduction of activity of factor II (or fibrinogen), V, VII or X
82
when the PT is used to monitor vitamin K antagonist anticoagulant therapy such as warfarin, how are results expressed and why?
expressed as international normalised ratio (INR)
involves a correction for different thromboplastin reagents used by different laboratories
therefore all laboratories should obtain the same INR result for a sample irrespective of the thromboplastin source
83
how does the activated partial thromboplastin time (APTT) work as a test of coagulation?
measures integrity of intrinsic pathway
performed by the contact activator of factor XII by a surface such as glass, or using a contact activator (e.g silica, kaolin)
blood combined with sodium citrate (chelates calcium, prevents clotting) and spun to produce platelet-poor plasma
contact activator, phospholipid and calcium added to the plasma sample
time taken to clot is recorded
84
what 3 things could an isolated prolonged APTT (i.e normal PT) indicate?
haemophilia A (factor VIII deficiency)
haemophilia B (factor IX deficiency)
factor XI deficiency
may also be caused by factor XII deficiency, which does not cause bleeding
85
what does a prolonged APTT with an associated prolonged PT indicate?
deficiency in multiple clotting factors
86
what 3 things can bleeding be caused by?
reduction in platelet number/function (primary haemostasis - platelet plug)
reduction in coagulation factors (secondary haemostasis - fibrin clot)
increased fibrinolysis
87
what 3 things could cause a reduction in platelet number, which could lead to bleeding?
failure of platelet production (drugs, viruses, bone marrow infiltration, megaloblastic anaemia resulting from B12 or folate deficiency, hereditary thrombocytopenia)
shortened platelet survival
(immune thrombocytopenia, disseminated intravascular coagulation/DIC)
increased splenic pooling
88
what 2 things could cause a reduction in platelet function, which could lead to bleeding?
inherited causes
antiplatelet drugs (e.g aspirin)
89
what are the congenital causes of reduced coagulation factors and how are they inherited?
most common bleeding disorder - reduction in level of function of VWF (known as von Willebrand disease) - autosomally inherited, affects males and females
the rest have deficiencies of one of the clotting factors
- most have haemophilia A (factor VIII deficiency, X-linked)
- some have haemophilia B (factor IX deficiency, X-linked)
- a very small percentage have deficiencies of one of the other clotting factors (usually autosomal recessive inheritance, common in areas with high levels of consanguinity)
90
how are congenital causes of reduced coagulation factors treated?
replacement of missing clotting factor with specific clotting factor concentrates
91
how is the risk of pathogen transmission eliminated when replacing missing clotting factors due to congenital causes?
factor concentrates are recombinant
92
what are the 3 acquired causes of reduced coagulation factors?
liver disease
anticoagulant drugs
disseminated intravascular coagulation (DIC)
93
what is disseminated intravascular coagulation (DIC)?
process where there is generalised and uncontrolled activation of coagulation, followed by a marked activation of the fibrinolytic system
activation of fibrinolytic system due to TF within the circulation, leading to:
- generation and dissemination of large amounts of thrombin
- activation and consumption of platelets
- widespread formation of thrombi in the small blood vessles (micocirculation)
clotting factors and fibrinogen in plasma become depleted - impairs haemostatic activity, may result in severe bleeding
high levels of fibrin degradation products (FDPs) as a result of fibrinolysis activation
94
what are the causes of increased fibrinolysis, which may lead to bleeding?
seen in DIC (disseminated intravascular coagulation)
due to administration of thrombolytic therapy e.g tPA
95
what are some of the causes of DIC (disseminated intravascular coagulation) and how is it controlled/treated?
bacterial sepsis
advanced cancer
variety of obstetric emergencies
bleeding can be controlled by replacing missing clotting factors and platelets
underlying cause needs to be addressed in order to switch off the unregulated coagulation activation
96
what is thrombosis?
formation of a blood clot within an intact blood vessel
results in obstruction of blood flow with serious consequences
97
what are the 3 contributing factors to thrombosis (Virchow's triad) and where are they dominant?
blood (dominant in venous thrombosis)
vessel wall (dominant in arterial thrombosis)
blood flow (contributes to both arterial and venous thrombosis)
98
what are the 3 changes in blood that may increase the risk of venous thrombosis?
reduced level of anticoagulant proteins
reduced fibrinolytic activity
increased levels of clotting factors or platelets
99
what can cause reduced levels of anticoagulant proteins, which may increase the risk of venous thrombosis?
usually genetic basis
e.g inherited antithrombin deficiency (inherited thrombophilia)
100
what can cause reduced fibrinolytic activity, which may increase the risk of venous thrombosis?
pregnancy
inhibition of plasminogen activation through production of a specific inhibitor by the placenta
101
what can cause increased levels of clotting factors, which may increase the risk of venous thrombosis?
factor VIII levels rise when pregnant
factor V activity increased by a point mutation known as factor V Leiden
(makes factor V more resistant to inactivation by protein C)
many people are carriers - most common of inherited thrombophilias
platelets increased in some myeloproliferative disorders
