Haemostasis Flashcards
What are the different components of haemostasis
– i.e. vessel wall, platelets and clotting factors
Clotting factors can be found in the blood, on the surface of the endothelium or expressed on the surfaces of some extravascular cells
How can clinical bleeding manifest
e.g. thrombocytopenia, haemophilia, VWD
How can thrombosis manifest
.g. thrombophilia, venous/arterial thrombosis, cancer
In many diseases (cancers, myocardial infarction) a haemostatic/thrombotic challenge is often the cause of death for these patients
Summarise the therapeutic intervention to manage the disorders of haemostasis
anticoagulant/antiplatelet drugs, replacement therapy (for haemophilia)
What is haemostasis
“the cellular and biochemical processes that enables both the specific and regulated cessation of bleeding in response to vascular insult”
Essentially the balance between bleeding and thromboisis
Don’t want to bleed excessively once we cut ourselves, so it acts rapidly to prevent blood loss.
What is haemostasis needed for
“to prevent blood loss from intact and injured vessels, enable tissue repair”
meshwork of fibrin which is a temporary structure- to allow subsequent repair mechanisms to kick in
Describe a delicate balance in normal haemostasis
Maintaining the integrity and patency of the vascular system is essential to life. Although there are many regulators that ensure this process operate within the normal range
Balance between pro-fibronylytic factors and pro-coagulant factors to ensure balance between bleeding and thrombosis.
Essentially, what is meant by normal haemostasis
Equilibrium: normal haemostasis is a balance between bleeding and thrombosis
Describe how the balance can be tipped towards bleeding
Increase in fibrinolytic factors or anticoagulant proteins
Decrease in coagulant factors and platelets
Describe how the balance can be tipped towards thrombosis
Decrease in fibronlytic factors or anticoagulant proteins
Increase in coagulant factors and platelets
What is a major complication of DVT and what will most people die from
Pulmonary embolism is a major risk factor of DVT- clots float away and get trapped in the lungs (DVT not a risk factor for strokes)
Most people die with a ‘haemostatic end-point’! And this is modifiable with therapeutic intervention.
What is the first step in the response to injury to the endothelial cell lining
Vessel constriction
Vascular smooth muscle cells contract locally
Limits blood flow to injured vessel
But vessel constriction (although it reduces the blood flow)- is strictly not a haemostatic response
Summarise primary haemostasis
Formation of an unstable platelet plug
platelet adhesion
platelet aggregation
Limits blood loss + provides surface for coagulation
Platelets act as a physical barrier to stop the flow of blood
Summarise secondary haemostasis
Stabilisation of the plug with fibrin
blood coagulation
Stops blood loss
Activation of coagulation cascade and deposition of fibrin
Only stop bleeding when you get fibrin mesh on top of the platelets (i.e once you get primary and secondary haemostasis).
Summarise vessel repair and dissolution of the clot
Vessel repair and dissolution of clot
Cell migration/proliferation & fibrinolysis
Restores vessel integrity
What is important to remember about vessel constriction
§ Vessel constriction:
o Mainly important in SMALL blood vessels.
o Local contractile responses to injury but the precise mechanisms are uncertain.
This is in itself sometimes sufficient to temporarily restrict blood loss from a wound in small blood vessels.
Describe a key property of the arterial endothelial cells
Anti-coagulant by nature- provide a surface for the blood to irrigate
EC - anticoagulant barrier
TM, EPCR, TFPI, GAG
Describe the key properties of the sub-endothelium
Sub-endothelial structures are pro-coagulant in nature
So the tunica intima, media and adventitia (which also has its own microvessels) have tissue factors and various ECM proteins which helps to recruit platelets to the site of injury.
Summarise the subendothelium
Subendothelium - procoagulant Basement membrane Elastin, collagen VSMC - TF Fibroblasts - TF
Internal elastic lamina seperates the tunica intima and media
External elastic lamina seperates the tunica media and tunica adventitia.
Describe the normal intact blood vessel wall
Intact blood vessel with endothelium expressing anti-coagulant factors such as EPCR and TM.
Various plasma proteins (clotting factors) whicha re present constituitively in the blood, but normally in a latent form.
VWF, PI
FVII, FX, Prothrombin (FII), FV, FVIII, FIX, FXI, Protein C, Protein S etc…
How can we trigger a haemostatic response in mice
Shine a laser through a microscope onto their blood vessels to initiate a haemostatic response.
See platelets arrive before fibrin deposition (BUT IN REALITY primary and secondary haemostasis occur simultaneously).
State the characteristics of platelets
Small (2-4µm)
Anuclear
Life span: ~10 days
Platelet count: 150-350 x 109/L
Can be stained by wright-Giemsa- will become purpley
Outline the differentiation pathway of platelets
Haematopoietic stem cell – promegakaryocyte
Promegakaryocytes – megakaryocytes (proliferation of DNA 2N-16N)
Megakaryocytes- -proplatelets
Proplatelets- - platelets
Describe how megakaryocytes give rise to platelets
MK looses its ability to divide, however continue to replicate its DNA becomes polyploid, cytoplasm enlarge. MK matures, becomes granular and form platelets that will be released in the circulation.
model MK migrate from endosteal niche to the vascular niche where they do not pass the EC of sinusoidal blood vessels and remain in vascular niche. They form pseudopodia-like extensions (proplatelets) that extend in the lumen plt are released from tip of these long extensions by shear forces.
Other model is following migration from endosteal niche to vascular niche mature MK pass the E.C barrier and enter the circulation. Due to large size of MK they get trapped in the microvasculature of lungs mechanical force induce fragmentation of MK
Describe the key properties of megakaryocytes
Haemopoietic stem cells give rise to megakaryocyte precursors which undergo nuclear replication without cytoplasmic division, then maturation before migrating to the marrow sinusoids, extending proplatelets through the endothelial wall and fragmenting into platelets in the circulation.
Can have 8-16 nuclei- multipleudic
Each megakaryocyte produces ~4000 platelets.
- 1011 platelets are produced each day
- Lifespan ~10 days
Describe how platelets are responsive to agonists in their micro-environment
TP receptors- respond to thromboxane
PAR receptors- respond to thrombin
P2Y1/12- responds to ADP
State the two ways in which platelets can bind to collagen
It can bind via vWF to collagen (via the GlpIb/V/IX receptor)
It can bind directly to the collagen (via the GlpIa receptor or to the GPVI or to Alpha2Beta1 receptor)
How can the platelet bind to fibrinogen
Via the alpha2bBeta3 receptor
alpha beta- are integrins
Describe the other key features of platelets
Alpha granules which include growth factors (cytokines to be release upon activation), fibrinogen, FV and VWF
Dense granules- ADP, ATP, serotonin, Ca2+ and polyphosphates
Phospholipid membrane- quiescent in resting state of platelet. However, when platelet becomes activated, we see a flip-flop reaction- where the negative phospholipids normally on the I.C leaflet are now expressed on the E.C leaflet- which makes the surface of the platelet attractive for clotting factors.
Cytoskeleton- dynamic microtubules and actin cytoskeleton, helps the platelet change shape rapidly from quiescent to active upon activation.
What else do alpha granules contain
Alpha granules also contains extra pool of alphaIIbb3 also in the open canalicular system
Describe the platelet cytoskeleton
Important for platelet morphology, shape change, pseudopods,
contraction and clot retraction.
Platelet activation:
conversion from a passive to an interactive cell
inactive (flat) - rounded- flattened egg-shaped active cell
Summarise the roles of platelets
Haemostasis and thrombosis
Cancer
Atherosclerosis
Infection and inflammation- interact with leukocytes to clear infections
Vastly dynamic cells which will have different roles depending on the agonists that they are exposed to.
Describe the normal circulation of VWF (i.e when the vessel wall is intact)
Multimeric VWF circulates in plasma in a globular conformation. Binding sites are “hidden” from platelet GpIb
Describe the consequences of vascular injury on VWF
Vascular injury damages endothelium & exposes sub-endothelial collagen
several components of the subendothelium (e.g. collagen, fibronectin, laminin) become exposed, to which platelets will get recruited
Exposed sub-endothelial collagen binds globular VWF (tethering it to the endothelium)
Tethered VWF unravelled by rheological shear forces of flowing blood
VWF unravelling exposes platelet binding sites
(GpIb) - platelets get tethered
Binding of VWF to platelet GpIb recruits platelets to site of vessel damage
How else can platelets be recruited
Platelets can also bind directly to collagen via GPVI & α2β1
(only at low shear – i.e. not in arteries/capillaries)
Describe the importance of collagen for platelet activation in primary haemostasis
Activate platlets via GPVI and A2b1
Platelets change shape, release their granule contents and undergo the membrane flip-flop reaction (essentially, they become activated).
Thrombin can also activate platelets
Activated platelets release agonists (ADP and thromboxane) that will further activate platelets and allow recruitment of additional platelets
Via which domain does VWF bind to collagen
VWF can bind to collagen via its A3 domain
