Haemostasis COPY Flashcards
What is haemostasis
the cellular and biochemical processes that enables both the specific and regulated cessation of bleeding in response to vascular insult
What is the function of haemostasis
to prevent blood loss from intact and injured vessels, enable tissue repair
What are the 4 stages of haemostasis
- Vessel constriction
- Formation of an unstable platelet plug
- Stabilisation of the plug with fibrin
- Vessel repair and dissolution of clot
Describe the vessel constriction response in haemostasis
Vascular smooth muscle cells contract locally
Limits blood flow to the injured vessel
Mainly important in small blood vessels
Local contractile response to injury
What is involved in unstable platelet plug formation and what is the purpose
Platelet adhesion and aggregation
Limits blood loss + provides surface for coagulation
What is involved in stabilisation of a platelet plug and what is the purpose
Blood coagulation
Stops blood loss
What does vessel repair involve and what is the function
Cell migration/proliferation and fibrinolysis
Restores vessel integrity
Describe the normal vessel wall
Endothelial cell - anticoagulant barrier (TM, EPCR, TFPI, GAG) Subendothelium - procoagulant Basement membrane - elastin, collagen VSMC - TF Fibroblasts - TF
Which pro-haemostasis factors are carried in the blood
FVII, FX, Prothrombin (FII), FV, FVIII, FIX, FXI, Protein C, Protein S etc…
Describe platelets
Small (2-4µm) Anuclear Lifespan - 10 days Platelet count = 150-350 10^11 produced every day
Describe the megakaryocytic
Contains nuclear lobes and a granulated cytoplasm
Describe the development of platelets
- Haematopoietic stem cell from the bone marrow develops into a promegakaryocyte.
- Promegakaryocyte proliferates and forms megakaryocytes
- Megakaryocytes mature into platelets (1 MK - 4000 platelets)
Describe the platelet cytoskeleton
Important for morphology, shape change, pseudopods, contraction and clot retraction
What are the ultrastructural features of the platelet
On membrane: Alpha granules Dense granules Phospholipid membrane Microtubules and actomyosin GbIb/V/IX GPVI TP, PAR
What is contained in the alpha and dense granules
Alpha - growth factors, fibrinogen, FV, VWF
Dense - ADP, ATP, Serotonin, Ca2+
What are the roles of platelets
Haemostasis and thrombosis Cancer Atherosclerosis Infection Inflammation
Describe the normal blood vessel
Platelets circulate in close contact with the endothelial cell lining of the blood vessel wall
VWF circulates in a globular conformation. Binding sites are hidden from GpIb
Describe what occurs in the blood after injury of the vessel
- Collagen, fibronectin and laminin become exposed and platelets become recruited via sub-endothelial collagen
- VWF binds to collagen via its A3 domain
- VWF is unravelled by the shear stress
- GPIba binding site of VWF is exposed
- Binding of VWF to to platelets recruits other platelets (also Bia GPVI and a2b1 on collagen at low shear)
- Activation of platelets
- Further recruitment of platelets (also by thrombin and collagen)
- Release of platelets bound to collagen/VWF
- ADP and thromboxane activates platelets
- Aggregation via GpIIb/IIIa + fibrinogen binding
Describe the changes in platelet shape
Flowing disc-shaped
Rolling ball-shaped platelet
Hemisphere-shaped platelet
Spreading platelet
At what platelet counts will spontaneous bleeding and bleeding occur
Severe spontaneous - <10x10^9
spontaneous - <40x10^9
With trauma - 100x10^9
What are the cutaneous features of immune thrombocytopenia
Purpura
Multiple bruises
Ecchymoses
Where are clotting factors produced
Liver - most
Endothelial cells - VWF, TM, TFPI
Megakaryocytes - VWF, FV
Describe the state of clotting factors in the blood
Circulates as inactive precursors
Either serine protease zymogens of cofactors
Activated by specific proteolysis
What are some serine protease domain-containing proteins/clotting factors
FVII FX Prothrombin FIX FXI Protein C
Describe serine protease domains
Catalyses proteolysis of target substrate
Serin protease contain a catalytic triad of His,Asp,Ser
Serine proteases cleave substrates after specific Arg and Lys residues
Describe the initiation of coagulation
- FVII/FVIIa binds to cell surfaces via GIa domain
- TF and FVII form TF-FVIIa
- FVIIa becomes more active
- Activation of FX and FIX via removal of the activation peptide to yield an active enzyme
- Prothrombin activation by FXa (insufficient)
- Thrombin activates FV and FVIII
- FIXa and FVIIIa activate FX
- Enhanced activation of ProT by FXa and FCA
What are tissue factors
Cellular receptor and cofactor for FVII/VIIa
Only procoagulant factor that does not require proteolytic activation
47kDa integral membrane
Normally locates at extravascular sites (VSMS, fibroblasts)
Where are tissue factors highly expressed
Lungs Brain Heart Testis Uterus Placenta
Describe factor VII
serine protease zymogen 48kDa plasma glycoprotein Expressed/secreted by the liver GIa domain, 2xEGF_like domains, serine protease domain Circulates in plasma at 10nM 1% in active form (VIIa)
What do FVII, FIX, FX and PC share
A homologous modular structure (4 domains)
Gla domain - binding to phospholipid surfaces
EGF domain is involved in protein-protein interactions
all circulate in plasma in zymogen form
activated by proteolysis
Which proteins contain GIa domains
FVII FX Prothrombin FIX Protein C Protein S
Describe the GIa domain
Defines vit K-dependent proteins
Contains 9-11 gamme-carboxyglutamic acid residues
Binds 6/7 Ca2+ ions -> structural transition
What may be the consequence of deficiency in procoagulant factors
Haemophilia
Haemophilia A - FVIII
Haemophilia B - FIX
X-linked
Draw a diagram that represents the initiation of coagulation
-
Draw a diagram that represents the regulation of coagulation
-
Describe how coagulation is regulated
Antithrombin inhibits FXa, FIXa and thrombin
TFPI inhibits TF-FVIIa anf FXa
APC and protein S inhibits FVa and FVIIIa
Describe TFPI
Tissue factor pathway inhibitor
TFPI-FXa can bind and inactivated TF-FVIIa
Active site = Kunitz domain (K1)
Describe the protein C pathway
Protein C is activated by thrombin-TM complex
Activated protein C (APC) inhibits thrombin generation by proteolytically inactivating procoagulants FCA and FVIIIa
Describe the process of protein C activation
Usually Serine protease cleaves fibrinogen to form fibrin
- Thrombin binds to TM with high affinity
- Serine protease activates protein C
- Protein C becomes localised to endothelial surface
- Thrombin cleaves protein C -> activation peptide released
- Activates protein C zymogen to APC
- release of APC so PC can bind to EPCR
- APC down-regulates thrombin generation
What is the role of protein S
Co-factor for protein C
Describe antithrombin
58kDa serine protease inhibitor (SERPIN)
Inactivates activated coagulation serine proteases (FXa, thrombin, FIXa, FXIa)
Frees serine proteases that escape the vessel damage site
Describe fibrinolysis
- Plasminogen is converted to plasmin via tissue plasminogen activator (tPa) on the fibrin clot surface
- Plasmin breaks down fibrin clot
- Fibrin degradation products are produced
What are the therapeutic uses of tPA
Thrombolysis for:
Myocardial infarction
Ischaemic stroke
Give examples of anticoagulant drugs
Heparin
Warfarin
DOACs
Give examples of anti platelet agents
Aspirin
P2Y12 blockers
Which tests are done for haemostat disorders
Coagulation (PT, APTT)
platelet function tests
d-dimer
