Haemostasis Flashcards
What is haemostasis
the cellular and biochemical processes that enables both the 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
Mainly important in small blood vessels
Local contractile response to injury
Limits blood flow to the injured vessel
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
Describe platelets (size, organelles, lifespan, count)
Small (2-4µm) Anuclear Lifespan - 10 days Platelet count = 150-350 10^11 produced every day
What is the role of 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 Thrombin receptor Phospholipid membrane Microtubules and actomyosin Glycoproteins (GbIb/V/IX)
What is contained in the alpha granules of platelets
Growth factors,
Fibrinogen
FV
VWF
What is contained in the dense granules of platelets
ADP
ATP
Serotonin
Ca2+
Describe platelets and VWF in a 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
What is vWF
vWF is a blood-borne glycoprotein which also adheres to endothelial collagen, and binds Gp1b especially well under high shear stress.
What occurs in the adhesion stage of platelet plug formation
- Platelets bind to exposed endothelial collagen with their Gp1a.
- Platelets bind to von Willebrand factor (vWF) with their Gp1b.
- vWF unravel under high shear stress
- Binding of VWF to to platelets recruits other platelets (also Bia GPVI and a2b1 on collagen at low shear)
- Activation of platelets
What occurs in the aggregation stage of platelet plug formation
Release of ADP and thromboxane by platelets causes them to stick together via fibrinogen using their GpIIb/IIIa. Ca2+ has to be present.
What are prostaglandins important metabolites of and which structures are involved in their formation
Arachidonic acid
Platelets and endothelial cells
What do platelets convert arachidonic acid to
Thromboxane A2 via COX-1
Thromboxane A2 is a potent inducer of platelet aggregation.
What do endothelial cells covert arachidonic acid to
Prostacyclin (PGI2) via cyclooxygenase-2 (COX-2).
Prostacyclin inhibits platelet aggregation.
What are the ways to monitor platelet function
platelet count
Bleeding time (40m pressure + incision, normal = 3-8)
Platelet aggregation （vWF)
At what platelet counts will spontaneous bleeding and bleeding occur
Severe spontaneous - <10x10^9
spontaneous - <40x10^9
With trauma - 100x10^9
Normal - >150
Describe vessels disorders and give examples
Bleeding due to impaired interaction of platelets with the compromised vessel
e.g. scurvy, allergic vasculitis
Give examples of platelet disorders
Low platelet count (thrombocytopenia)
Drug-induced
What are the 3 mechanisms from which thrombocytopenia can arise
Failure to produce platelets
Shortened platelet half life
Increased pooling of platelets in an enlarged spleen
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 coagulation cascade
- Tissue Factor, exposed by vessel damage, forms a complex with FVII
- Formation of an active TF-VII complex
- Activation of the extrinsic pathway via FX -> FXa
- Activation of the intrinsic pathway via FIX -> FIXa
- Xa leads to conversion of prothrombin -> thrombin (IIa)
- Activation of FV and FVIII by thrombin and platelets
- Enhanced thrombin formation
- Thrombin then converts fibrinogen to fibrin, which is cross-linked by the XIIIa
enzyme (activated by thrombin) to form cross-linked fibrin
Where are tissue factors highly expressed
Lungs Brain Heart Testis Uterus Placenta
Draw a diagram that represents the initiation of coagulation
-
Describe how coagulation is regulated (and which each mechanism inhibits)
Antithrombin = FXa, FIXa and thrombin
Tissue factor pathway inhibitor = TF-VIIa + FXa
Protein C + protein S = FVa, FVIIIa
Describe the protein C pathway
- 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 warfarin and how does it work
Anti-coagulant - long-term anticoagulation after venous thrombosis and for atrial fibrillation patients
Inhibits thrombin production by inhibiting vitamin K epoxide reductase
Explain how inhibition of vitamin K epoxide reductase (warfarin) causes an anticoagulant effect
Before factors II, VII, IX, and X can bind to platelet membrane phospholipids, their glutamate residue needs to be gamma-carboxylated in the liver, using vitamin K.
What is heparin and how does it work
an anticoagulant
Accelerates the action of antithrombin
Heparin binds to
antithrombin and makes it easier to bind thrombin/Xa.
Heparin is used for immediate anticoagulation in venous thrombosis or pulmonary embolism.
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 anti platelet agents
Aspirin (COX-1)
P2Y12 blockers
What are the 3 ways of monitoring blood coagulation
Activated partial thromboplastin time (APTT) Prothrombin time (PT) Thrombin time (TT)
What is the Activated partial thromboplastin time (APTT)
This tests the intrinsic and common pathways. Coagulation is initiated via factor XII and any abnormalities can be detected. Used to monitor heparin treatment.
What is prothrombin time
This tests the extrinsic and common pathways. Coagulation is initiated via tissue factor and any abnormalities can be detected. Used to monitor warfarin treatment.
What is thrombin time
This tests the common pathway only. Thrombin is added and the conversion of fibrinogen to fibrin can be checked for abnormalities. Also known as thrombin clotting time (TCT).
