Lecture 7/8 - Haemostasis Flashcards
Haemostasis: what is it and what are the 5 main components of it
The ability to minimise blood loss after injury
Blood vessels - vasoconstriction
Platelets
Coagulation factors
Coagulation inhibitors
Fibrinolysis
What is the process of hemostatic plug formation?
Vessel injury - collagen exposure - platelet adhesion - platelet activation - platelet aggregation - primary hemostatic plug
Vessel injury + platelet acivation - vasoconstriction - reduced blood flow - primary hemostatic plug
Vessel injury (tissue factor) + platelet phsopholipid (platelet activation) - thrombin - fibrin + primary hemostatic plug - hemostatic plug
Endothelial cells: what do they do, how do they affect hemostasis, and what do they produce
Maintain vascular integrity
Have potent anti-hemostatic influence (prostacyclin, nitric oxide, vasodilatory properties, platelet adhesion inhibition)
Von Willebrand factor (vWF), tissue plasminogen activator (fibrinolysis)
Vasoconstriction
Immediate (few minutes) smooth muscle cell activation before other mechanisms take over
Platelets
Produced in bone marrow as a fragment (2-4µm diameter) of a megakaryocyte (1 ->4000)
Lifespan of 9-10 days, normal platelet count 150-400x10^9/L
Destroyed in spleen and liver by Kupffer cells
700-40-1
Platelet production
Endomitotic synchronous nuclear replication
Nuclear lobes increase in multiples of two. Cytoplasmic volume increases and becomes more granular and platelets are produced
Negative feedback - number of platelets circulating
Stimulated by thrombopoietin (TBO) and stimulated by IL-3 and GM-CSF
Platelet structure: what are the components, what do they do, and what do glycoproteins do?
- Glycocalyx (contain glycoproteins (GPIa, GPIb, GPIIb, etc))
- Glycogen (energy supply), dense tubular system (prostaglandin and thromboxane A2 synthesis)
- Platelet contractile proteins (make aggregation irreversible)
- Electron dense (delta) granules (contain Ca²⁺, ADP, ATP, and serotonin)
- Alpha granules (platelet-derived growth factor, platelet factor 4, fibrinogen, vWF, factors V and VIII)
- Canalicular system - allow granule release
The glycoproteins allow adhesion to surfaces like collagen
Platelet function
Adhesion, secretion (granule release reaction), and aggregation
vWF often binds with GPIIb and GPIIIa
Hemostatic stable plug
Secretes ADP - and thromboxane A2 - increasing aggregation
Platelets swell due to the ADP presence
Positive feedback occurs
Coagulation factor I: name and active form
Fibrinogen
Fibrin
Coagulation factor II: name and active form
Prothrombin
Serine protease
Coagulation factor III: name and active form
Tissue factor
Co-factor
Coagulation factor V: name and active form
Labile factor
Co-factor
Coagulation factor VII: name and active form
Proconvertin
Serine protease
Coagulation factor VIII: name and active form
Antihaemophilic factor
Co-factor
Coagulation factor IX: name and active form
Christmas factor
Serine protease
Coagulation factor X: name and active form
Stuart-Prower factor
Serine protease
Coagulation factor XI: name and active form
Plasma thromboplastin antecedent
Serine protease
Coagulation factor XII: name and active form
Hageman factor
Serine protease
Coagulation factor XIII: name and active form
Fibrin stabilising factor
Serine protease
Coagulation cascade initiator
TF is the main initiator and binds with coagulation factors (VII-VIIa)
The process of fibrin generation
Injury -> Factor VII is activated and binds to tissue factor forming factor VIIa TF
VIIa TF -> activates both factor X and IX
IXa and VIIa -> activate factor X
Xa and Va -> convert prothrombin into thrombin
Thrombotic activation -> Va, XIa, and IXa and VIIIa formation
Thrombin converts fibrinogen and fibrin
XIa -> activates more XI and IX
Tissue factor pathway inhibitor (TFPI)
Coagulation cascade initiation: what are the steps?
Initiation:
* Extrinsic tenase complex
* Generation of trace thrombin
* Down-regulation of initiation by TFPI
Amplification
Propagation:
* Formation of intrinsic tenase complex
* Formation of prothrombinase complex
* Burst of thrombin generation
Initiation: Extrinsic tenase complex
Involves FVIIa, TF, FX, phospholipids, Ca²⁺
Initiation: Generation of trace thrombin
Factor IIa is activated, Ca²⁺ is present, and factor Xa activates a small amount of thrombin
Initiation: Down-regulation of initiation by TFPI
Factor Xa is limited by TFPI
Amplification
The produced factor IXa and thrombin, although not enough to convert fibrinogen into fibrin, can amplify the process by feedback and activate factor VIII, a co-factor for factor IX, and factor V, a co-factor for factor X
This results in an amplified cascade
Propagation: Formation of intrinsic tenase complex
FXa and FVIIa activate FIX and the intrinsic tenase complex is formed, consisting of FIXa, FX, FVIIIa, Ca²⁺, and Phospholipids (obviously meaning this is occurring on the membrane, near the site of injury(?))
Propagation: Formation of prothrombinase complex
The prothrombinase complex is composed of FXa, FVa, FII, Ca²⁺, and phospholipids
Since the process has been amplified, the inhibitor has less of a role in inhibiting FX
This process ends with prothrombin being activated
Propagation: Burst of thrombin generation
FXa, FVa, FIIa, Ca²⁺
FVa and FXa - convert prothrombin into thrombin
Thrombomodulin, a thrombin co-factor, has procoagulant and anticoagulant properties
The produced thrombin increases the process to be stimulated by positive feedback
The end result of this process is a stable fibrin clot, only degradable by fibrinolysis
The core components of the extrinsic tenase, intrinsic tenase, and prothrombinase pathways?
Extrinsic tenase: FVIIa, TF, FX, Ca²⁺, and K⁺
Intrinsic tenase: FIXa, FVIIIa, FX, Ca²⁺ and K⁺
Prothrombinase: FXa, FVa and FII, Ca²⁺ and K⁺
Fibrin clot: when is it added, what happens to the platelet plug, what is the final form of the hemostatic plug, and what mediates clot retraction?
Fibrin is added to the primary hemostatic plug, causing platelet mass and platelet-induced clot retraction/compaction
Fibrin increases as the platelet plug degranulates/autolyses
After a few hours, the entire haemostatic plug is composed of cross-linked fibrin
Clot retraction is medicated by glycoprotein receptors (GpIIb/IIIa)
What are the inhibitors present in the activating of the tissue factors?
Tissue Factor Pathway Inhibitor (TFPI) – VIIa and Xa
Antithrombin III – Xa and thrombin
Protein C and Protein S – PS inactivate Va and VIIIa, the action of PS is enhanced by PC and PC also enhances fibrinolysis
Tissue factor pathway inhibitor: how quickly does it act, where is it found, what causes its accumulation, and what is inhibited?
1st inhibitor to act
Found in plasma and platelets
Accumulates due to platelet activation
Inhibits FVIIa and FXa
Antithrombin: where is it synthesised and what does it inhibit/inactivate?
Synthesized in the liver and endothelium.
Inactivates serine proteases, principally factor Xa and thrombin
Protein C: what is it, how is it activated, how is its action enhanced, and what does it do?
Vitamin K-dependent protein made in the liver
Activated via a complex which inactivates factors Va and VIIIa
The action of protein C, enhanced by the action of protein S, binds protein C to the platelet surface
Activated Protein C enhances fibrinolysis by inactivating the tissue plasminogen activator (tPA) inhibitor
Protein S: what is it, how is it activated, and what does it do?
Vitamin K-dependent protein made in the liver
Activated via a complex which inactivates factors Va and VIIIa
Enhances the action of protein C
Fibrinolysis: what is it, how does it occur, and what is generated?
The degradation of fibrin (clot)
tPA activates plasminogen to plasmin which degrades fibrin
Generates soluble fragments called fibrin degradation products (FDPs)
Fibrinolytic systems
Plasminogen is activated into plasmin by tissue plasminogen activator (tPA) as well a factor XIIa, and then plasmin breaks fibrinogen down into fibrin
Streptokinase: what is it?
A drug used to break down clots extremely effectively
What inhibits the fibrinolytic system?
Plasminogen activator inhibitor (PAI-1) inhibits tPA and factor XII
α2-antiplasmin inhibits the action of plasmin
What methods are used to detect haemolytic issues?
Bleeding time: how is it monitored, how long should bleeding normally last for, and in what conditions is the bleeding time normal/abnormal?
Application of pressure to the upper arm (blood pressure cuff) with small incisions made in the forearm surface skin
Bleeding normally stops in 3-8 mins
Prolonged in thrombocytopenia (reduced platelet count) but normal in cases of abnormal vascular bleeding
Platelet count and function: how are platelet counts done and how is platelet function monitored?
- Full blood count – platelet count
- Bone marrow examination - when platelet issues are suspected
- Platelet aggregometry – most valuable
- Measure the change in light absorbance as the platelets aggregate
- Adhesion studies
- Von Willebrand assay
Prothrombin time: what does it do, what is the normal PT, and how is it standardised?
Measure factors VII, X, V, prothrombin, and fibrinogen and their integrity
Normal PT is 10-14 seconds
Prolonged in liver disease, oral anticoagulant treatment (OAT), e.g. warfarin
PT standardised as International Normalised Ratio (INR) – used to monitor OAT
Activated partial thromboplastin time: what does it do, what is the normal time, and when is it prolonged?
APTT measures factors VIII, IX, XI and XII as well as X, V, prothrombin and fibrinogen
Normal 3-40 seconds
Prolonged in haemophilia, heparin therapy
Which measuring technique measures which haemolytic pathway?
Extrinsic pathway - PT
Intrinsic - APTT
Contains a little overlap when it comes to factors X, V, and prothrombin and fibrinogen
This allows for some diagnosis as if either PT or APTT is fine but the other isn’t then you know there is an issue with the other
