Haemostasis Flashcards
What is haemostasis?
- Life-preserving processes designed to maintain blood flow. Prevent further haemorrhage after, for example, a vascular injury
- Responds to tissue injury, curtails (reduces) blood loss, restores vascular integrity and promotes healing, and limits infection.
- Protective process evolved in order to maintain a stable physiology.
The response to injury depends on closely linked interaction between:
- blood vessel wall,
- circulating platelets
- coagulation factors
Response needs to be tightly controlled
- to prevent extensive clots developing
- to be able to break down such clots once damage is repaired
SIMPLIFIED STEPS
Tissue injury
Vasoconstriction
Platelet activation
Haemostatic plug formation
Coagulation
Stable clot formation
Clot dissolution
What are the four key components of haemostasis?
- endothelium of blood vessel
- coagulation factors/inhibitors
- platelets - large SA to absorb coagulation factors. Many membrane receptor agonists such as GPIa/IIb complex & GPVI which are receptors for collagen. AlsoGlycoproteins (GPIb & IIb/IIIa) allows attachment of platelets to von Willebrand Factor (vWF), then to endothelium.
- fibrinolysis
What makes a blood clot?
- fibrin mesh
- platelets
- red blood cells
What are the three stages of the haemostatic system?
PRIMARY HAEMOSTASIS;
- vasoconstriction (immediate) - responsible for initial slowing of blood flow near site of injury. Reduced blood flow allows activation of platelets and associated haemostatic factors.
- platelet adhesion (within seconds)
- platelet aggregation and contraction (within minutes)
SECONDARY HAEMOSTASIS:
- activation of coagulation factors (within seconds)
- formation of fibrin (within minutes)
FIBRINOLYSIS:
- activation of fibrinolysis (within minutes)
- lysis of the plug (within hours)
Describe primary haemostasis.
- With haemostasis at rest, the triggers and cofactors are separated by endothelial cells. Coagulation is inhibited and platelet aggregation is prevented
If the endothelial tissue is gone, blood is exposed to components of the subendothelial tissue at site of injury. These tissues express tissue factor at their surface, which binds to endogenous activated Factor VII. When collagen, which was in the subendothelial tissue, is exposed, it attracts Von Willebrand factors (vWFs). - Normally, vWFs circulate in the blood as globules, but when there is an injury, it exposes a sticky part of its molecule which recognises collagen, so it will stick to the site of injury.
- The vWFs bind platelets to the damaged vessel wall. When the platelets bind, they change morphologically (become activated); they become flatter for a larger surface area. They also release important mediators of coagulation. vWFs also stabilise and protect Factor VIII from rapid clearance. Due to their role in acting as a connection between platelets and vessel wall through the GP1b receptor, loss of vWF function causes bleeding disorders. vWFs are synthesised at endothelial cells Weibel Palade bodies - stored in megakaryocytes.Plasma VWF entirely derived from endothelial cells
- More platelets aggregate to site of injury and become activated due to binding of subendothelial collagen to platelet receptors, and eventually, coagulation occurs on the platelet surfaces. Growth of plug occurs due to aggregation which covers the exposed connective tissue.
- Activated platelet releases a number of important cytokines and chemical mediators from its granules.
- Include ADP, vWF, thromboxane A2, serotonin and coagulation factors
- ADP promotes platelet aggregation to form primary haemostatic plug.
- Thromboxane A2 also potentiate aggregation.
- This primary haemostatic plug of platelets, localised to the site of injury, provides the phospholipid surface upon which secondary haemostasis takes place. The primary plug is temporary and unstable.
- vWF activity occurs under shear stress because, in areas of injury, the blood flow becomes more turbulent.
What is secondary haemostasis?
- Stabilisation of the platelet plug with fibrin(blood coagulation), contracting the clot.
- When a vessel wall is damaged, various signalling molecules are expressed / exposed, including tissue factor and collagen. The tissue factor leads to the production of a small local amount of thrombin. The exposed signalling molecules attract circulating platelets, which attach themselves to the exposed sub-endothelial tissue. These platelets become activated – principally through the presence of the thrombin – and release further attractant chemicals, which attract more platelets as well as other plasma-borne clotting factors. These new platelets bind to the adhered platelets and themselves become activated. Through the conformational changes inherent in activation, the loose platelet plug contracts to form a dense, adherent plug
- After primary haemostasis, there is then the activation of coagulation factors which then form the fibrin clot. This is blood coagulation. The complex of factors binds small amounts of Factors X and V to the exposed endothelial surface, which produces small quantities of thrombin. The activated factors (among them FVIII and FIX) enable the binding of activated FX and FV to the surface of platelets whose activation has produced conformational changes in their surface membranes to expose the ‘reaction sites’ necessary for the continuation of the process. This leads to the ‘thrombin burst’ that is necessary for the large-scale production of fibrin and so the development of an effective clot Prothrombin is converted into thrombin. Thrombin, along with Ca2+, causes fibrinogen to be converted to fibrin. Fibrin mesh binds to the platelet plug and helps form the clot.
Conversion of fibrinogen to fibrin is COAGULATION
- These three stages are called the initiation, amplification and propagation phases of coagulation
Where are most coagulation proteins made?
Most coagulation proteins are made in the liver. Vitamin K is also needed, because some factors require activation via Vitamin K.
If someone has liver disease, they may have bleeding problems because no factors are being made.
Describe fibrinolysis (tertiary haemostasis).
It is the process of clot dissolution, which allows for repair and the proper healing process (ie. replacing the clot with normal tissue).
Its main functions are the clot limiting mechanism and the repair and healing mechanism.
There are a series of tightly regulated enzymatic steps, such as feedback, potentiation and inhibition.
The key players in this process are:
- Plasminogen
- Tissue plasminogen activator (t-PA) and urokinase (u-PA)
- Plasminogen activator inhibitor -1 and -2
- α2-plasmin inhibitor
Protein C and S also act to prevent coagulation, mainly by inactivating factors
V and VIII
Describe the process of fibrinolysis.
Plasminogen is converted into Plasmin, with the help of tissue plasminogen activator (t-PA). Plasmin involved in degradation of fibrin to FDPs.
D dimers are generated when cross-linked fibrin is degraded.
FDP (fibrin degradation products) are generated if non-cross-linked fibrin or fibrinogen is broken down.
Blood can be tested for D dimers to see if a blood clot is degrading properly.
t-PA and a bacterial activator, streptokinase, are used in therapeutic thrombolysis for myocardial infarctions (to help remove the blood clot).
What are the two ways in which a blood clot can form?
- through Tissue Factor-driven clot formation
- through surface activation
Describe the balance of haemostasis, and what happens when it is unbalanced.
In normal haemostasis, fibrinolytic factors and anticoagulant proteins (preventing blood clot), and coagulation factors and platelets (creating blood clot) are in equilibrium.
If there is an increase in coagulation factors and platelets and decrease in fibrinolytic factors and anticoagulant proteins, this can lead to thrombosis (formation of a blood clot inside a blood vessel).
Thrombosis is the pathological process whereby platelets and fibrin interact with the vessel wall to form a haemostatic plug to cause vascular obstruction
- may be arterial, causing ischaemia
- or venous, leading to stasis
- Possible conditions due to thrombosis are ischaemic heart and various cerebrovascular conditions
One of the conditions this could lead to is chronic venous insufficiency (CVI), which can cause atrophic changes, hyperpigmentation, ulceration and infection. This occurs because the legs don’t get the nutrients, etc. needed.
If there is an increase in fibrinolytic factors and anticoagulant proteins and decrease in coagulation factors and platelets , this can lead to easy bruising (ecchymosis) and increased bleeding.
Defective haemostasis with abnormal bleeding may result from:
- A vascular disorder
- Thrombocytopenia or a disorder of platelet function
- Defective blood coagulation
Outline how haemostatic abnormalities can be tested for.
Describe a coagulation screen.
Simple tests are employed to assess the platelet, vessel wall and coagulation
components of haemostasis
e.g. Blood count and blood film examination, screening tests of blood coagulation,
individual coagulation factor assays, assay of von Willebrand factor
A coagulation screen can assess for deficiences of coagulation factors. There are three times that can be measured:
PROTHROMBIN TIME (PT)
- sensitive to the extrinsic pathway and, to a lesser extent, the common pathway
- tissue factor driven - prolongation indicates deficiency or inhibition of one or
more of the following coagulation factors: VII, X, V, II and fibrinogen
- most common causes of disorder are liver disease , Warfarin therapy and DIC
ACTIVATED PARTIAL THROMBOPLASTIN TIME (APTT):
- sensitive to the intrinsic pathway and, to a lesser extent, the common pathway
- contact activated - prolongation indicates deficiency or inhibition or one or
more of the following coagulation factors: XII, XI, IX , VIII X, V, II, fibrinogen
- common cause of disorder is haemophilia
THROMBIN TIME (TT);
- sensitive to defects in conversion of fibrinogen to fibrin
- prolongation indicates deficiency or abnormalities of fibrinogen or inhibition of thrombin by heparin or FDPs
- most common cause of disorder is heparin therapy and DIC(disseminated intravascular coagulation)
Platelet count and the tests of platelet function are also used in screening patients with a bleeding disorder
Outline the name and function of the various coagulation factors.
Fibrinogen (I)
- Forms clot (fibrin)
Prothrombin (II)
- Its active form (IIa) activates I, V, VII, XIII, protein C, platelets
Tissue factor (III)
- Co-factor of VIIa
Calcium
- Required for coagulation factors to bind to phospholipid (formerly known as factor IV)
Factor V (proaccelerin, labile factor)
- Co-factor of X with which it forms the prothrombinase complex
Factor VII (stable factor)
- Activates Factors IX, X
Factor VIII (antihaemophilic factor)
- Co-factor of IX with which it forms the tenase complex
Factor IX (Christmas factor)
- Activates Factor X: forms tenase complex with factor VIII
Factor X (Stuart-Prower factor)
- Activates Factor II: forms prothrombinase complex with factor V
Factor XI (plasma thromboplastin antecedent)
- Activates Factors XII, IX and prekallikrein
Factor XII (Hageman factor)
- Activates prekallikrein and fibrinolysis
Factor XIII (fibrin-stabilizing factor)
- Crosslinks fibrin
von Willebrand factor
- Binds to Factor VIII, mediates platelet adhesion
What occurs with the following deficiencies?
Factor VII deficiency
Factor XII deficiency
FVII deficiency causes bleeding
FXII deficiency not associated with bleeding
Outline the role of tissue factors in coagulation
- TF is outside the lumen
- Formation of TF-FVIIa complex
- Recruitment of Factor X and formation of thrombin
