Day 5- Lecture 1- Haemostasis Flashcards
What is haemostasis?
The stopping of a haemorrhage
Haemostasis has to act within seconds- how can this be helped along therapeutically to slow down blood loss and allow the clotting process to take effect?
- Apply pressure to a bleeding point
- Suturing of an injury
- Application of a topical agent that aids clotting
What is clotting?
The process whereby blood (a liquid in normal blood vessels) becomes a solid mass when it makes contact with connective tissue
What is the end result of activation of the clotting system?
- Production of the enzyme thrombin
- This acts on fibrinogen (circulating plasma protein and is soluble)
- Produces fibrin filaments (insoluble) which are then deposited and trap red blood cells
What is the name given to the system that destroys clots?
Fibrinolysis
Blood will clot as soon as it is spilled from the vessel- what 4 cells can be in contact with blood without clotting it?
- Endothelial cells
- Red blood cells
- White blood cells
- Unactivated platelets
Blood has to be maintained in a fluid clot-free state but be poised to produce a rapid and localised solid plug at the site of any vascular injury
What are the 3 steps of haemostasis?
VASOCONSTRICTION
- Severed artery constricts
- Not enough to stop the bleeding but enough to decrease the pressure downstream
- Contraction does not occur in veins but the pressure in them is much lower
PLATELET PLUG FORMATION (seconds to minutes)
- Primary haemostatic plug of activated platelets form at the hole in the vessel
- They stick to the injured vessel and the connective tissue around it
- Fragile but may control bleeding
BLOOD CLOT FORMATION (around 30 minutes later)
-Secondary haemostatic plug forms as fibrin filaments stabilise the fragile platelet plug into a blood clot
What are the 3 main ‘players’ in haemostasis?
- Platelets
- Process of blood clotting
- The vascular wall
Name 4 substances that activate platelets?
- Collagen surfaces: within the extravascular space
- ADP: released by activated platelets and injured red blood cells and amplifies the platelet response
- Thromboxane A2: a powerful platelet aggregator which is also released by activated platelets
- Thrombin: informs platelets that the clotting sequence is activated
When platelets are activated, what do they do?
- Stick to the exposed subendothelium (basement membrane or collagen) specifically to von Willebrand factor which is concentrated on the subendothelial basement membrane
- Aggregate with other platelets. This is how the platelet plug, and then the secondary haemostatic plug grows. Fibrinogen binds to the platelets and sticks them together.
- Swell and change shape to sticky, spiny spheres
- Secrete factors from platelet granules that help the platelet plug to grow and aid clotting e.g. Some fibrinogen, ADP, thromboxane A2
What does aspirin do?
- Aspirin irreversible activates cyclooxygenase (one of the enzymes responsible for the production of thromboxane A2)
- It therefore decreases platelet aggregation
Production of what protein is a mark of the endpoint of the clotting cascade?
Fibrin
What enzyme cleaves fibrinogen to fibrin?
Thrombin
Thrombin cannot circulate in the blood in the active state or blood would be solid, so what activates thrombin?
A group of circulating molecules called clotting factors numbered I to XIII
What substances require vitamin K for their synthesis?
Clotting factors:
- Factors II
- Factors VII
- Factors IX
- Factors X
Anticoagulants:
- Protein C
- Protein S
Most clotting factors are proenzymes- what is a proenzyme?
A biologically inactive substance which is metabolised into an enzyme (each proenzyme activates the next in line and amplifies the effect)
Give 2 examples of co-factors for the enzymes that are required in effective clotting
Phospholipids and calcium
Many of the interactions involved in clotting require assembly of the components on a surface- what provides this surface?
Provided by the platelet membranes when they swell and change shape during activation
What are the two pathways for clotting?
Intrinsic pathway and extrinsic pathway
Explain the intrinsic pathway
- It is called intrinsic because it involves factors, all of which are contained within the blood
- Triggered by a negatively charged surface (e.g. Subendothelium or glass)
- No vessels need to be broken for it to occur
Is the vascular wall passive in haemostasis?
No
Describe the extrinsic pathway
- Needs a tissue factor (thromboplastin- clotting factor III) which is present outside of the blood
- Pathway is triggered by thromboplastin released from damaged cells adjacent to the area of haemorrhage
When does the arterial media contract?
When an artery is damaged
What does the subendothelium do?
Trap platelets
The endothelium performs a balance between opposing and favouring clotting- name some substances involved in each
Endothelium secretes substances which oppose clotting e.g.
- Plasminogen activator which activates fibrinolysis
- Thrombomodulin which interferes with the clotting cascade by activating protein C
Endothelium secretes substance which favour clotting e.g.
- Von Willebrand factor
- Tissue factor
What factors oppose clotting?
The dilution of clotting factors by blood flow and natural anticoagulants
Natural anticoagulants: oppose the formation of fibrin (they do not destroy it after it has been formed- that is fibrinolysis)
Fibrinolysis: Fibrin degradation products inhibit clotting
What are the 3 main anticoagulants and what will occur if they are missing any of these proteins?
- Antithrombin III
- Protein C
- Protein S
If a person lacks any of these protein they will experience repeated episodes of thrombosis
Explain briefly ‘the evolution of the clot’
- Platelets in the clot die
- As they die they cling to the fibrin and pull by their actin-myosin filament system (same as muscle contraction)
- Clot retraction helps to pull sides of small wounds together
- May also toughen the clot by squeezing out fluid
What is fibrinolysis and when does it occur?
Once the hole in the vessel has been repaired the blood clot is dissolved by fibrinolysis
What is the enzyme responsible for fibrinolysis and what does it destroy?
Plasmin- fibrin builds up and is no longer needed, macrophages recognise it and break it down, the destruction of this fibrin is by the enzyme plasmin
Like thrombin, plasmin circulates as an inactive precursor- what is the name of this precursor and where is it made?
Plasminogen
In the liver
Name 3 plasminogen activators and where are they found?
Tissue plasminogen factor (tPA) which also circulates in the blood
Urokinase- found in the urine
Streptokinase- obtained from streptococci so not usually present in the body
Plasminogen activates are used therapeutically as they dissolve fibrin and therefore thrombi and thromboemboli- which plasminogen activators are used?
Streptokinase is antigenic- therefore a person should only be given it once
Tissue plasminogen activator (tPA)- has a higher affinity for fibrinogen than streptokinase and is not antigenic so it can be given more than once
What is a common side affect of therapeutically given plasminogen activators?
Bleeding (commonly occurs from the gums or nose, but more seriously it can occasionally occur in the brain)
What sets fibrinolysis in motion and how does it do so?
- The clotting cascade
- Fibrin increases the activity of tissue plasminogen activator (tPA)
- This produced plasmin
- Plasmin breaks down fibrin to fibrin degradation products (FDPs e.g. D-dimer)
When are fibrin degradation products increased?
In conditions where there is thrombosis- for example:
- Disseminated intravascular coagulation
- Deep vein thrombosis
- Pulmonary embolism
After surgery what happens to fibrinolytic activity?
It drops and remains low for around 7 to 10 days- so in this time period there is an increased risk of post-operative thrombosis
What is the final result of a clot?
- Clot becomes organised by fibrous repair
- Replaced initially with granulation tissue and then a tiny scar
Name 2 inherited bleeding disorders-
- Haemophilia A and B
- Von Willebrand Disease
What step of haemostasis do people with haemophilia lack?
They have normal platelets but cannot produce an adequate amount of fibrin- hence, impaired clotting
What is haemophilia A?
A deficiency of factor VIII - it is the most common hereditary disease associated with serious bleeding - patients have either a decreased amount or decreased activity of factor VIII.
What is the genetic nature of haemophilia A?
-X linked recessive (commonly affects males)
When are mild symptoms seen in haemophilia A?
When there is 6-50% of factor VIII amount/activity
When are severe symptoms seen in haemophilia A?
When there is less than 1% normal amount/activity of factor VIII
What are the symptoms of haemophilia A?
- Easy bruising
- Massive haemorrhage after trauma or surgery
- ‘Spontaneous’ haemorrhages in areas subject to minor trauma e.g. Joints (haemarthrosis is a haemorrhage into a joint-> recurrent bleeding into joints leads to joint deformities
Tip: petechiae (pinpoint haemorrhages) are not seen because they are caused by blood leaking from capillaries, which is typically a result of vasculitis or abnormalities in the number or function of platelets
What is Haemophilia B (Christmas disease)
Factor IX deficiency- clinically indistinguishable from Haemophilia A
It is also X linked recessive and has variable clinical severity
What is the test results for haemophilia?
- Normal platelet count
- Normal bleeding time (as this is a measure of platelet activity)
- Normal PT (prothrombin time)
BUT
-Prolonged APTT (which is the measure of the intrinsic pathways which factor VIII and factor IX is part of)
What is the treatment for haemophilia?
Haemophilia A: recombinant factor VIII
Haemophilia B: recombinant factor IX
What is von willebrand disease?
-Most common inherited bleeding disorder due to the deficiency or abnormality of von willebrand factor
What are the symptoms of von willebrand disease?
- Many asymptomatic
- Can have severe bleeding disorder
Common pattern of bleeding is mucosal bleeding reflecting the inadequate platelet function and adhesion
What are the 2 functions of von willebrand factor?
- Assists in platelet plug formation by attracting circulating platelets to the site of vessel damage
- Stabilised factor VIII protecting it from premature destruction
What is the test results of von willebrand disease?
Bleeding time and APTT can both be raised in this condition
What is thrombocytopenia?
A normal platelet count is 150-400x10^9/L -> a count less than 100x10^9/L is classified as thrombocytopenia
Under what platelet count does spontaneous bleeding occur?
20x10^9/L
What step of homeostasis will a low or non-functional platelet count lead too?
Lack of step 2- platelet aggregation and primary haemostatic plug formation
What is the test results for someone with thrombocytopenia?
- Low platelet count
- Prolonged bleeding time
- Normal PT
- Normal APTT
(As both PT and APTT assess the clotting cascade and not the platelet function)
What type of spontaneous bleeding occurs in thrombocytopenia and where?
The bleeding appears as petechiae- the spontaneous bleeding is seen from small vessels in places such as:
- Skin
- Gastrointestinal tract
- Genitourinary tract
Occasionally intracerebral bleeding can occur
What are the 4 causes of thrombocytopenia? Explain them briefly
- Decreased production of platelets
- Due to bone marrow infiltrated by malignancy
- Drugs e.g. Cytotoxic drugs
- Infections e.g. Measles or HIV
- Folate deficiency (which are needed for platelet production) - Decreased platelet survival
- Due to immunological destruction e.g. Immune thrombocytopenia purpura
- Non-immunological destruction e.g. Disseminated intravascular coagulation - Sequestration - in a enlarged spleen (hypersplenism)
- Dilutional- due to massive blood transfusions (blood stored for more than 24 hours does not contain platelets)
What is disseminated intravascular coagulation?
A thrombohaemorrhagic disorder occuring as a secondary complication in a variety of disorders
What happens in DIC?
- An activator of clotting gets into the blood and microthrombi are formed throughout the circulation
- This process consumes platelets, fibrin and coagulation factors and activates fibrinolysis
What any be the result of DIC?
May experience haemorrhage
DIC never occurs as a disease itself, but secondary to another condition- give some examples?
- Sepsis (especially gram negative sepsis as such bacteria produce endotoxins which activates clotting)
- Severe trauma (especially in the brain as it contains large amounts of thromboplastin)
- Extensive burns
- Complications of childbirth (e.g. Amniotic fluid embolism, retained dead foetus)
- Malignancy
- Snake bite
So treat the underlying cause!
If bleeding is a prominent feature in DIC what treatment may be needed?
Transfusions of:
- Platelets
- Fresh frozen plasma (FFP which contains clotting factors)
- Cryoprecipitates (which contain factor VIII, fibrinogen, von willebrand factor and factor XIII)
- Red blood cells
Occasionally treatment with an anticoagulant such as heparin is required
List some conditions that can arise from microvascular thrombosis in DIC?
- Neurological impairment
- Gangrene of the skin
- Renal failure
- Respiratory distress
- Gastrointestinal ulceration
List some conditions that can arise from haemorrhagic component in DIC?
- Intracerebral bleeding
- Petechiae
- Haematuria
- Epistaxis
- Gastrointestinal bleeding
What is a test result that can be used in DIC?
-As the fibrinolytic system is activated FDPs such as D-diner are released in large numbers and can be measure in the blood
Red blood cells are often traumatised and fragmented as they squeeze past the microthrombi- results in anaemia. What is the name given to this type of anaemia?
Microangiopathic haemolytic anaemia
What is a thrombophilia?
Inherited or acquired defects of haemostasis resulting in predisposition to thrombosis e.g. Deep vein thrombosis
Give some examples of thrombophilia
- Factor V Leiden -> which there is an abnormal factor V which isn’t deactivated resulting in thrombosis
- Antithrombin deficiency
- Protein C or Protein S deficiency
- Antiphospholipid syndrome
