Week 3-haemostasis Flashcards
What does the term haemostasis mean?
Arrest of bleeding and the maintenance of vascular patency.
What are the four requirements of haemostasis?
Permanent state of readiness
Prompt response
Localised response
Protection against unwanted thrombosis.
What makes up primary haemostasis?
Formation of the platelet plug
What makes up secondary haemostasis?
Formation of the fibrin clot.
What occurs after secondary haemostasis?
Fibrinolysis- the clot is broken down (to maintain patency of the blood vessels)
What are anticoagulant defences?
They are proteins that switch off the coagulation pathway.
Where are platelets formed?
In the bone marrow, they bud off from megakaryocytes.
Do platelets have a nucleus?
No they are anucleated.
What is the life span of a platelet?
7-10 days.
Describe the process that leads to platelet adhesion (not aggregation) after injury?
Endothelial (vessel wall) damage leads to exposure of collagen and releases Von Willebrands Factor and other proteins to which platelets have receptors for. This leads to platelet adhesion at the site.
Describe how the platelets then aggregate to form the platelet plug?
The platelets already adhered to the site release chemicals which cause other platelets to stick to them. Forming the platelet plug.
What kind of injury is primary haemostasis in response too?
Small injuries e.g. paper cuts. If you have a larger injury the platelet plug won’t hold so you need a fibrin clot to form.
When might you have failure of primary haemostasis (platelet plug formation)?
Vascular issue (the vessel itself could be deficient in collagen)
Platelets- reduced number (thrombocytopenia), reduced function (drug induced)
Von Willebrand Factor (this acts as a glue that sticks the platelets together, without this you cant get aggregation).
What is a condition when you get decreased collagen in vessel walls?
Henloch Schonlein Purpura
What are the consequences of having a failure in platelet plug formation?
Spontaneous bruising and purpura (mostly lower limbs due to gravity- this is where the blood leaks out)
Mucosal bleeding- e.g. nose bleeds, GI bleeding, conjunctival bleeding, heavy periods
If the platelets arent working at all- intracranial haemorrhage
Describe the initiation stage of forming the fibrin clot?
Platelets secrete calcium onto their surface which is positively charged. Circulating clotting factors are all negatively charged so they will come and stick to the platelets. They start off inactive, and when they stick they become active.
When you get tissue damage, tissue factor (TF) is released from damaged tissues. TF activates clotting factor number VII, which in turn activates factor V and X.
Describe the propagation stage of forming the fibrin clot?
Factor X and V work to break down prothrombin into its active form, thrombin. Thrombin cleaves fibrinogen to form fibrin. However only a small amount of fibrin is produced.
Describe the amplification stage of the formation of the fibrin clot.
The thrombin generated also activates factor VIII and IX which then activates more factor V and X which then creates more thrombin and therefore more fibrin. This means the plug formed is of substantial size.
If there is a single clotting factor deficiency, where is the bleeding likely to occur?
Likely to occur into joints and muscles. Classicly the ones that weight bear more e.g. ankle, knee, elbow. This is due to small tears occuring in the muscles all the time, normal people respond to this by forming a fibrin clot however in patients with a clotting factor deficiency, this doesnt occur.
