Anatomy and physiology of blood cells 2 - platelets
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What is the name of the process used by the body to stop bleeding from a ruptured vessel?
What are the three main physiological processes involved in haemostasis?
- Vascular response
- Platelet plug formation
- Coagulation to form clot
Describe what happens during the vascular response in haemostasis
Damage to the endothelium releases local substances which act on the chemoreceptors in the smooth muscle to bring about contraction of the smooth muscle to cause a reduction in the opening of the blood vessel to reduce blood loss
Describe what happens during platelet plug formation in haemostats
If there is a break in the blood vessel wall then the platelets can get access to the surrounding tissue around the blood vessel. If platelets get out and make contact with collagen they react with collagen and become active. Activated platelets contract and release enzymes that act on arachadonic acid and convert it into thromboxane A which activates more platelets. This means a POSITIVE FEEDBACK SYSTEM has been set up. Activated platelets become sticky and stick together and to the wall of the blood vessel with the help of von Willedbrand factor (a type of protein anchored in the blood vessel wall) to form a platelet plug which is held on to the site of damage.
How is platelet plug formation limited to the site of damage?
Healthy endothelium is able to generate and enzyme which converts arachadonic acid to prostaglandin 1 / prostacyclin which is a profound inhibitor of platelet plug formation. Therefore the positive feedback system is limited to the place where there is damage to the tissue.
Briefly describe what happens during coagulation in haemostasis
Fibrinogen is an inactive soluble component of plasma which is converted by the enzyme thrombin into loose fibrin threads. These insoluble fibrin threads will form a mesh. Inactivated Factor XIII is activated by thrombin and cross links the fibrous threads to form a stable structure - the clot.
What are the three pathways involved in coagulation?
The intrinsic and extrinsic pathways which both feed into the final common pathway
What activates the intrinsic pathway?
Tissue damage (exposure to collagen)
Why is the coagulation pathway referred to as a cascade reaction?
As each step in the pathway causes an amplified response - each factor can activate multiple factors at the next stage
What are some of the anticlotting mechanisms used to limit clot formation to the site of damage?
- 1 - The same mechanism which prevents platelet aggregation also has an effect to inhibit clot formation. It has a negative effect on factors VIII and V which inhibits thrombin formation.
- 2 - If thrombin is present it will act on protein C activator which catalyses the conversion of protein C into activated protein C which inhibits factors VIII and V, which consequently inhibits thrombin formation. This is a negative feedback system.
- 3 - Antithrombin III reacts with heparin and inhibits thrombin formation.
What two processes does dissolution of a blood clot depend on?
- Clot retraction - as a blood clot dries out it starts to pull the two edges of the wound together
Describe what happens during fibrinolysis
- Endothelial cells secrete t-PA which converts plasminogen (an inactive clotting factor which is constantly in circulation) to the enzyme plasmin; plasmin breaks up fibrin into soluble fibrin fragments.
- Fibrin has a positive action on t-PA and thrombin inhibits an inhibitor of t-PA
Give some examples of anticlotting drugs/agents
- Ca2+ chelating agents eg EDTA
Give some examples of common diseases associated with blood clotting
- Von Willebrands disease
- Haemophilia (A or B)
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