Platelets I Flashcards
- What is the diameter of platelets
1-4 µm in diameter
- What is the blood concentration of platelets
Blood concentration = 150,000-400,000 per µl of blood
- How long do platelets circulate in the blood
Circulate for 7-10 days
- How are platelets removed from the blood
Removed by macrophages in the liver and spleen
- What is the ultrastructure of platelets enclosed by
Enclosed by plasma membrane with open canalicular system
- What is expressed on the surface of platelets
Surface expression of glycoprotein receptors
- What type of ring of cytoskeleton does a platelet have
Ring of microtubule cytoskeleton
- What organelles can be found in platelets
Mitochondria
Golgi apparatus
Endoplasmic reticulum
- What does the endoplasmic reticulum store in platelets
stores Ca2+ (calcium)
- What do platelet granules store
stores LOTS of mediators important for haemostasis
- What 8 mediators are found in platelet granules
Clotting factors (see later)
ADP/ATP
Calcium
Serotonin
Prostaglandins
Thromboxanes
Fibrin-stabilising factor
Platelet-derived growth factor (PDGF)
- Define coagulation
the process through which blood changes from a liquid to a clot (thicker)
- Define fibrinolysis
removal of a clot (thrombus) after it has formed
- Outline what happens during the vascular spasm (phase 1) phase of primary haemostasis
- Endothelial cell contraction
- Release of chemical mediators/hormones
- What does endothelial cell contraction allow to happen
allows exposure of underlying collagen matrix to bloodstream
- What happens after the release of chemical mediators/hormones
- smooth muscle contraction
- proliferation of endothelial, smooth muscle and fibroblast cells
- What does phase 1 result in and what does this do for the injured vessel
- Results in vasoconstriction.
- It reduces blood flow through the injured vessel
- Outline what happens during platelet phase (phase 2) of primary haemostasis
Platelet adhesion
Platelet activation
Platelet aggregation
- What happens during platelet adhesion
Platelets adhere to exposed collagen and themselves via platelet glycoprotein receptors
- What does von Willebrand factor do
Encourages adherence to endothelium
- What happens during platelet activation
- Platelets become activated and:
- develop finger-like projections
- release granule contents
- ADP, thromboxane A2 (TxA2) and thrombin activate more platelets
- Serotonin and TxA2 maintain vasoconstriction
- What happens during platelet aggregation
- ADP makes platelets become sticky
- clotting factors are released
- Conformational change in the platelet receptor IIb/IIIa allows binding of fibrinogen which forms bridges between platelets
- The formed mass forms the platelet plug
- What does the coagulation of blood depend on
depends on the balance between procoagulants and anticoagulants
- When does coagulation occur
occurs when vessel injury initiates a chain reaction resulting in activation of clotting factors
- What are clotting factors
Clotting factors are serum proteases
- Explain how the common pathway works
- Activated Factor X combines with prothrombinase, Factor Va and calcium to convert prothrombin to thrombin
- Thrombin converts soluble fibrinogen into insoluble fibrin
- Factor XIIIa cross links the fibrin fibres and fibrin combines with calcium too to form a stable fibrin clot
- Give features of the extrinsic pathway
- Cascade of reactions involving factors that originate OUTSIDE of the blood
- Triggered with tissue damage
- FAST (~15 sec!)
- Give features of the intrinsic pathway
- Cascade of reactions involving factors that originate WITHIN the blood
- Triggered when blood contacts with a negatively charged surface (e.g. glass, activated platelets)
- Slow (1-6 mins)
- Explain how the extrinsic pathway activates Factor X
- Peripheral tissues have membrane glycoprotein called Tissue Factor (thromboplastin)
- When the endothelium is damaged, Tissue Factor comes in contact with Factor VII, activating it.
- Factor VIIa, Tissue Factor and calcium combine to activate Factor X
- Explain how the intrinsic pathway activates Factor X
- Factor XII is activated when blood comes in contact with negative charges (e.g glass, activated platelets, collagen)
2.activated platelets also release Platelet Factor 3
- Factor XIIa initiates a series of proteolytic steps that are calcium dependent which lead to the formation of Tenase complex
- This Tenase complex along with Platelet Factor 3 activates Factor X
- What 4 clotting factors undergo post-translational modification when in the presence of the reduced form of Vitamin K
Factor:
- II (prothrombin)
- VII
- IX
- X
- What does vitamin K reductase do
catalyses the conversion of vitamin K to its reduced form
- How does warfarin lead to reduced clotting
inhibits vitamin K reductase which reduces the action of Factors II, VII, IX and X
- Explain how clot retraction works to contribute to haemostasis
- Platelets attached to fibrin fibres contract
- This compresses the fibrin meshwork into a smaller mass
- As the clot retracts, it pulls the torn edges of the vessel closer together
