Platelets (Thrombocyte)
Developed from myeloid stem cells 150,000 – 400,000 per µL of blood
2-4 µm in diameter
No nucleus or organelles
Have granules that release chemicals promoting blood clotting Lifespan: 5-9 days
Function : help stop blood loss from damaged blood vessels by forming a platelet plug.
Hemostasis : a sequence of responses that stops bleeding.
There are 3 mechanisms that reduce blood loss: vascular spasm, platelet plug formation, and blood clotting or coagulation. The ultimate goal is to prevent Hemorrhage , the loss of a large amount of blood from the vessels.
1) Vascular Spasm
In response to damage, the smooth muscle in the walls of arteries and arterioles vasocontrict immediately. This temporarily reduces blood loss for several minutes to hours allowing other hemostatic mechanisms to take action.
2) Platelet Plug Formation
Within platelets are an array of chemicals: enzymes, clotting factors, ADP, ATP, Ca++, Serotonin, lysosomes, some mitochondria. The formation of a platelet plug is to stop external bleeding.
Platelet plug formation occurs in 3 steps :
Platelet Adhesion : endothelial cells produce Von Willebrand factors (VWF) which binds to collagen fibres of underlying layers of the damaged vessel. Platelets contact and stick to the VWF & the damaged blood vessel forms a ‘bridge’. Platelet Release Reaction : platelets become activated through binding, extending projections that enable them to release the contents of their vesicles. This includes chemical agents & thromboxane A2. Platelet Aggregation : the release of chemical agents makes other platelets sticky causing a gathering of platelets forming a mass called the platelet plug.
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