Haemostasis is the arrest of bleeding from the interaction between blood vessels and
platelets
Coagulation is the formation of fibrin that involves plasma proteins and calcium ions
Fibrinolysis is the dissolution of fibrin
There are 4 main systems to arrest blood loss
o Vascular system
o Platelets
o Coagulation system
o Fibrinolytic system
Platelets
Platelets are essential for haemostasis
o Form primary haemostatic plug
They facilitate coagulation by providing a phospholipid surface and clotting factors
o They also promote healing of the vascular endothelium using platelet derived
growth factor
Platelets consist of irregular smooth discs and have a life span of 7-10 days
The components of platelets consist of
o Alpha granule
o Electron dense granule
o Glycogen
o Submembranous filaments
o Dense tubular system
o Mitochondria
o Glycocalyx
Surface glycogens play role in platelets including
o GPIA - for adhesion to collagen on exposed subendothelial
o GP1B and GPIIb/IIIa - for attachment to platelets to vWF
, o GPIIb/IIIa - for the binding site of fibrinogen
Phospholipid Membrane
Arachidonic acid is bound to membrane phospholipids
It is liberated by the action of specific phospholipases allowing it to take part in the
prostaglandin pathway
Neutral phospholipids on the outside and amino phospholipids on the inside
o Neutral phospholipids include phosphatidylcholine and sphingomyelin
On injury Flip by PL scramblase allows coagulation complexes to assemble
Prostaglandins
Activated adenylyl cyclase has a positive effect on cAMP levels
This lowers Ca2+
Therefore inhibiting membrane hydrolysis
Thrombopoiesis
Thrombopoiesis is the formation of platelets in the bone marrow by megakaryocytes
Megakaryocytes arise from CFU-Meg
Platelet formation is regulated by thrombopoietin formed by the liver and kidney
o It acts through C-MPL receptors
o It increases maturation and release of platelets
Thrombocytopaenia means low numbers of circulating platelets
o Its caused by leukaemia, aplastic anaemia and cytotoxic drugs
Granules
Dense granules contain non protein materials
o ADP for aggregation
o ATP for energy
o Serotonin as a vasoconstrictor
o Ca2++ for coagulation and actin and myosin coupling
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