Hemostasis Foundations Flashcards
what is hemostasis?
the mechanism by which bleeding comes to an end is called hemostasis (or, simply, clotting)
requires the precise and collective activity of plasma, vascular, and platelet factors
also need an anti-hemostasis system to control hemostasis
How is hemostasis balanced?
Hemostasis involves a delicate balance between pro-hemostasis (clotting) and anti-hemostasis (anti-clotting) forces.
Disruptions in this equilibrium can lead to either bleeding or thrombosis.
what are the steps to forming a clot?
when a hole appears in a blood vessel, pro-clotting forces immediately rush to the scene and form a clot
- blood vessel constriction
- platelet plug formation (primary hemostasis)
- fibrin formation (secondary hemostasis)
what happens during blood vessel constriction?
immediately after a vessel is damaged, regional small arterioles constrict
constriction reduces the amount of blood loss at the site of injury, and makes it easier for the coagulation factors and platelets to come in direct contact with each other and with the injured endothelium, so that the processes of platelet plug formation and fibrin formation can begin
what happens during a platelet plug formation?
aka primary hemostasis!
- at the injury site, platelets adhere to the blood vessel wall = adhesion
- then the platelets pile up on top of each other = aggregation
end result is a soft platelet plug that covers the hole of the damaged vessel
platelet plug on its own is not a permanent solution, because without fibrin to glue it together, it quickly falls apart
what is primary hemostasis?
platelet plug formation!
what happens during fibrin formation?
aka secondary hemostasis
involves sealing the platelet plug in place with a molecule called fibrin
fibrin is made via the coagulation cascade
where is tissue factor found?
TF is normally “hidden” from the bloodstream until a vessel is damage
then it is released from endothelial cells (and some other places), and it initiates the coagulation cascade along the extrinsic arm
what happens as soon as a little bit of factor X is activated?
tissue factor pathway inhibitor (TFPI) shuts down the extrinsic pathway
the tiny bit of thrombin that was formed along the extrinsic pathway is just enough to kick off the intrinsic pathway because the thrombin activates factor XI
coagulation continues now through the final common pathway to form fibrin
what stops the coagulation cascade?
- TFPI: switches off the extrinsic arm
- antithrombin
- proteins C and S
as soon as enough fibrin has been formed to stabiliize the clot, these guys turn the cascade off by inhibiting certain enzymes, effectively shutting down any further formation of fibrin
how do you dissolve a clot?
once the vessel wall is sealed and repaired, excess clot needs to be eliminated = clot remodeling
clots are destroyed in a process called thrombolysis or fibrinolysis via plasmin
what’s a pathologic clot?
one that you didn’t intend to make and that is serving no good purpose
what enzyme dissolves clots? how does it do it?
plasmin
it cleaves the fibrin surrounding the clot, leaving little fragments of clot which simply wash away
