Blood - Platelets and Hemostasis Flashcards
How long do platelets circulate in the blood?
9-12 days
What is thrombocytopenia?
Low platelet count
What is thrombocytosis?
Excessive amount of platelets
Where are platelets produced?
Red bone marrow
What 3 factors influence platelet production?
Thrombopoietin, interleukin-6, and multi-CSF
What are the three phases of hemostasis?
Vascular phase, platelet phase, and coagulation phase
How long does the vascular phase last?
Around 30 minutes
What happens during the vascular phase of hemostasis?
- Vascular spasm occurs, which decreases the diameter of the vessel at the site of injury
- Endothelial cells contract and expose the basement membrane to the bloodstream
- Endothelial cells begin releasing chemicals which promote smooth muscle contraction and promote vascular spasms
When does the platelet phase start in terms of time after injury and triggers?
15 seconds after injury and when platelets start sticking to endothelial surfaces, to the plasma membrane, and to exposed collagen fibers
What is platelet aggregation?
As the number of platelets at the site of injury increases, they start sticking to each other
What is platelet adhesion?
The attachment of platelets to exposed surfaces
When do platelets activate?
Platelets become activated when they arrive near the injury site
What is a sign of platelet activation?
First sign of activation is that they get more spherical and they develop cytoplasmic processes that extend toward nearby platelets
What compounds to platelets release during activation and what do they do?
- Adenosine diphosphate: stimulates platelet aggregation and secretion
- Thromboxane A2 and serotonin: stimulate vascular spasms
- Calcium ions: Help platelet aggregation and several steps in the clotting process
- Prostacyclin: Inhibits platelet aggregation and is released by endothelial cells
When does coagulation phase start?
After 30+ seconds
What are the three coagulation pathways?
Extrinsic, intrinsic, and common
How does the extrinsic pathway start?
Starts when damaged endothelial cells release factor III
What does the extrinsic pathway do?
It then combines with Ca2+ and Factor VII to form an enzyme that activates factor X, the first step in the common pathway
How does the intrinsic pathway start?
Begins with the activation of proenzymes (Usually factor XII) exposed to collagen fibers at the injury site
What does the intrinsic pathway do?
Platelets release many other things that accelerate the reactions of the intrinsic pathway, ultimately making an enzyme complex capable of activating Factor X
What is the common pathway?
When factor X combined with 5a and calcium activates thrombin, causing further activation of platelets and the enhanced formation of fibrin, which holds the platelets together in strands
What is the speed of both pathways in relativity to the other and which one is more important?
Intrinsic is slower than the extrinsic but the intrinsic is more important
What proteins and chemicals stop clotting?
- Plasma contains anticoagulants which inhibit clotting
- Heparin: accelerates the activation of antithrombin-III
- Thrombomodulin: a protein released by endothelial cells that binds to thrombin and converts it to an
enzyme that activates protein C
What is fibrinolysis and how does it happen?
- Clot gradually dissolving
- Plasminogen is activated by thrombin and tissue plasminogen activator
- Activated plasminogen produces plasmin, which digests the fibrin strands in the clot
