Hemostasis & Coagulation Flashcards
What does hemostasis mean?
“Stop Blood”
“Haima/hemo-” = Blood
“-stasis” = Stop
What does hemorrhage mean?
Abnormal blood flow
“Hemo/haima-“ = blood
“-rhagia” = abnormal flow
What are the 3 critical elements of hemostasis?
- vascular wall
- platelets
- coagulation cascades
What is the central regulator of hemostasis?
The endothelium.
At steady-state the endothelium promotes anticoagulant factors:
- inhibit platelet aggregation
- inhibit coagulation
- promote fibrinolysis
During hemostasis (after injury or pathogen exposure for instance), the endothelium promotes pro-coagulant factors:
- hemodynamic forces
- inflammation
What percentage of the blood is made up of platelets?
About 4%
After centrifuge of whole blood, what layer are the platelets contained in?
Platelets can be found in the buffy coat layer of centrifuged whole blood.
What progenitor cells do platelets arise from during development?
Megakaryocytes in the bone marrow
What is Thrombopoietin?
Thrombopoietin is a hormone produced primarily by the liver that encourages platelet development.
What are thrombocytes?
Another name for platelets
True or False:
Platelets have a nucleus/nuclei.
False.
Platelets contain proteins and some other cell organelles such as mitochondria, ER, & granule-filled vesicles, but they do NOT contain a nucleus.
What are the two major types of granules that platelets contain? What are their roles?
α granules - contain fibrinogen, fibronectin, factors V and VIII, Platelet factor-4, platelet derived growth factor (PDGF), and transforming growth factor β (TGF β)
δ granules - contain ATP, ADP, Ca2+, histamine, serotonin, and epinephrine
Both of these granule types play critical roles in hemostasis & coagulation.
What are the 4 steps of hemostasis?
- Arteriolar Vasoconstriction
- Primary Hemostasis
- Secondary Hemostasis
- Anti-thrombotic counter-regulation
What hormone do damaged endothelial cells release to trigger vasoconstriction?
Endothelin
Once endothelin is released, it will bind to smooth muscle cells and trigger an influx of calcium into the cell, leading the smooth muscle tissues to contract, which causes vasoconstriction.
What happens during step 1 of hemostasis?
Arteriolar Vasoconstriction
- Endothelin released from damaged epithelial cells = increase in intracellular calcium = local smooth muscle contraction
- local myogenic reflex mechanisms = local smooth muscle contraction
- inflammatory mediators stimulate local neurons/ANS = local smooth muscle contraction
All of these events lead to vasoconstriction.
What happens during step 2 of hemostasis?
Primary Hemostasis
- Steady-state platelet inhibition (mediated by endothelial cells) is impaired once endothelial cells are damaged.
- The damage to the endothelium exposes subendothelial extracellular matrix (basement membrane) which contains collagen fibers.
- Damaged endothelial cells release von Willebrand factor (vWF) which allows platelets to then adhere to the site of damage & initiate the “Platelet Response” (Adhesion, Activation, Aggregation)
- The platelet response is what leads to the formation of the primary hemostatic plug (primary hemostasis)
What is the von Wille brand factor (vWF)?
A clotting factor that is released from the damaged endothelial cells of blood vessels when damaged.
It binds to exposed collagen fibers in the area where the endothelium has been damaged, acting as the “molecular glue” that allows platelets to adhere to the damaged vessel wall.
What is a myogenic reflex?
The vascular smooth muscle automatic reflexive response to contract in response to injury or mechanical stress.
It is one of the mechanisms of vasoconstriction that occurs during step 1 of hemostasis.
What are the 5 events that occur during primary hemostasis (step 2 of Hemostasis)?
1) Platelet adhesion to damage site via vWF
2) Platelet shape change
3) Platelet granule release
4) Further platelet recruitment
5) Platelet aggregation & primary hemostatic plug formation
What occurs during the first step of the platelet response?
Platelet Adhesion
- vWF binds to exposed collagen
- platelets contain a high affinity receptor for vWF, a protein called Glycoprotein 1b (Gp1b), that allows them to bind tightly to collagen-bound vWF
What are the 3 major steps of the “Platelet Response” during hemostasis?
The 3 A’s
1) Adhesion
2) Activation
3) Aggregation
What occurs during the second step of the platelet response?
Activation
- After adhesion, an irreversible conformational change in platelet shape occurs to “seal” the damage
- Then the platelets release δ granules, α granules, & Thromboxane A2, which induce further coagulation factor recruitment, platelet activation, aggregation, & vasoconstriction
What are the major factors released by α granules?
What functions do these factors serve in the coagulation cascade?
Fibrinogen - crucial for platelet aggregation & coagulation cascade. Found in platelets but also circulating freely in plasma. Once released, will bind the fibrinogen receptor GP2B on the platelets surface to complex with other platelet molecules, causing platelet aggregation.
- Thrombospondin
- Platelet Factor IV
- Platelet Factor V
What are the major factors released by δ granules? What functions do these factors serve in the coagulation cascade?
Calcium - an important cofactor for the enzymatic coagulation cascade
ADP - in addition to being activated by vWF, platelets also contain receptors for ADP that, once bound, can activate them as well, AND cause platelet aggregation as more platelets pass by & become activated by the released ADP. ADP also induces a conformational change in the fibrinogen receptors on the surface of platelets, Glycoprotein 2B/3A (GP2B/3A), which allow fibrinogen (once released by the alpha granules of platelets to then bind to platelets surface themselves.
Serotonin - enhances the vasoconstriction ( first step of hemostasis) & acts as a chemoattractant as well, by recruiting immune cells to the damage site
What occurs during the third step of the platelet response?
Aggregation
Platelets aggregate via the GP2B/3A receptor proteins on their surface that form platelet-fibrinogen complexes.
What happens during step 3 of hemostasis? What are the 4 major events occuring?
Secondary Hemostasis (aka the coagulation cascade)
Enhances clot formation & tethers clot in place via these major events:
1) Tissue factors released from damaged endothelial cells (extrinsic pathway)
2) Phospholipid complex expression on platelet surface
3) Thrombin activation
4) Fibrin polymerization
What is the major goal of the coagulation cascade?
To generate/activate thrombin, a major clotting factor, from prothrombin.
What are the two major pathways of the coagulation cascade/secondary hemostasis (step 3 of Hemostasis)?
- Intrinsic Pathway - begins with the activation of Factor XII
- Extrinsic Pathway - begins with the exposure of Tissue Factor (which includes TF, Thromboplastin, & Factor III)
What is Tissue Factor?
An activating factor in the extrinsic pathway of the coagulation cascade during secondary hemostasis (step 3 of hemostasis).
It is a procoagulant glycoprotein that is synthesized by endothelial cells & released in response to damage.
Once released, Tissue Factor will then activate Factor VII in the coagulation cascade
What are the basic requirements of the coagulation cascade?
Three basic elements are required for the coagulation cascade to take place:
- Enzyme (active coagulation factor)
- Substrates (proenzymes/next factors to be activated in series; including circulating clotting factors from the liver & locally synthesized proteins)
- Co-Factors (which accelerate/regulate the enzymatic reactions)
- Calcium (Ca2+)
- Vitamin K
Essentially a proenzyme is initiated & activated and that enzyme will initiate & activate the next enzyme in the sequence, and so on & so forth, until thrombin is generated.
What is Factor I & where is it synthesized?
Factor I = Fibrinogen
Fibrinogen is synthesized in the liver.
What is Factor II & where is it synthesized?
Factor II = Prothrombin
Prothrombin is synthesized in the liver.
What is Factor III & where is it synthesized?
Factor III = Tissue Factor (TF)
Tissue Factor is synthesized in endothelial cells.
What is Factor IV & where is it synthesized?
Factor IV = Calcium ions
Calcium ions are sourced from the plasma.
What is Factor XII & where is it synthesized?
Factor XII = Hageman factor; activates intrinsic pathway
Factor XII is synthesized in the liver.
What is Factor XIII & where is it synthesized?
Factor XIII = Fibrin Stabilizing Factor (FSF); cross-links fibrin to tether/stabilize secondary clot formation.
What are the factors involved in the intrinsic pathway of the coagulation cascade from initiation to fibrinogen activation?
12 —> 11 —> 9 + 8 —> 10 + 5 —> 2 (prothrombin cleavage) —> 2a (thrombin) —> 1 (fibrinogen cleavage) —> 1a (insoluble fibrin)
What are the factors involved in the extrinsic pathway of the coagulation cascade from initiation to fibrinogen activation?
7 + 3 (Tissue factor) —> 10 + 5 —> 2 (prothrombin cleavage) —> 2a (thrombin) —> 1 (fibrinogen cleavage) —> 1a (fibrin)
Define Embolus
An embolus is when a thrombus (clot) breaks away and moves freely in the blood stream.
Define Embolism
An embolism is when the embolus obstructs a blood vessel
What occurs during step 4 of hemostasis?
Clot Retraction & Fibrinolysis
- Platelet Contraction (pulls damaged endothelial cells closer together)
-Endothelial/smooth muscle tissue repair (Platelet derived growth factor, PDGF, & Vascular endothelial growth factor, VEGF) - Fibrinolysis (plasminogen is activated to plasmin, which breaks down fibrinogen & fibrin)
How is blood kept thin 7 clot-free during steady-state conditions?
Various Anticoagulation mechanisms:
Nitric oxide & prostacyclin - released by healthy endothelial cells; inhibit platelet aggregation.
ADPase - produced by healthy endothelial cells; degrades ADP, preventing platelet activation.
Thrombomodulin - Excess Factor 2a (thrombin) binds to thrombomodulin on healthy endothelial cells inactivating thrombin itself, & Factors V & VII.
Antithrombin III - binds and inactivates thrombin & factor X, as well as Factors VII, IX, XI, & XII.
Tissue-type plasminogen activator (tPA) - released by healthy endothelial cells to activate plasminogen to plasmin & keep fibrinogen & fibrin degraded
Tissue factor pathway inhibitor (TFPI) - released by healthy endothelial cells; inhibits factor VIIa.
