The Healthy Circulation and Physiologic Hemostasis Flashcards
Blood is
A complex liquid flowing under pressure
Normally, blood will not interact with each other
What happens to blood under injury?
Blood is stopped by clot formation and normal flow is restored
Physiologic Clot is
Just right in size and strength and persists just long enough
influences on blood flow
Vessels,
Flow
Blood itself
Virchow’s triad
Blood flow normally
Denser RBCs in the middle with laminar flow….platelets and plasma at periphery
Blood flow in anemia
Narrower central column, less viscous, flows faster, and thicker outer sleeve with more opportunities for endothelial interaction…more prone to excess bleeding
Polycythemia affects on blood flow
More viscous so thicker central column…more opportunities for endothelial interaction so more likely to thrombosis
Healthy endothelium
Keep flow moving and avoid interaction with passing bloodstream components except for purpose of promoting normal flow…if endothelium activate, it can secrete procoagulant substances (normally held in reserve)
Endothelial cells and NO
Endothelium forms NO from L-arginine via NOS…diffuses across cell membranes and causes vasodilation while inhibiting platelet adhesion and aggregation
NO destroyed by hemoglobin and short half life (local interactions)
Intravascular hemolysis effecto n NO
Causes local depletion of NO
What is normally bound to endothelial surface
Heparans and thrombomodulin - part of natural anti-coag system
Heparans
Expressed on endothelial surface bound to glypican and syndecan glycoproteins
Similar to heparin
Bind and activate antithrombin, enable anticoagulant effect confined to normal endothelial surface
Effect is to inhibit unwanted clotting in the intact circulation
Thrombomodulin
Expressed on intact endothelium, cooperates with thrombin to activate protein C
Effect is to confine clot formation to the site of the injury by inhibiting the spread to adjacent normal endothelium
Prostacylcin
INtact endothelial cells adjacent to disrupted endothelium enzymatically release arachidonic acid
COX2 converts to prostacyclin
COX2 predominates on endothelium, induced by laminar blood flow
Effect of PGI2 is vasodilation and maintenance of healthy blood flow
Thromboxane
COX1 converts AA into thromboxane A2 in platelets
Stimulator of platelet activation and aggregation and produces vasoconstriction
Endothelial COX2 generates PGI2 which blocks [platelet aggeregation and antagonizes TxA2 mediated vasoconstriction
NET effect is an adjustable tension between vasodilatory of PGI2 and vasoconstrictive of TxA2
tPA
Tissue plasminogen activator is secreted by endothelium under stress or vascular occlusion…initiates fibrinolyis
Helps limit clot size
Endothelin
Endothelium secretes vasoconstrictive peptides called endothelins…immediate but temportary
In healthy flow vs. acute bleeding
Healthy - vasodilatory of NO predominantes
Acute bleeding - endothelin mediated vasoconstriction precomes predominant
Out of balance in fascular dz
vWF, ADAMTS-13, FV8, Tissue factor
All help to clot (pro-coagulants)
vWF - primary hemostasis
ADAMTS-13 - cleaves vWF
F8 - secondary hemostasis
TF - not normally expressed but exposed with endothelial acitivation to begin secondary hemostatic process
Primary and secondary hemostasis
Primary - platelet interactions with vWF and each other (platelet plug)
Secondary - interactions of procoagulant proteins with cell surfaces, platelet plug, and each other (fibrin clot)
Occur at same time
Modulated by natural anticoagulant and fibrinolytic systems
Overview of primary hemostasis formation
Damage exposes vWF…circulating platelets bind to vWF, collagen activates platelets…platelets release substances ot recruit other platelets…activated platelets sitck together via fibrinogen bridges
Enhancing the platelet plug
Damaged or activated endothelial cells release additional high weight vWF factors that are cleaved by ADAMTS-13 protease
Key structures of a platelet
Phospholipid bilayer
Contractile proteins that flex into its activated form
Dense bodies - contain smaller molecules and ions like ADP, serotonin and calcium
Alpha granules - contain 300 proteins and peptides with various functions
Platelet receptors
Throbin, epinephrine, ADP, and collagen
AA released from platelet membrane and converted to TXA2
GPIb/9/5 receptor - platelet adhesion to GPIb binding domain on vWF molecule (always exposed)
GP2b/3a recepotr - binding platelets into aggregated platelet plugs (concelaed on quiescent platelet but exposed on activated)
Can also make NO
Activation of platelets causes
Granule release, shape change, recruitment, and aggregation
vWF
Produced in endothelial cells and megakaryocytes
Linear monomer that dimerize via disulfide bonds…dimers form multimers
Circulating ADAMTS-13 cleaves into smaller ones
vWf binding sites
FV8, collagen, GB1b and GB2b/3a
Cleavage site of ADAMTS-13 q
3 pools of vWF
Circulating - rolled up to conceal GPIb binding domain…binds F8 and prevents prevents degradation by protein C (chaperone function)
Subendothelial deposit fomr - stretched out by collagen binding…poised for action if injury occurs (by binding platelets)
Endothelial storage granule form - ultra large monomers unroled that can be extruded for rapid hemostasis…cleaved by ADAMTS-13
Once adherent to subendothelial vWF via GPIb interaction
Platelet also comes in contact with subendothelial collagen which activates platelet…platelet undergoes shape change and exposes GP2b/3a receptors, release of granule contents
How is ADP imprtant in platelet recruiting process
Interacts with P2Y12 ADP receptor on nearby resting platelet
Aggregation
Occurs once Gp2a/3b is exposed and ready to engage with a fibrinogen molecule…as many as six platelets can engage with a fibrinogen molecule
PLatelet plug function
Initially stops bleeding and serves as scaffolding for secondary hemostasis
Disorders = thrombosis and bledding
Secondary hemostasis
Fromation of fibrin strands on and witin the platelet plug which knit plug together int oa fibrin clot
Activation of fibrin forming coagulation cascade
Begins with exposure of TF…on all cells but RBCs
TF location
Subendothelial, not on actual endothelium…exposed with damage…heavy in the brain and placental villi
Tissue factor and F7
Factor 7 wraps around TF…activated to become protease (F7a)…cleaves and activates factor X…factor X cleaves prothombin into thrombin
Coagulation facotrs
Made in liver
F8 made in endothelium
Most are serine proteases that cleave
Factors 5 and 8 are cofactors for Fx and FIX
Secondary hemostasis general process
Latent but poised (coag factors criculate as zymogens)
Multistep cascade that is amplified
Localized to site of injury
Requires platelet and cell membrane surfaces for efficient factor interaction
Coordinated choreography
Cell membranes (including platelet plug) is where secondary hemostasis takes place
Sticking to the stage (secondary)
Factors 7 and other serine proteases employ carboxylated glutamates to bind calcium ions
Calcium helps bind factors to cell membvranes so they can function properly
Vitamin K role
Factors 7, 9, 10, prothrombin dependent and also anticoagulant proteins C and S
Glutamic acid rich tdomains in 3 structure
Gamma carboxy glutamic acids needed for interaction with membranes and calcium
Vit K dependent protein creation
Gamma carbons of glutamate residues are carboxylated by Vit K dependent enzyme…needed to make functional prothrombin as well
Vit K dependent protein structure
Gamma carboxylation permits interactions with calcium and cell/platelet membranes
No vitamin K—- no gamma carboxylation —- no membrane bound factor complexes
Vit K
Dietary sources VK2 made by gut bacteria Def with diet or AB use Replacement given orally Subq - poor absorption in obese IM - hematoma risk IV - anaphylaxis risk
Recycling of VK2
VK2 catalyzes gamma carboxylase and in process oxidized to VKO…VKO restored to normal form beginning with VK epoxide reductase (VKOR)
Warfarin
Inhibits VKOR the VK2 recylcing function
In vivo cascade
Factor 7a activates factor X which cleaves prothrombin into thrombin…these are on the phospholipid bilyaer ot TF cells
TF bearing cells are
Sequestered away from vessel lumen by intact endothelium
Initiation phase
INjury exposes TF…circulating F7 complexes with exposed TF and calcium…converts F7 to F7a…cleaves F10 and F9 into active forms…F10a cleaves prothrombin (F2) into thrombin (F2a)
TFPI
Tissue factor pathway inhibitor
Modulates early secondary hemostasis by reversibly inhibiting FXa…the FXa-TFPI can then inhibit the F7a-TF complex
Thrombin difference
Not membrane bound and catalyzes a bunch of reactions
Thrombin function
Platelet activating substance that stimulates platelet activation and construction of platelet plug
Activates F11…begins acceleration or porpogation phase of secondary hemostasis
Cleaves fibrinogen to fibrin
Activates F13…F13 cross links fibrin polymers domain E to D
Fibrin monomers
Spontaneuously dimerize and polymerize
F12 does NOT
Participate in secondary hemostasis in vivo…improtant in the test tube
Key events of secondary hemostasis
Interaction of F7 and TF to initiate the process and generation of thrombin to propogate it
2 parts of natural anticoag system
C/S and antithrombin system
C protein
Serine protease activatedby thrombin during secondayr hemostasis…thrombin complexes with thrombomodulin bound to uninjured endothelial membrane adjacent to injury
Protein S
Cofactor for protein C
Activated complex cleaves F5a and F8a
Sequence of C/S systme
F7a-TF complex cleaves F10 into F10a at site of injury
F10a complexes with activated cofactor F5a and cleaving prothrombin into thrombin
Thrombin complexes with TM on uninjured membrane and cleaves protein C to APC…APC complexes with protein S to cleave F5a and F8a (cofactors to F10 and F9) into inactive forms
Inhibition of F5 and F8 slows down the clotting cascade just adjacent to the injury
Basically makes sure clot doesn’t spill over
Antithrombin system
Keeps circulation moving by preventing unwanted clot formation from happening in first place
AT is a weak inhibitor of serine proteases but when complexed with heparans, it becomes much more active and can inhibt all of the serine proteases in procoag cascade
Heparans
Natural ligand for AT
Bound to surface of endothelial cells
Role in inhibiting unwanted coagulation
Circulation is naturally “heparanzed” in vicinity of endothelium
Fibrinolytic system
Limit size of clot formation and lyse the clot when job is done…also role in resolution of pathologic thrombosis
Plasmin
Serine protease that lyses clots by cleaving fibrin molecules…can also cleave fibrinogen
Circulates as plasminogen and activated by tPA (tissue type plasminogen activator) which is made inthe endotheluum
tPA
Made by neodthelial cells and Release in response to stress or vascular occlusion
Low affinity for circulating plasminogen but high affinity for fibrin wihtin a thrombus…binds and activates local plasminogen resulting in fibrin spcific physiologic fibrinolysis
Fibrinolysis does not occur diffusely but localized othe clot
PAI-1
Inhibits tPA
PAI-1 def would allow overactivity of tPA leading to pleeding
alpha 2 antiplasmin
Serine protease inhibitor which inactivates circulating plasmin…when complexed with fibrin, helps retard firbrin degradation
Summation of fibrinolysis
Fibrin holds togehter platelet plug with attached plasminogen and plasmin…tPA released andbound to fibrin…cleaves plasminogen to plasmin which in turn cleaves fibrin and releases fibrin degradation products into circulation…free plasmin escaping clot is captured by alpha 2 antiplasmin and not allowed to float away
How is fibrinolysis modulated
At the beginning by PAI-1 that degrades tPA
D-dimer
Specific cleavage product of cross linked firbin that is normally measured
Fibrinogen made into mesh by thrombin and then crosslinked by factor 13…plasmin then cuts into D dimer
D dimer measurement
Elevation demonstrates PRIOR formation of cross linked fibrin
Expected to be elevated in known VTE, traume, or recent surgery
Negative predictive value - if not elevated, helps rule out VTE in cases of suspected PE
