test 6 Flashcards
Circulatory Homeostasis (hemostasis)
Normal blood circulation is maintained by a system of
checks and balances
preservation of blood fluidity
ability to seal off any site of bleeding
Balance of circulatory homeostatis is determined by the number and function of (2)
anticoagulant factors
procoagulant factors
Arterial circulation and response
-high pressure, narrow, flow velocity high
requires rapid response system to seal off any bleeding sites
platelets take leading role followed by fibrin formation (antiplatelet agents used to prevent coronary thrombosis)
-use antiplatelet drugs
Venous circulation and response
-low pressure, low velocity of flow
slower response acceptable
rate of thrombin generation takes leading role (antithrombin agents used to prevent deep venous thrombosis)
-platelets not as important as they are on the arterial side
-use antithrombin agents
anticoagulant factors
released by endothelial cells that line the vascular system (prostacyclin, vascular plasminogen activator)
procoagulant factors
platelets
plasma proteins – inactive state (zymogen)
Tend to be activated when the lining of the vascular system is disrupted (endothelial cells damaged or destroyed)
-endothelial lining is not conducive to clot formation
-everything inside and behind the endothelial cells are procoagulants
What Happens When An Arterial Blood Vessel Is Damaged?
Vascular constriction
Platelet adhesion
Platelet activation
formation of the platelet plug (clump and first blockage to prevent leakage)
Activation of coagulation and formation of fibrin clot**
Clot retraction
Activation of fibrinolytic cascade (stops additional coagulation)
Vessel repair / regeneration
The importance of platelet aggregation and what they do
platelets localize & accelerate process
provide essential negative phospholipid surface for various reactions
provide receptor sites for specific cascade factors
release some of the cascade factors
-not just the number of platelets that is important, but the platelets MUST be activated at the site of the injury
two groups the clotting factors broken down into
Proenzymes (also called zymogens)
Procofactors
Proenzymes (also called zymogens)
Factors that become enzymes when activated
-then participate in further activity within the clotting process (converting inactivated substances to activated substances)
Procofactors
- once activated they have no enzymatic activty
- must bind with another active substance (enzyme) to create some type of a complex which enhances the action of the enzyme itself
Additional clotting factors
High-molecular-weight kininogen
Antithrombin; Antithrombin III
Antiplasmin
Plasminogen activator inhibitor
cascade model
Factors are activated in a specific sequence creating
cell-based model
Seems to provide a better representation of what is happening in vitro
Cascade defined as a series of steps where enzymes and their cofactors cleaved other proenzymes (zymogens) to create the next enzyme in the step
Some enzymes need to bind with appropriate cofactor to function properly
Most of the steps were postulated to occur on the surface of phospholipid membranes and required calcium
Interaction of thrombin with fibrinogen the major exception
Three pathways (2 entry, 1 common) lead to the production of the fibrin clot
What is the Coagulation Cascade
Series of proteolytic reactions that lead to the formation of a fibrin clot
Driven by serial activation of various plasma proteins (normally inactive)
Two points of initiation
-intrinsic and extrinsic
Common pathway (prothrombin time – PT)
intrinsic initiation
contact activation; exposure to collagen, basement membrane, microfibrillar substance (activated partial thromboplastin time - aPTT)
extrinsic initiation
release of thromboplastin (tissue factor) from damaged tissue (prothrombin time – PT)
What is factor XII activated by and what does it activate?
1) Factor XII (Hageman factor) is activated by contact with subendothelial structures. XIIa also interacts with other cascades.
2) Activated XII and cofactor High Molecular Weight Kininogen activate factor XI (Plasma thromboplastin antecedent) making it XIa
- physical contact activates factor XII
FXIIa also activates
- Intrinsic pathway
- Neutrophils
- Platelets
- Fibrinolytic system
- Compliment cascade
- Endothelial cells
Additional FXIIa activities
• Interacts with HMWK to produce bradykinin (vasodilator)
• Interacts with pre kallikrein to form kallikrein
• Potent activator of
neutrophils & fibrinolysis
What does factor XIa (Plasma thromboplastin antecedent) do?
- cleave factor IX (Plasma thromboplastin
component) to IXa
What does factor IXa (Plasma thromboplastin
component) do?
- in the presence of cofactor phospholipid (has to occur on a cell membrane) and in the presence of activated factor VIII (Antihemophilic factor A)(that whole complex) activates factor X (Stuart factor)
- thrombin activates factor VIII
Extrinsic pathway process
- initiated by the interaction of tissue factor with factor VII (Proconvertin)
- has to happen on a phospholipid membrane
- ability to cleave IX to IXa
- ability to cleave X to Xa
Extrinsic pathway
- most common entry way into the cascade
1) Release of tissue factor (thromboplastin) appears to be the primary entry point into the cascade.
2) Cells not normally in contact with blood but may be exposed following damage produce tissue factor at rest.
3) Cells in contact with blood express tissue factor (protein) on membrane surface only after activation.
4) Tissue factor binds with circulating factor VII
prothrombinase complex
-Xa in the presence of Va and Ca++ on a phospholipid membrane to convert (factor II) prothrombin into thrombin (factor IIa)
What does activated prothrombin (thrombin, factor IIa) interact with and cause?
- interacts with fibrinogen (factor I) cleaving it into fibrin (weak) which begin to bind together and platelets
- cleaves factor XIII (Fibrin stabilizing factor) to XIIIa which interacts with fibrin strands to slightly change the structure of the fibrin to turn the weak bonds into strong bond of platelets and fibrin
what does factor XIIIa do?
- Cross-links fibrin chains & binds alpha2-antiplasmin to fibrin. Protects fibrin from plasminmediated breakdown.
high concentration of fibrin, what will happen and what are the threads like
- form a clot
- fibrin threads narrower but a denser aggregation of them which are harder for the fibrinolytic system to break down
low concentration of fibrin, what will happen
- still form a clot but the structure of the clot will have a looser mass of fibrin threads and the fibrin threads tend to be bigger
- easier for fibrinolytic system to break down
Pro’s of Cascade Model
Allowed understanding of how the enzymatic steps occur in plasma-based in vitro coagulation
Allowed understanding of calcium dependence and the development of calcium chelating anticoagulants
Allowed the development of useful clinical testing and interpretation of the testing results
Identification of specific factor deficiencies
Allowed close examination of the properties of the specific enzymes and clearer understanding of associated cofactors
Con’s of Cascade Model
Provides a reasonable explanation of how coagulation takes place in plasma or purified protein-based fluid systems where the fluid in NOT moving and there is NO interaction with the endothelial cells of the vascular wall and NO interaction with other cells surfaces
Does not describe the hemostatic process that is occurring in our bodies (i.e. in vivo)
States that the extrinsic and intrinsic pathways are able to operate independently in the generation of Xa
in vivo it has been shown that bleeding will occur if there are factor deficiencies in the extrinsic pathway even of the intrinsic pathway is intact
Need for an In Vivo Model
Vascular system is dynamic not static
Blood comes into contact with many different types of cells
Some factors may participate in the enzymatic cascade of coagulation in a static situation, but in a dynamic system they appear to have other roles NOT related to hemostasis
Inflammatory response
Function of vessel wall
Cell proliferation
Needs to be cell bases
Cell-Based Model: Thrombin generation occurs during three phases
Initiation phase
Amplification phase
Propagation phase
Initiation Phase
The activity of the factor VIIa/Tissue Factor complex is the most significant event to initiate coagulation.
-these two are driving forces
Initiation Phase (Step 1)
-Activation of VII
Endothelial cell damage (i.e. vascular damage) exposes tissue factor on the surface of the cell
TF is a receptor and a cofactor for VII / VIIa
When VII binds TF it is converted to VIIa (don’t know why) and forms VIIa/TF complex (extrinsic tenase complex)
Initiation Phase (Step 2): Extrinsic tenase complex (VIIa/TF complex) does what
catalyzes conversion of IX and X to their activated forms (IXa and Xa respectively)
Within the circulation, IXa is rapidly inhibited by tissue pathway inhibitor (TFPI) and antithrombin (AT)
If ACTIVATED platelets are close by, IXa will diffuse to those platelets, bind to their surface and in conjunction with cofactor VIIIa activate X to Xa
Initiation Phase (Step 3): Xa formed on TF-bearing cell interacts with what and forms
Cofactor Va to form a prothrombinase complex
Prothrombinase complex generates a small amount of “priming” thrombin on the surface of the TF-bearing cells
II (Prothrombin) converted to IIa (Thrombin)
Xa remains on the cell surface
-amount of thrombin isn’t enough to create a clot
-PURPOSE: to recruit platelets
Amplification Phase: thrombin generated on the TF-bearing cell serves as
the “signal” for the start of the Amplification Phase
Amplification Phase sets the stage for large scale thrombin production by activating platelets
When enough thrombin (IIa) is generated on or near the TF-bearing cells, platelet activation begins
-activation begins when we have enough “priming” thrombin
At this time there is NOT ENOUGH thrombin present to initiate clot formation
-process taking place in the tissue
To generate the amount of thrombin needed to form a clot
the injury must allow the movement of platelets and large proteins to leave the vascular space so they can bind with TF-bearing cells in the extravascular area
Factors involved in the amplification and propagation phases are larger and cannot pass through an intact vascular membrane
Amplification Phase – Step 1: thrombin (IIa) generated during initiation phase
is very busy with
Binds with platelet surface receptors causing platelet activation
Catalyzes the conversion of V to activated Va
Catalyzes the splitting of the VIII/vWF (von Willebrand factor) complex
vWF will mediate platelet adhesion and aggregation
Thrombin catalyzes conversion of VIII to VIIIa
Catalyzes the conversion of XI to XIa
Amplification Phase – Step 2: Va binds to? VIIIa binds to? XIa binds to? IXa will complex with
Va binds to the activated platelet membrane
VIIIa binds to the activated platelet membrane
XIa binds to the activated platelet membrane
IXa will complex with VIIIa to form the Intrinsic Tenase Complex
IXa created during initiation phase but not deactivated because activated platelets available
Amplification Phase – Platelet Activation: Binding with IIa causes extreme changes to the surface of the platelet
Shape change
Rearrangement of membrane phospholipids creating procoagulant membrane surface
Promotes binding of coagulation proteins to activated membrane
Release of granule contents
Raw materials for clotting reactions
Agonists that induce further platelet activation
-these changes are mediated by thrombin activation
Propagation Phase
Purpose of propagation phase is to generate huge amounts of thrombin on the surface of the activated platelets
Remember, platelets are able to leave the blood vessel as a result of vascular injury and are able to bind to underlying collagen, vWF, blood vessel wall, and other extravascular material. This binding (and the “priming” dose of thrombin) start platelet activation
The adherence of platelets to the damaged tissue is the first step in the formation of the platelet plug which is necessary for primary hemostasis
Propagation Phase (Step 1):
Va, VIIIa, and XIa all bind to the surface of the activated platelet
XIa catalyzes conversion of IX to IXa
IXa (produced by Extrinsic Tenase Complex AND conversion by XIa on platelet surface) attaches to the bound VIIIa to create the Intrinsic Tenase Complex (also called Platelet Tenase Complex)
Propagation Phase (Step 2 & 3)
Intrinsic Tenase Complex begins massive conversion of X to Xa
Xa binds with Va to form large number of Prothombinase Complexes
The large number of Prothombinase Complexes creates an explosive conversion of prothrombin (II) to thrombin (IIa) – the “thrombin-burst”
Clot formation
The “thrombin-burst” generates enough thrombin so there is a significant amount of fibrinogen converted to fibrin resulting in the formation of the fibrin clot
Majority of thrombin produced actually produced after the initial fibrin clot has been formed
Additional thrombin continues to activate XIII and other factors that affect the strength and structure of the fibrin clot
