Hemostasis Physiology Flashcards
What are the platelet precursors and how do they develop into platelets?
megakaryocytes
fragment into a lot smaller platelets
Can platelets produce ATP?
Yes, have mitochondria
What is the purpose of platelets’ growth factor?
induces fibroblast activity after clot formation to cause connective tissue formation and closing of the vessel defect
What are the contractile proteins of platelets?
myosin
actin
thrombosthenin
How do platelets prevent binding to healthy endothelial cells?
surface is coated by glycoproteins - repulse adherence to normal endothelium - causes only adherence to injured endothelial cells
What is the half life of platelets and how are they eliminated?
about 10 days
dogs: 5-7 days
destroyed by macrophages
Describe the formation of a platelet plug
tissue injury - exposed collagen and vWF - platelets bind to this and become activated
* form pseudopods
* swell and become sticky
* release granules –> attract more platelets
* release TXA2, ADP, PAF –> attract more platelets
enough to stop bleeding of tiny inuries - otherwise fibrin required
What platelet receptors bind vWF?
Gp1b
What are the 2 possible fates of a blood clot?
- dissolution
- connective tissue integration via fibroblasts
What are the 2 stages of fibrin monomer binding?
- early stages - fibrin monomers held together by weak noncovalent hydrogen bonding - no cross-linking - weak clot and can be broken apart easily
- within next few minutes - fibrin stabilizing factor (XIII, activated by thrombin) - forms covalent bonds ebtween fibrin monomers - strong meshwork
Explain the steps of clot retraction
within few min of clot formation - contraction, expression of serum withn 20-60 min
platelets activate thrombosthenin, actin, myosin - cause strong contractions
Describe the steps of the extrinsic pathway
TF exposure - activates and binds to circulating FVII –> activates FX —> binds with FV to form prothrombin activator complex with Ca2+ and phospholipids
How is FV activated?
Initially inactive as part of the prothrombin activator complex
once small amounts of thrombin activated –> will activate FVa which accelerates the prothrombin activation
Describe the Intrinsic pathway
exposure to collagen or blood trauma => FXII activation and phospholipid release from platelets
=> FXIIa activated FXI, this step requires kallikrein and is accelerated by prekallikrein
=> FXIa activates FIX
=> FXIa + VIIIa + phospholipids + platelet factor 3 => activate FX
=> FXa + FV => prothrombin activator
What factor do platelet phospholipids contain?
Platelet factor 3
How do the speeds of the extrinsic versus intrinsic pathway compare?
Extrinsic pathway is very fast, can happen within seconds
Intrinsic pathways takes minutes to cause clotting
What are characteristics of the endothelium that help keep clotting at bay
- smooth surface - prevents contact activation
- glycocalyx - repels clotting factors and platelets
- thrombomodulin on the endothelial surface - binds thrombin and also activates APC
- endothelial cells produce NO and PGI2 –> vasodilation and platelet inhibition
What are the 2 ways thrombin is removed from the blood to prevent excessive clotting?
- 85-90% of activated thrombin is absorbed by the fibrin fibers of the clot
- bindings with antithrombin III
what factors does the heparin-antithrombin complex bind and inhibit?
II
IX
XII
What cells produce heparin and in what organs are they most abundent?
basophils
mast cells
within the tissues surrounding capillaries of the lungs and liver
Describe the role of vitamin K for clotting factors
within the liver => cofactor of carboxylase =>
- gamma-carboxylation of glutamic acid of clotting facotrs => adds carboxyl group to glutamic acid residues on II, VII, IX, X, PC, PS => facilitates addition of extra negative charge on glutamic acid
- allows these factors to bind Ca++ which enables them to bind Phosphatidylserine of the phospholipid layer
in the process Vitamin K becomes oxidized - inactive
recycled by Vitamin K epoxide reductase complex 1 (VKORC1)
How is vitamin K produced and what is needed for its absorption?
intestinal bacteria produce Vitamin K
fat absorption - e.g., lack of bile can prevent fat absorption and therefore reduce vitamin K absorption
Describe how Prothrombin testing works
blood drawn - citrated or oxalated (i.e., binds Ca+)
for test - large amount Ca+ added + TF (usually grown from placentas)
=> activates extrinsic pathway => time to clotting measured
What can be used to standardize PT measurements between laboratories and machines?
International normalized ratio (INR)
tissue factor batches come with an international sensitivity index (ISI) - i.e., indicates activity of the TF produced
INR = (PT patient/PT control) ^ISI
What is the desired INR for warfarin treatment?
2-3
Name the PLT receptors for these ligands
Describe the open canalicular system of platelets
membrane-lined invaginations that let the PLT secrete products from the cytoplasm and take up products from plasma
What is another name for the PLT receptor GPIIbIIIa?
integrin alpha-II-b-beta-3
List the contents of alpha granules
proteins
- fibrinogen
- fibronectin
- vWF
- FXIII, FIX, VIII, V, Protein S
- P selectin
- Albumin
- Immunoglobulins
List contents of dense granules
- Calcium
- ATP/ADP
- Serotonin
- Histamine
Where is vWF stored?
- Weibel Palade bodies in the endothelial cells
- alpha granules PLT
What receptor does vWF bind to on PLTs?
GP1balpha - subunit of GP1bIXV
Describe the three-stage mode lof platelet activation
I. Initiation
- tissue injury - PLT and endothelial cells release vWF
- shear stress causes conformational change of GPIbalpha
- => thethering and rolling of PLTs
- => binding of PLTs to vWF and collagen
- => PLT monolayer formation
II. Extension
* PLTs release TXA2 and ADP => recruit more platlets
* activation of integrin alpha-IIb-beta-III (GPIIb3a) => fibrinogen binding => platelet aggregation
III. Stabilization
* PLT contraction => strengthens connection + prevents diffusion of activators away from the PLT
* clot retraction
Describe how the cells’ phospholipid layer is changed to allow clotting factor binding
- need to bind to phosatidylserine (PS)
- location of this is mediated by flippase, floppase, scramblase
- normal homeostasis => PS kept inside by flippase
- apoptotic cells or activated platelets => scramblase moves PS to the cell surface to bind with clotting factors
Where is tissue factor synthesized?
in adventitial fibroblasts
Tissue injury - exposure of TF-bound fibroblasts -> FVIIa binding
Describe the initiation phase of the cell based model
Tissue damage => TF exposed => binds with circulating VIIIa => TF-FVIIa-complex
=> activates small amounts of IX and X
=> FXa activates FV and binds
=> forms prothrombinase complex (FXa-FVa)
=> activates prothrombin to thrombin
Describe the amplification phase of the cell based model
Platelet activation => exacerbated by thrombin binding to PLTs
* procoagulant membrane surface
* release FV -> supplies large amounts for prothrombinase formation
* VIII dissociates from the PLT bound vWF
Describe the Propagation phase of the cell based model of coagulation
- initially formed thrombin activates FXI
- TF-FVIIa also activated small amounts of FIX
- FXIa ==> FIX activation
- FIXa activates and binds with FVIIIa
- FIXa-FVIIIa - tenase complex - activate FX
- FXa combined with FVa => thrombin burst
List the natural anticoagulants
- endothelial cell secreted Tissue factor pathway inhibitor (TFPI) => inhibits TF-FVIIa
- Antihrombin => complexes with and inhibits IIa, FIXa, FXa, FXIa, FXIIa
- Thrombomodulin => bins with thrombin => activates APC => binds with cofactor Protein S => inhibits FVIII and FV
Describe how tPA versus uPA activate plasminogen
tPA
* activates plasminogen only in the presence of fibrin - tertiary complex
* fibrin binds to the lysine binding site of plasminogen
uPA
* activates plasminogen without needing fibrin present
* binds to cell-bound uPA receptors -> enhances activation of cell-bound plasminogen
What triggers tPA release?
- thrombin FXa
- beta-adrenergic agents
- histamine
- bradykinin
What are the 3 most important fibrinolysis inhibitors and how do they work?
- PAI-1 (plasminogen activator inhibitor) - binds to uPA and tPA
- alpha-2 antiplasmin - binds noncovalently to plasminogen or crosslinks fibrin - prevents their binding
- TAFI - activated by thrombin/TM complex => removes the lysin on fibrin => can’t bind to plasminogen anymore
Describe how NETs partake in clot formation
cfDNA - induces XIII activation and initiates contact pathway of coagulation (XII)
histones - activates platelets
histones - increase thrombin generation
histones - inhibit fibrinolysis
