Normal Hemostasis Flashcards
Hemostasis/Coagulation
- processed involved when blood clots in response to an injury
- interaction of blood vessels, platelets, coagulation factors, fibrinolysis
Hemostasis function
- to keep blood within veins and arteries
- prevent blood loss from injuries by formation of thrombi
- re-establising blood flow during the healing process
- maintain a complete balance between bleeding and clotting
3 Stages of Hemostasis
- Normal hemostasis
- Secondary hemostasis
- Fibrinolysis
Primary hemostasis
- vasoconstriction upon injury of vessel to minimize blood loss
- platelets accumulate/aggregate at site forming platelet plug
Secondary hemostasis
- coagulation factors produce fibrin
- fibrin stabilizes fragile platelet plug (making it a clot)
Fibrinolysis
breakdown of fibrin to remove clot after healing of wound
Primary hemostasis involvement
- blood clotting in response to vascular injury
- blood vessels and platelets involved
Structure of blood vessels
- blood flows through central cavity (lumen)
- endothelial cells line lumen to protect vessel from injury
Activation of hemostasis
- initiated by contraction of vessel
- brings hemostatic components closer to vessel wall
- damaged endothelial cells release factors that aid in hemostasis
Endothelial cells role in primary hemostasis
- produce and secrete vWF (aids platelets)
- produce tissue factor
- expose collagen (secretes platelet activating factor)
- release plasminogen activator inhibitor (inhibits fibrinolysis)
Platelet maturation
stem cell –> CFU-GEMM –> BFU Meg –> CFU Meg –> MK1 (megakaryoblast) –> MK2 (promegakaryocyte) –> MK3 (Megakaryocyte) –> platelets
MK1-MK3
- undergoes endomitosis (DNA doubles, but no division occurs)
- cell gets larger and larger
- usually stop at 16N (DNA content)
Megakaryoblast (MK1)
- 6-24 microns
- scant basophilic cytoplasm
- no visible granules
- round nucleus
- visible nucleoli
Promegakaryocyte (MK2)
- nuclear division ends
- cytoplasmic granule development begins
- granules spread throughout
- membrane demarcation begins (channel system)
- multilobed nucleus
Megakaryocyte (M3)
- cytoplasm becomes more purple
- DMS finishes packaging platelets
- proplatelets break off into circulation
- large multilobed nucleus
- no visible nucleoli
Mature megakaryocyte
- all that’s left is nucleus when all proplatelets are released (metamegakaryocyte)
- average platelet lifespan is 10 days
Platelet structure
- peripheral zone: outermost zone
- structural zone: microtubules, cytoskeletal network, microfilaments that provide support
- organelle zone: mitochondria, glycogen particles, granules
- membrane system: two systems of membranes
Platelets in Hemostasis
- must be adequate in number and function
- normally don’t interact with other cells
Formation of Platelet Plug (Platelet action)
- platelet adhesion
- platelet activation
- platelet shape change
- platelet secretion of granules
- platelet aggregation
Platelet adhesion
- damaged vessel exposes blood flow to subendothelial connective tissue
- connective tissue is composed of adhesive molecules
- involves: vWF, platelet membrane receptor (GP1b), and collagen fibers
vWF
- synthesized by endothelial cells and megakaryocytes
- released into plasma and binds to GP1b receptor
- forms a bridge connecting platelet to collagen fibers
collagen
- adhesion of platelets promotes platelet spreading along vessel wall
- platelet adhesion to wall activates platelets
Platelet activation
- morphological and functional change of platelets
- secretion of granules, formation of aggregates
Platelet agonist
- agent that induces platelet activation
- some generated by platelets, others by cells/molecules at site of injury
- binds to specific receptor on platelet
Platelet derived agonists
ADP
Serotonin
Platelet activating factor
Thromboxan A2 (TXA2)
Non-platelet derived agonists
Collagen
Thrombin**
Epinephrine
Platelet shape change
- triggered by adhesion to collagen via vWF
- discs –> spheres with projections (pseudopods)
- allows greater chance of contact, become sticky
Platelet secretion of granules
alpha and dense granules released into surrounding area
Alpha granules
- thrombospondin: promotes plt to plt interaction
- vWF: platelet adhesion
- Platelet-derived growth factor: promotes smooth muscle growth
Dense granules
- ADP: promotes platelet aggregation
- Calcium: regulates platelet activation/aggregation
- serotonin: promotes vasoconstriction
Platelet aggregation
- attachment of platelets to one another
- 10-20 secs after vessel injury
- platelets arrive and become activated by TXA2, ADP, etc
- platelets become sticky and clump together = aggregation
- platelets also secrete granules
Platelet plug
- primary hemostatic plug
- clumping acts as a plug to stop bleeding
- fragile and easily dislodged
- anchored to vessel wall by secondary hemostasis
Secondary hemostasis
- reinforcement of platelet plug with stable fibrin clot
- initiation of coagulation cascade (ending in fibrin formation)
Hemostatic reactions
- occurs in a cascade/waterfall-like manor
- zymogens (inactive factors) are sequentially activated
- end result if fibrin clot
Coagulation factors
- proteins, most are made in the liver
- normally present in plasma as zymogens
- factor deficiencies produce bleeding tendency disorders
- Fibrinogen group, Prothrombin group and Contact group
Fibrinogen group
- I, V, VIII, XIII
- consumed during coagulation process
- present in plasma, absent in serum
Prothrombin group
- II, VII, IX, X
- consumed during coagulation process (except II)
- Vitamin K dependent (so they can bind Ca2+)
Contact group
- XI, XII, Prekalikrein (PK), High molecular weight kininogen
- not consumed during coagulation
- requires contact with a surface for activation
The Coagulation cascade
- initiation occurs either via the intrinsic (contact) pathway or via the extrinsic (tissue factor) pathway
- converge to a single pathway (common pathway)
- single, tissue factor pathway in vivo
- intrinsic pathway necessary for large amount of fibrin
Extrinsic pathway
- tissue factor (thromboplastin) is exposed when vessel is injured and it initiates the pathway
- tissue factor complexes with factor VII to start pathway
- continues until clot is formed (end of common pathway)
Intrinsic pathway
- substances normally present in circulation
- exposure to collagen, subendothelium activates factor XII and XI (contact group)
- continues until clot is formed once activated
Common pathway
- convergence of intrinsic and extrinsic pathways
- starts with initiation of factor X by either pathway
- factor X activates prothrombin (II) to thrombin (IIa)
- thrombin cleaves fibrinogen (I) to make fibrin strands AND activates factor XIII
- factor XIII links and stabilizes fibrin strands
Fibrinolysis
- slow process of breaking down fibrin when clot is no longer needed
- enzymatic cleavage into soluble fragments
- initiated upon fibrin formation (so we don’t overly clot)
- gradually dissolves clot as tissue repair occurs
Plasminogen (PLG)
- circulates as inactive molecule
- binds to fibrin throughout clot
- activated to plasmin by tissue plasminogen activator (tPA) and urokinase type plasminogen activator (uPA)
Plasmin (PLN)
- initiates fibrinolysis
- digests fibrin by hydrolysis
FDP/FSP
- fibrin degradation products/fibrin split products
- fragments formed as fibrin is broken down
- cleared by the liver
- detection is diagnostic for hemostatic disorders
- plasmin splits fibrin to X and Y fragments (early FDPs)
- Y fragment is split into D and E fragments (late FDPs)
- fragments exert antithrombin effect (prevents clotting)
Regulators of coagulation
regulated by activators and inhibitors
Regulation of Secondary Hemostasis
- limit the amount of fibrin clot formed to prevent ischemia
- localize clot formation to site of injury to prevent widespread thrombosis
Secondary Hemostasis inhibitors
Tissue factor pathway inhibitor (TFPI) Activated Protein C and Protein S Antithrombin III (AT) Alpha2-macroglobulin Alpha1-antitrypsin Heparin cofactor II (HCII) C1-inhibitor Protein Z (PZ) ZPI
Tissue factor pathway inhibitor (TFPI)
- glycoprotein on endothelial cell surfaces
- inhibits factor VIIa and Xa
Activated Protein C and Protein S
- C is major inhibitor in coagulation pathway
- Vitamin K dependent inhibitors
- synthesized in the liver
- inhibit Va and VIIIa
Serine Protease Inhibitors (serpins)
- inhibit by trapping enzyme with serpin so it loses activity
- Antithrombin III (AT), Alpha2-macroglobulin, Alpha1-antitrypsin, Heparin cofactor II (HCII)
Antithrombin III (AT)
- most clinically relevant serine protease inhibitor
- enhanced by heparin (heparin won’t work without it)
- synthesized by the liver and endothelial cells
- inhibits thrombin, factors IXa, Xa, XIa, XIIa, kallikrein and plasmin
Alpha2-macroglobulin
inhibits kallikrein, plasmin, thrombin, factor Xa
Alpha1-antitrypsin
inhibits factor XIa, thrombin, kallikrein, plasmin
Heparin cofactor II (HCII)
inhibits thrombin
C1-inhibitor
- inhibits esterase activity of C1 from complement cascade
- inhibits contact system proteases (XIIa, IXa, kallikrein, PLN)
- main inhibitor of XIIa
Protein Z (PZ)
- Vitamin K dependent protein
- enhances inhibitory function of ZPI against factor Xa
ZPI
- plasma serpin that inhibits factor Xa
- PZ-dependent
Fibrinolytic activator
- Tissue plasminogen activator (TPA)
- produced by endothelial cells
- aids in regulation of fibrinolysis
- clot-busting drug
Fibrinolytic inhibitors
Plasminogen activator inhibitor 1 and 2 (PAI-1 and PAI-2)
Thrombin activated fibrinolysis (TAFI)
Alpha 2 - antiplasmin inhibitor
Alpha 2 - macroglobulin
Plasminogen activator inhibitor 1 and 2 (PAI-1 and PAI-2)
inhibits tPA and uPA
Thrombin activated fibrionolysis (TAFI)
suppresses plasminogen binding to fibrin
Alpha 2 - antiplasmin inhibitor
inhibits plasmin
Alpha 2 - macroglobulin
inhibits plasmin
