Hemostasis/Coagulation
Gregory S. Travlos, DVM, DACVP
National Institute of Environmental Health Sciences
Research Triangle Park, NC 27709
919-541-0653
Travlos@niehs.nih.gov
Hemostasis
The process by which bleeding is arrested.
• It is a series of physiological and biochemical events which
terminate in the formation of an insoluble fibrin clot
Hemostatic Sequence:
• Interaction between vessel wall and platelets
• Blood coagulation
• Fibrinolysis
Hemostatic Component Interactions
Thompson &
Harker, 1983
Blood Vessels
Intact endothelium forms a thromboresistant surface
• Required for the free flow of blood; does not promote platelet adherence or
activate coagulation
• Passive mechanisms:
• Endothelial glycocalyx (negative charge - repels like-charged particles, e.g.,platelets).
• Presence of a2-macroglobulin at cell surface (protease inhibitor).
• Active mechanisms:
• Endothelial cells remove platelet aggregation promoters from circulation (e.g., PGF1,
bradykinin, serotonin, adenine nucleotides).
• Secretion of PGI2 - potent inhibitor of platelet aggregation, induces vasodilation.
Proteoglycan matrix of the vessel wall influences thrombogenicity.
• Heparin, heparan sulfate and dermatan sulfate have anticoagulant activity; other
glycosaminoglycans and hyaluronic acid do not.
• Veins have the highest concentration.
Endothelium
Besides their role in thromboresistance, endothelial
cells have additional synthetic functions.
• Produce Von Willebrand’s factor
• Absorbed by platelets; needed for adherence to collagen
• Produce plasminogen activator (tPA)
• Mediates fibinolysis
• Injured cells release thromboplastin (factor III)
• Activates the “extrinsic” coagulation cascade
• Others (e.g., type III and IV collagens, elastin, fibronectin, etc.)
Blood Vessel Structure
Thompson &
Harker, 1983
Platelets
Adhere to exposed collagen (platelet plug)
• Occurs in seconds; can control hemorrhage of minute injuries
Secretory functions; mediators of coagulation and
fibrinolysis
• Releases ADP; sticky and promotes platelet adherence
• ADP activates phospholipase A2 which stimulates
thromboxane A2 synthesis
• Release of membrane fibrinogen, factor V, factor VIII and
calcium
• Release of membrane platelet phospholipid.
Platelet - TEM
microtublules
OCS
granules
mitochodrion
Ultrastructural and Functional Platelet
Anatomy
Platelets - cont.
The role of platelets in hemostasis is as important as the
coagulation mechanism.
• Thrombocytopenia, thrombasthenia or thromobopathia - impair hemostasis
• Thrombocytosis or thrombocythemia - may impair, but usually promotes
clotting (predisposes to thrombosis).
Platelets promote hemostasis by:
• Release of ADP and other agonists; promotes adherence.
• ADP activates phospholipase A2 which stimulates thromboxane A2
synthesis
• Thromboxane A2 - stimulates vasoconstriction and platelet aggregation
• Release of membrane fibrinogen, factor V, factor VIII and calcium
• Components of coagulation localized at site of injury
• Release of membrane platelet phospholipid.
• Accelerates the “intrinsic” and “common”pathways of coagulation
Prostaglandin Metabolism
Harlan &
Harker, 1981
Hemostatic Platelet Functions
Thompson &
Harker, 1983
Platelet Response
When a vessel is injured or severed a brief, local, reflex
vasoconstriction occurs.
• Reduces blood flow at site.
• Maintained by vasoactive compounds (platelets, surrounding tissues).
Passing platelets adhere to exposed collagen.
• Occurs in seconds; initially adhere in a single layer and become activated.
• Severe injury - collagen serves as a potent platelet activator.
• Less severe injury - vWF and fibrinogen become the major activators.
The adhered platelets undergo a conformational change.
• From discoid to development of long filopodia.
• Activation of GP receptors for fibrinogen and/or vWF (GPIIb/IIIa and
GPIb/IX/V).
Structure of the GPIb-IX-V receptor
Tablin, 2000
Platelet Response to Agonists
Platelets - unstimulated Addition of ADP Addition of thrombin
(mild stimulation) (strong stimulation )
Characteristic discoid shape Shape change (elongation Increased spreading,
and crescents) and filaform filaform process extension
process formation (arrows) (arrows) and aggregate
formation (stars)
SEM plates;
Gentry, 2000
Platelet Response cont.
Activated platelets release their a-granule and dense body
contents inducing additional platelet recruitment.
• Dense granules - ADP, serotonin and epinephrine.
• alpha-granules - fibrinogen (and vWF in human and pig).
• Synthesis and release of PAF and TxA2.
The agonists accelerate the development of an irreversible
platelet aggregate (platelet plug).
• Reversible v. irreversible responses.
• Thrombocytes of birds and reptiles do not respond to ADP.
• Serotonin and epinephrine:
• Serotonin - shape change (rat, g. pig and dog); aggregation (human, rabbit, cow,
horse, pig, sheep and cat).
• Epinephrine - only human, primate, cat and horse platelets appear responsive.
• Either serotonin or epinephrine combined with another agonist - strong response in
all species.
Platelet Response cont.
More about agonists.
• Platelet Activating Factor (PAF).
• Cow, horse, sheep, primate, dog, g. pig and rabbit respond to PAF.
• Human less sensitive and rat and mouse are insensitive to this agonist.
• Thromboxane A2 (TxA2).
• Strong agonist - human, g. pig and rabbit.
• Weak agonist - horse.
• Insensitive - rat, cow, pig.
In real life, however, platelets are exposed to multiple agonists from
platelets and other cells (e.g., red cells, ADP; white cells, PAF).
Platelet Aggregation to Thrombin
Harlan &
Harker, 1981
Hemostatic Plug Formation
Baumgartner
& Muggli, 1980
Coagulation System
Consists of a cascading system of proteins
• Primarily originating from liver (except factor III)
• Circulate in inactive form (except, possibly, factor VII)
• System includes:
• Enzymatic factors
• Non-enzymatic factors
• Tissue thromboplastin (factor III)
• Calcium (factor IV)
• Platelet phospholipid (PF 3) - structural component; accelerates factor
activation
• Anticoagulant factors
The coagulation system consists of three pathways (intrinsic,
extrinsic and common)
Procoagulant Factors
Coagulation Systems - cont.
Enzymatic factors
• Circulate as non-active zymogens - must be activated to function
• Activated enzymatic factors are not consumed during clotting (except
factors II and XIII)
• Partial deficiency results in partial loss of clotting ability
• Activated enzymatic factors inhibited by antithrombin III (complexed with
heparin) and some alpha-2-glycoproteins
• Enzymatic factors:
• XI and XII (contact factors)
• II, VII, IX and X (vitamin K-dependent factors)
• XIII (clot stabilizing factor or fibrin-stabilizing factor)
Coagulation Systems - cont.
Non-enzymatic factors
• Originate from liver but associate with platelet membranes (also found in
plasma)
• Normal clotting with partial deficiency; almost total absence needed to
affect hemostasis or clotting
• Clotting consumes these factors - absent in serum
• No known natural inhibitors
• Considered reactive proteins - increased during inflammatory and
neoplastic processes (except factor III)
• Non-enzymatic factors:
• Fibrinogen (factor I)
• Factor V
• Factor VIII:C (associated with Von Willebrand’s factor)
Coagulation Cascade Interactions
Does this turkey have factor XII?
Of course, he does
But, his feathered companion does not
Coagulation Systems - cont.
Clot stabilization
• Fibrin stabilizing factor (factor XIII) forms fibrin strand cross-links.
• Synthesized by monocytes and hepatocytes.
• Zymogen is activated by thrombin (plus calcium).
• A very small amount of factor XIII (2 - 10%) is adequate for hemostasis.
• Converts soluble fibrin monomers (unstable) to a fibrin polymer (stable).
• Lead, silver, zinc and snake venoms are known inhibitors.
Coagulation Inhibitors
The activity of coagulation system must be attenuated.
• Numerous inhibitors are found in blood.
Coagulation is controlled by three types of actions.
• Inhibition of converting enzymes (e.g., AT III, C1 esterase inhibitor, a2-
macroglobulin, a2-antiplasmin, a1-antitrypsin, HC-II).
• Act on one or more of the converting enzymes (activated factors).
• AT III-heparin pathway: major system - 80% of the thrombin inhibitory action in plasma.
• Destruction of protein cofactors (e.g., TM-PC-PS system).
• TM-PC-PS system degrades cofactors V & VIII:C, inhibiting prothrombinase and tenase
complexes, respectively.
• Blocking receptor availability needed for complex formation (e.g., Tissue factor
pathway inhibitor (TFPI) and annexin V).
Proposed Mechanism of AT III-Heparin
System
Lysine
sites
Serine site
H
AT III
Arginine
site Th
Thrombin Heparin
Antithrombin
III
H
AT III
Th
Proposed Mechanism of Thrombomodulin,
Protein C and Protein S (TM-PC-PS) System
F-Xa Prothrombin
Activated
F-Va
platelet
PS Thrombin
Ca++
x
Ca++
Protein C
Activated Thrombin
Protein C
Thrombomodulin
Proposed Mechanism of Tissue Factor
Pathway Inhibitor (TFPI) Activity
F-Xa
F-Xa
TFPI
TFPI
F-Xa
TFPI
F-VIIa Tissue factor
Endothelium
Anticoagulant Factors
Fibrinolytic System
Method for removing clots and maintenance of a patent vascular system and fibrin
deposited during inflammation and tissue injury must be removed.
• Plasmin (serine protease) primarily responsible for fibrinolysis.
• Produced in the liver and kidney, it circulates in an inactive form (plasminogen).
• Activators: tissue plasminogen activator (tPA), cytokinases-urokinases (urine, CSF, tears, saliva, milk,
bile, synovial, prostatic and amniotic fluids), erythrocyte erythrokinase, neutropil activator and factor
XII-dependent activator (XII-prekallikrien-hageman factor cofactor complex).
• In addition to fibrin and fibrinogen, plasmin will hydrolyse a variety of proteins.
• While plasminogen is normally found in blood and body fluids, plasmin is usually absent due
to numerous antiplasmins.
• Inactivators: antithrombin III, a2-macroglobulin, a1-antitrypsin and C1 inactivator.
Fibrinolytic System and Factors Regulating
Fibrinolysis (Fibrinogenolysis)
Activation Plasminogen Inhibition
Damaged Kallikrein
endothelium Plasminogen activator inhibitor
e-aminocaproic acid
FHIIa Prekallikrein
tPA
Streptokinase
Urokinase
Plasmin
a2-Antiplasmin
a2-Macroglobulin
Biodegradation of
FV, FVIII, FIX, FXI Complement activation
fibrinogen
Fibrin/fibrinogen
Firbrinogen/fibrin
Degradation products
Degradation of Fibrin/Fibrinogen
Fibrinogen or Fibrin
Plasmin
Fragment X Small Peptides
Plasmin
Fragment Y Fragment D Small Peptides
Plasmin
Fragment E Fragment D Small Peptides
Evaluation of Hemostasis
Fundamental physiology and pathophysiology of
hemostasis is similar in mammalian species.
• Variables identical for laboratory animals and human patients
Platelets
• Platelet count - detection of thrombocytopenia
• Clot retraction - non-anticoagulated blood
• Failure to separate - platelet function defect or thrombocytopenia
• Bleeding time (BT)- in vivo test; simple; low sensitivity
• Used to evaluate platelet function defects
• Thrombocytopenia - prolongs BT
• Clotting factor deficiency does not alter BT
• Vascular disease (eg., scurvy) can prolong BT (humans, guinea pigs)
Considerations for Blood Collection
Clean/smooth surfaces
• Want to avoid platelet clumping or activation of factor XII
• Use plastic or siliconized glass for sample collection
• Animal blood clots faster than human blood - prime needle with anticoagulant
Collect sample from an endothelial-lined vessel and
careful venipuncture
• Want avoid contamination with tissue juice (factor III)
• Small clot activates coagulation system invalidating results
• Samples from indwelling catheters are usually unacceptable
Sample Handling/Anticoagulants
Plasma samples separated from cells within 30 minutes
• Perform analyses immediately
• Plasma samples may be quickly frozen (dry ice/alcohol or liquid nitrogen)
and stored at -70o for analysis at a later date
• Activity of factors V and VIII is lost rapidly in samples held at room
temperature
Citrate (trisodium salt) is the anticoagulant of choice.
• Oxalate anticoagulants are acceptable - not commonly used
• Heparin - unacceptable
• EDTA - unacceptable (except for indirect evaluation of fibrinogen
concentration by heat precipitation and refractometry)
Evaluation -cont.
Activated Coagulation Time (ACT) - in vivo test
• Measures (seconds) time to clot formation in fresh whole blood
• Careful attention to sample collection/handling
• Platelet counts <10,000 cause slight increase in ACT
• Results from lack of platelet phospholipid for test
• Increased ACT suggests factor deficiency in intrinsic or common pathways
• Deficiency must be 5% of normal to prolong ACT
Activated Partial Thromboplastin Time (APTT)
• Measures (seconds) time to clot formation in citrated plasma
• Increased APTT - factor deficiency in intrinsic or common pathways
• Deficiency must be 30% of normal to prolong APTT
• Fibrinogen <50 mg/dL will prolong APTT; inflammation may shorten APTT
• Sensitivity increased with saline-diluted plasma
• Heparin therapy prolongs APTT - differentiate using a 1:1 dilution with normal
plasma
Evaluation -cont.
One-Stage Prothrombin Time (OSPT, PT)
• Measures (seconds) time to clot formation in citrated plasma
• Rabbit or synthetic tissue thromboplastin preferred; human origin reagent
gives longer PT times
• Increased PT - factor deficiency in factor VII or common pathway
• Deficiency must be 30% of normal to prolong PT
• Fibrinogen <50 mg/dL will prolong PT
• Sensitivity increased with saline-diluted plasma
Russel’s Viper Venom Time (RVVT)
• Measures (seconds) time to clot formation in citrated plasma
• Increased RVVT - in or common pathway but insensitive to factor VII
deficiency
• Deficiency must be 30% of normal to prolong RVVT
• Fibrinogen <50 mg/dL will prolong RVVT
• Sensitivity increased with saline-diluted plasma
Evaluation -cont.
Thrombin Clotting Time (TCT)
• Measures (seconds) time to clot formation in citrated plasma
• Increased TCT - decreased fibrinogen concentration (<100
mg/dL), dysfibrinogenemia, increased FDP concentration,
heparin therapy
Fibrinogen Concentration (factor I)
• In most species, fibrinogen is 100 - 400 mg/dL
• Fibrinogen decreases in DIC, severe liver insufficiency and
hereditary hypofibrinogenemia
• Inflammation can increase fibrinogen concentration
Evaluation -cont.
Fibrin-Fibrinogen Degradation Products (FDP)
• Measures, by latex agglutination, the concentration of products of
fibrinolysis; D-dimer assay is another method for measuring FDP
• Increased FDP - occurs with disseminated intravascular coagulation or
severe internal bleeding
• In most species, normal FDP is <10 micrograms/mL
Example
Acute oral study in dogs
Animals given 3 X LD50 in food
• Brodifacoum
• Bromadiolone
• Diphacinone
Coagulation studies
• ACT
• RVVT
• PT