HEMOSTAZA
Curs an IV limba engleza
2012-2013
HEMOSTASIS
• Hemostasis is defined as a property of
circulation whereby blood is maintained within
a vessel and the ability of the system to prevent
excessive blood loss when injured.
• Process is rapid and localized
HEMOSTASIS
• Hemostasis can be divided into two stages
– Primary hemostasis
• Response to vascular injury
• Formation of the “platelet plug” adhering to the
endothelial wall
• Limits bleeding immediately
– Secondary Hemostasis
• Results in formation of a stable clot
• Involves the enzymatic activation of coagulation
proteins that function to produce fibrin as a
reinforcement of the platelet plug
• Gradually the stable plug will be dissolved by
fibrinolysis
HEMOSTASIS
Following an injury to blood vessels several actions
may help prevent blood loss, including:
Formation of a clot
HEMOSTASIS
• The primary players in hemostasis include
– Blood vessels
– Platlets
– Plasma proteins
• Coagulation proteins – involved in clot formation
• Fibrinolysis – involved in clot dissolution
• Serine protease inhibitors
• Other minor players include
– Kinin system
– Complement system
VASCULAR SYSTEM
• Smooth and continuous endothelial lining is designed to facilitate
blood flow
• Intact endothelial cells inhibit platelet adherence and blood
coagulation
• Injury to endothelial cells promotes localized clot formation
– Vasoconstriction
• Narrows the lumen of the vessel to minimize the loss of blood
• Brings the hemostatic components of the blood (platelets and plasma proteins)
into closer proximity to the vessel wall
• Enhances contact activation of platlets
– Von Willebrand factor
– Collagen fibers
– Platlet membrane glycoprotein Ib
• Activated platlets enhance activation of coagulation proteins
LOCAL VASOCONSTRICTION
• is due to local spasm
of the smooth muscle
(symp. reflex)
• can be maintained by
platelet
vasoconstrictors
PRIMARY HEMOSTASIS
• Platelets
– Interact with injured vessel wall
– Interact with each other
– Produce the primary hemostatic plug
• Primary platelet plug
– Fragile
– Can easily be dislodged from the vessel wall
PLATLET FUNCTION
 Provide negatively charged surface for factor X and
prothrombin activation
 Release substances that mediate vasoconstriction,
platlet aggregation, coagulation, and vascular repair
 Provide surface membrane proteins to attach to other
platlets, bind collagen, and subendothelium
ADHESION
• Damage to endothelium exposes blood to the
subepithelial tissue matrix with adhesive
molecules
• Platlet receptor GPIb binds to subendothelium
collagen fibers through von Willebrand’s
factor (vWF)
• Platlet adherence stops the initial bleeding
ADHESION
SHAPE CHANGE
• Following vessel injury and platlet exposure to
external stimuli, platlets change shape from
circulating discs to spheres with pseudopods
• Exposure of platlet membrane phospholipids
promotes the assembly of vitamin-K dependent
factors on the platlet membrane surface
• Activated platlets adhere to exposed collagen
AGGREGATION
• Platlet-to-platlet interaction
–
–
–
–
–
Begins 10-20 seconds after vascular injury and platlet adhesion
Requires dense granule release from the adhering platlets
Requires Ca++ and ATP
Requires fibrinogen and fibrinogen receptors GPIIb and IIIa
Mechanism:
• ADP released from platlet cytoplasm upon adherence induces exposure of
fibrinogen receptors GPIIb and IIIa
• Fibrinogen binds to the exposed GPIIb and IIIa
• Extracellular Ca++-dependent fibrinogen bridges form between adjacent
platlets, thereby promoting platlet aggregation
• This is primary or reversible aggregation
• Secondary aggregation begins with the release of dense granules
• Secondary aggregation is considered irreversible
SECRETION
• Secondary aggregation begins with platlet
secretion of dense granules
• Dense granules contain large amounts of ADP
• ADP binds to the platlet membrane triggering
the synthesis and release of TXA2
• The release of large amounts of ADP
combined with TXA2 amplifies the initial
aggregation of platlets into a large platlet mass
PRIMARY HEMOSTASIS
SECONDARY HEMOSTASIS
• Also called the fibrin forming coagulation system
• The purpose is to reinforce the platlet plug
• System mediated by many coagulation factors present
in an inactive form in blood.
– Factors are assigned Roman numerals, I through XIII
• Assigned in order of discovery, not place in reaction sequence
• Each one has one or more common names or synonyms
• Roman numeral followed by letter “a” indicates the activated form
of the factor
• All are produced in the liver. The von Willebrand factor is also
produced in endothelial cells and megakaryocytes.
SECONDARY HEMOSTASIS
• The cascade theory of blood coagulation
– Involves a series of biochemical reactions
– Transforms circulating substances into an
insoluble gel through conversion of fibrinogen to
fibrin
– Requires
• Plasma proteins
• Phospholipids
• calcium
COAGULATION - CASCADE
THEORY
• Each coagulation factor is converted to an
active form by the preceeding factor in the
cascade
• Calcium participates in some of the reactions as
a co-factor
• The blood coagulation cascade occurs on cell
surface membranes.
COAGULATION - CASCADE
THEORY
• Clotting factors bind to the phospholipid
membrane surface and rearrange until a complex
including enzyme, substrate, and cofactor is
formed.
• Subendothelial tissue exposed with vascular
injury and platlet surface provide the critical
membranes
• The membrane localizes the reaction to the site
of injury
SECONDARY
HEMOSTASIS
• Three different complexes assemble on the phospholipid
membrane
• The pathways for the formation of these complexes are
– Intrinsic
– Extrinsic
– Common -Both intrinsic and extrinsic pathways converge
to share factors in the common pathway
– Both intrinsic and extrinsic pathways require initiation
• Intrinsic - all factors involved in clot formation are in
the vascular compartment
• Extrinsic- is initiated when a tissue factor not found in
blood enters the vascular system
EXTRINSIC PATHWAY
• The extrinsic pathway is initiated when there is
an injury to a blood vessel wall
• In the presence of the tissue factor released from
the injured nonvascular tissue (factor III or
thromboplastin) and calcium (factor IV), factor
VII is activated to factor VIIa
• Factor VIIa activates factor X to Xa
• Factor VIIa can also activate factor IX in the
intrinsic pathway to IXa
EXTRINSIC PATHWAY
Extrinsic
pathway
Injured
Cells
= Calcium & PL complex
VII
* = active serine protease
unk. TF
X
*VIIa
*Xa
Va
prothrombin
Common
pathway
V
fibrinogen
*thrombin
XIII
CLOT
XIIIa
Fibrin monomer
Fibrin polymer
INTRINSIC PATHWAY
• Is initiated following exposure to negatively charged foreign
substances such as collagen, subendothelium, or
phospholipids.
• Factor XII is activated to XIIa.
• XIIa then interacts with the contact factors, prekallikrein and
kininogen, to activate Factor XI to XIa
• XIa then activates Factor IX to IXa in the presence of Ca++
• IXa participates along with co-factor VIII:C, Ca++, and PF3 (a
source of phospholipids), to activate Factor X which leads to
the generation of thrombin
• IXa -factor VIIIa-phospholipid-Ca++ is called the tenase
complex
INTRINSIC PATHWAY
• The complex of IXa, VIIIa, X, PF3,and Ca++
assembles on the surface of activated platlets (supply
the phospholipid).
• The surface provides the protective environment for
the enzymatic reactions to occur
• In plasma, VIII circulates as a complex in association
with C which has the procoagulant activity and von
Willebrand factor (vWF) which functions as a carrier
protein.
• VIII requires enhancement by the generated enzyme
thrombin to amplify its activity
Intrinsic Pathway
XII
XIIa
XI
PTT
Partial Thromboplastin Time
XIa
IX
IXa
VIIIa+Ca+Pl
X
Xa
Va+Ca+Pl
II
Fibrinogen
IIa
Fibrin
COMMON PATHWAY
• Intrinsic and extrinsic pathways
– Converge on the common pathway
– Both pathways activate Factor-X to Xa
• Xa in the presence of Factor V, Ca++ and
phospholipid converts prothrombin (Factor II) to its
active form thrombin (IIa)
• Thrombin then feeds back to activate factors VIII and
V, converts fibrinogen to soluble fibrin, and helps to
stabilize the fibrin monomer by converting factor
XIII to XIIIa.
• XIIIa cross-links the fibrin monomers to form a stable
fibrin polymer
FIBRINOGEN
• 3 stages of conversion of fibrinogen to fibrin
– Proteolysis
• Thrombin cleavage of fibrinogen results in fibrin monomers
– Polymerization
• Spontaneous self-assembly into fibrin polymers
– Stabilization
• Introduction of covalent bonds into fibrin polymers by XIIIa
COAGULATION
THE CLOT
FIBRINOLYSIS
• The last stage of coagulation is fibrinolysis,
which is the dissolution and localization of a
fibrin clot.
• Prevents excessive fibrin deposition
• Allows closely coupled with fibrin formation
• Localized surface bound phenomenon that is
catalyzed by fibrin formation
FIBRINOLYSIS
• Fibrinolytic system is a complement to the coagulation
system – a fine balance between the two systems must
occur
– Restricts fibrin formation to area of injury
– Dissolves clot by digestion of fibrin
• Initiated when coagulation begins – will ultimately
dissolve clot
• plasminogen
↓
• Fibrin plasmin → fibrin degradation products (FDPs)
FIBRINOLYSIS - COMPONENTS
• Plasminogen -> plasmin
• Plasminogen activators
• Inactivators of plasminogen
• Inhibitors of plasmin
FIBRINOLYSIS
• Plasminogen is activated and converts to plasmin by
factor XII, HMWK,and PK
• Plasmin = enzyme which dissolves fibrin clots into
protein fragments that are cleared from plasma by the
liver
• Fibrin degradation products are breakdown fragments of
fibrin or fibrinogen.
– The protein fragments are designated X, Y, D, and E
– Fragments are strong inhibitors of further coagulation by
• interfering with the action of thrombin
• interfering with platelet aggregation
FIBRINOLYSIS
• Inhibitors of fibrinolysis = antiplasmins.
• Used to regulate and limit plasmin activity and
fibrinolysis
– alpha-2-antiplasmin
– alpha-2-macroglobulin
FIBRINOLYSIS
FIBRINOLYSIS
HEMOSTATIC BALANCE
• The regulation of hemostatic and fibrinolytic processes is
dynamic
– Balance between
• Pro- and anti-hemostatic mediators
• Pro- and anti-fibrinolytic mediators
– Balance can be upset if any components are
• Inadequate
• Excessive
• Development of thrombi
– Excessive local or systemic activation of coagulation
• Sustained bleeding
– Excessive local or systemic fibrinolytic activity
ANTICOAGULANTS
• Although tissue breakdown and platelets
destruction are normal events in the absence
of trauma, intravascular clotting does not
usually occur because:
- the amounts of procoagulants released are very small
- natural anticoagulants are present (Antithrombin III,
Heparin, Antithromboplastin, Protein C and S, fibrin
fibers)
NATURAL ANTICOAGULANTS
• Antithrombin III – inhibits factor X and thrombin
• Heparin from basophils and mast cells potentiates effects of
antithrombin III (together they inhibit IX, X, XI, XII and thrombin)
• Antithromboplastin (inhibits „tissue factors” – tissue
thromboplastins)
• Protein C and S – activated by thrombin; degrade factor Va and
VIIIa
HEMOSTATIC BALANCE
• When hemostasis is delayed
– Either platelet disorder or a coagulation defect
• Bleeding episode may be prolonged
• Imbalance created between
– An abnormally slow hemostatic rate
– A normal rate of fibrinolysis
• An inadequate fibrinolytic response
– May retard lysis of a thrombus and even contribute
to its extension
LABORATORY EVALUATION
OF HEMOSTASIS
• Bleeding
disorders
present
differently
depending upon the causative problem
• Platlet disorders present as petechiae and
bleeding into mucous membranes because of
failure to form the platlet plug
• Patients with coagulation defects may develop
deep spreading hematomas and bleeding into
the joints with evident hematuria because of
failure to reinforce the platlet plug.
LABORATORY EVALUATION
• Three different categories of disorders may be
found
– Vascular and platlet disorders
– Coagulatioin factor deficiencies
inhibitors
– Fibrinolytic disorders
or
specific
LABORATORY EVALUATION
• Tests to differentiate between these include
– Platlet count
– Peripheral blood smear evaluation
– Ivy bleeding time (N=2.5-9.5 min) or platlet function
analyzer (PFA)
– Prothrombin time (PT) – test contains thromboplastin
and calcium chloride and measures measures the
extrinsic and common pathways (Normal=11-13 sec)
LABORATORY EVALUATION
– Activated partial thromboplastin time (APTT) contact activators and a platlet substitute and
calcium chloride are added to measure the intrinsic
and common pathways (Normal usually 23-35 sec,
may vary depending upon analyzer used, reagents
used, and patient population)
– Thrombin time (TT) – add thrombin and measure
the time required for thrombin to convert
fibrinogen to fibrin (common pathway) (N=15-22
sec)
LABORATORY EVALUATION
– Mixing studies with PT and APTT abnormal results patient plasma is mixed with normal plasma to
distinguish between factor deficiencies and
coagulation inhibitors
• If assay is corrected – due to factor deficiency
• If partially corrected or uncorrected – due to inhibitor
– Coagulation factor assays
– Assays for fibrin degradation products – evidence of
fibrinolysis