Haemostasis

Dr Mayank Agarwal
Assistant Professor
AIIMS, Raebareli
 Hemostasis

• Hemostasis is a complex physiologic process that

• keeps circulating blood in a fluid state, and
• if an injury occurs, produces a clot to stop the bleeding,
• confines the clot to the site of injury, and
• finally dissolves the clot as the wound heals
 Mechanism involved in hemostasis

1. Primary hemostasis
• Vascular constriction
• Platelet plug formation
2. Secondary hemostasis
• Fibrin clot formation
• Fibrinolysis
3. Tissue repair
 Vascular Constriction

1. Local myogenic spasm

2. Local autacoid factors from the traumatized tissues,

vascular endothelium (endothelin), and platelets
(serotonin, thromboxane A2)

3. Nervous reflexes (pain)

 Platelet plug formation

• Vascular injury → collagen exposure and von Willebrand factor

→ platelet adhesion → release of granules contents → ADP,
platelet-activating factor (PAF), and thromboxane A2 →
activation of nearby platelets → aggregation of platelets →
platelet plug
Summary of platelet
plug formation
its
 Importance of platelet mechanism for closing
vascular holes

• Platelet-plug closes minute ruptures in very small blood vessels

that occur many thousands of times daily

• Petechiae (small hemorrhagic areas, which appear as purple or

red dots on the skin) occurs throughout the internal tissues of a
person who have thrombocytopenia
 Blood coagulation/ Fibrin clot formation

• Clot is a meshwork of fibrin entrapping formed elements and

plasma
• Anticoagulants normally predominate, so blood does not
coagulate while it is circulating in blood vessels
• When a vessel is ruptured, procoagulants from the area of
tissue damage become activated and override anticoagulants
Clotting factor Name
I Fibrinogen
II Prothrombin
III Thromboplastin, tissue factor
IV Calcium ion
V Proaccelerin, Labile factor
VI -
VII Proconvertin, stable factor
VIII Antihemophilic factor A
IX Antihemophilic factor B, Christmas factor
X Stuart Prower factor
XI Antihemophilic factor C, Plasma thromboplastin antecedent
XII Hageman factor, contact factor
XIII Fibrin stabilizing factor, Laki Lorand factor
HMW-K High molecular weight kininogen, Fitzgerald factor
Pre K Pre kallikrein, Fletcher factor
Ka Kallikrein
PL Platelet phospholipid
Three essential steps involved in clotting
Rate limiting
step

4 low molecular weight peptides

• Prothrombin activator is generally considered to be formed in
two ways:

1. Extrinsic pathway: begins with trauma to the vascular wall

and surrounding tissues. Clot formation can occur within 15
seconds (explosive).

2. Intrinsic pathway: begins with exposure of the blood to

collagen. Slow, requires 1-6 minutes for clot formation.
 Plasma based
cascade of
coagulation

HMW-K
Pre K

Ca++

PL: Platelet (factor 3) PL

and/or tissue phospholipid
 Plasma based
cascade of
coagulation

PL, Ca2+

PL: Platelet (factor 3) and/or

tissue phospholipid
 aPTT PT
activated partial Prothrombin time
thromboplastin time

TT
Thrombin time
aPTT measures the time PT measures the time
necessary to generate fibrin necessary to generate fibrin
after activation of factor XII after activation of factor VII

Measure time required to

convert fibrinogen to fibrin in
the presence of thrombin
 Role of calcium

• Ionized calcium is required for the coagulation complexes that

assemble on platelet or cell membrane phospholipids

• Coagulation factor bind to negatively charged phospholipid

surfaces through positively charged calcium ions

• Except for the first two steps in the intrinsic pathway, calcium
ions are required for promotion or acceleration of all the
blood-clotting reactions
 Few roles of thrombin in clot formation

1. Coverts fibrinogen to fibrin

2. Activates fibrin stabilizing factor (XIII)
3. Activates factor VIII (intrinsic pathway)
4. Activates factor V (common pathway)
5. Most potent activator of platelets in vivo
6. Accelerates conversion of prothrombin to thrombin (+ve feedback)
Role of Vitamin K
• Vitamin K is fat soluble and absorbed from intestine with
the help of bile
• Neonates lack enteric bacteria that produce vitamin K
 Role of platelets in secondary hemostasis

• Platelets directly binds fibrin monomers

• Platelets entrapped in clot continue to release procoagulant

substances (fibrin stabilizing factor)

• Platelets contribute directly to clot contraction by

thrombosthenin, actin, and myosin molecules
 Clot Retraction

• Within a few minutes after a clot is formed, it begins to

contract and usually expresses most of the fluid from the clot
within 20 to 60 minutes

• The fluid expressed is called serum because all its fibrinogen

and most of the other clotting factors have been removed

• Clot retraction time serves to test platelet function

• An abnormal intravascular clot attached to a vessel wall is
known as a thrombus, and freely floating clots are called
emboli (singular, embolus)
 Coagulation regulatory mechanisms

• Prevent intravascular thrombosis and maintains blood in fluid

state
• The principal regulators are
1. Tissue Factor Pathway Inhibitor (TFPI)
2. Antithrombin (AT)
3. Activated protein C (APC)
 TFPI

• inactivates X and complex of III and VII

Protein C Regulatory System
 Protein C Regulatory System

• Thrombomodulin + thrombin → activates protein C and

decrease thrombin concentration
• Protein S binds and stabilizes activated protein C
• APC:PS complex digests and inactivates factors V and VIII
 Antithrombin (AT-III) and Heparin

• 85-90% thrombin → absorbed on fibrin

• Most of the remaining thrombin combines with AT-III and

becomes inactive

• AT activity is amplified 100 to 1000 folds by binding to heparin

• AT neutralize IIa, Xa, IXa, XIa, XIIa

Thrombin role in Anticoagulation
 Fibrinolytic System

• Hemostasis requires not only the formation of a fibrin clot to

stop bleeding but also the lysis of the clot following repair to
the vessel wall, thus restoring normal blood flow through the
vessel
• The process of removing fibrin is called fibrinolysis
• Large amount of plasminogen is trapped in the clot
• Injured tissues and vascular endothelium release tissue type
plasminogen activator (t-PA)
• Urokinase type plasminogen activator (uPA) is secreted by
kidneys and macrophages
• tPA and uPA convert plasminogen into plasmin
• Plasmin breaks fibrin polymer and forms Fibrin Degradation
Products (FDPs) that inhibit thrombin
 Thrombin

D-dimer is a specific product of digestion

of cross-linked fibrin only and is therefore
a marker of thrombosis and fibrinolysis
 Control of Fibrinolysis

• Plasminogen Activator Inhibitor-1 or PAI inhibits tPA and uPA

• APC inhibits PAI
PAI Activated C protein
 Role of Endothelium in Haemostasis

Blood flow

Smooth,
glycocalyx
 Few haemostatic function tests
Test for Prolonged in
Thrombocytopenia
Bleeding time (BT) Platelet function
Thrombasthenia
Disorders of coagulation
Clotting time (CT) Coagulation cascade
like Haemophilia
Extrinsic + common
Prothrombin time (PT) Vitamin K deficiency
pathway
Activated Partial Intrinsic + common Haemophilia
Thromboplastin Time (aPTT) pathway Vitamin K deficiency
Formation of fibrin in
Thrombin time (TT) Afibrinogenemia
presence of thrombin
 Haemostatic disorders

• Broadly classified as

1. Vessel wall disorder

2. Platelet disorders

3. Clotting factor disorders

 Platelet disorders

1. Thrombocytopenia
a. Decreased platelet survival (hypersplenism)
b. Decreased production
c. Sequestration (splenomegaly)
d. Dilutional (massive transfusion of old stored blood)

2. Thrombocytosis
• Idiopathic (Immune) Thrombocytopenic Purpura (ITP):
increased destruction of platelets by immune mechanisms
 Coagulation disorders

• Characterised by:
• Massive bleeding after trauma
• Hematoma
• Hemarthrosis
 Coagulation disorders

• Haemophilia A (Classic): XR, 85% cases of haemophilia, VIII

deficient

• Haemophilia B (Christmas disease): XR, 15%, IX deficient

• Haemophilia C (Rosenthal syndrome): AR, XI deficient

• Vitamin K deficiency: II, VII, IX, X deficient

 von Willebrand disease

• vWF adheres platelets to endothelium and act as a carrier for

factor VIII
• Reduction in vWF results in reduced factor VIII concentration
and decreased platelet plug formation
Disseminated Intravascular
Coagulation
 Bleeding disorder Clotting disorder

Petechiae Usual Rare

Deep hematomas Rare Usual

Hemarthrosis Rare Usual

BT Prolonged Normal

CT Normal Prolonged
 Anticoagulants (in vivo)
1. Heparin: acts via AT III, negatively charged polysaccharide, produced
by mast cells (lungs and liver) >> basophils, exogenous heparin is
inactivated by heparinase
2. Hirundin: Natural anticoagulant, inhibits II
3. Warfarin (Coumarins): inhibits vitamin K dependent coagulation
factors
4. Thrombin inhibitors
5. Xa inhibitors
 Anticoagulants (in vitro)

1. Heparin

2. Sodium/ammonium/potassium citrate

3. Ammonium and potassium oxalate (toxic) Chelates calcium

4. Ethylene Diamine Tetra-acetic Acid (EDTA)

5. Sodium fluoride: for glucose estimation

 International normalized ratio

• For monitoring oral anticoagulant therapy (warfarin)

• Used to standardize measurements of prothrombin time

international sensitivity index (ISI)

 International normalized ratio

• High INR level indicates a high risk of bleeding, whereas a low

INR suggests that there is a chance of having a clot
• Patients undergoing warfarin therapy usually have an INR of 2.0
to 3.0