Basic Principles of Hemostasis
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Objectives:
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Describe the 4 steps of hemostasis
Describe platelets, their normal count, development,
and functions.
Describe the process of coagulation & the role of Vit K
in it
Define normal coagulation time
List the natural anti-coagulants present in blood
Define hemophilia and list possible symptoms of
hemophilia
Explain normal bleeding time and define Purpura
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Steps of Hemostasis
1. Vascular spasm (vasoconstriction)
2. Platelet plug formation
3. Clot formation (blood clotting)
4. Clot retraction
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Platelets (Thrombocytes)
Count 150,000-400, 000 /μL
Origin: megakaryocytes in bone marrow
develop under influence of
“Thrombopoeitin”
Disc shape- with vesicles
2-4 μ in size
No nucleus
Platelets
Residues of ER, Golgi apparatus
Mitochondria
Enzyme systems
Contain granules which secrete chemicals:
i. Serotonin; ii. endothelial growth
factor; iii. Prostaglandins; iv.
phospsholipids (membrane)
v. Fibrin stabilizing factor
vi. Platelet Derived Growth factor
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Natural destruction by macrophages in
spleen
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Functions of platelets:
Help in hemostasis (stoppage of bleeding) by
1. producing vasoconstriction
2. becoming sticky (activated) to form a platelet plug to close
small holes in blood vessels
3. initiate coagulation
Clinical implications
Low dose Aspirin reduces platelet adhesiveness:
Rationale for use in “blood thinning”
Low platelet count (< 50,000/μL) causes bleeding disorder
Simple investigation: Bleeding time measurement by finger
prick.
Normal bleeding time 3-6 minutes
Disease when platelet count is low: Thrombocytopenic purpura5
Vascular spasm
Contraction of vascular smooth muscle
because of
i. muscle damage; ii. serotonin from
platelets
Platelet plug formation
1. Platelet adhesion to damaged
endothelium
2. Platelet release reaction:
ADP: increase platelet
adhesiveness (stickiness)
3. Serotonin & TXA2 : vasoconstriction
4. Platelets stick to one another
to form a plug: Adhesion which is
good enough to stop a small puncture
from bleeding
(Prostacyclin from endothelium
opposes (‫ )يعارض‬stickiness)
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Blood coagulation: factors involved
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Principles of Hemostasis
Blood coagulation
Vit K
Prothrombin activator
PROTHROMBINASE)
(Factor Xa + Ca++ +
Phospholipid + Factor V)
-Source: dietary, large intestine bacteria
-Fat soluble;
-Synthesis of
prothrombin, factors VII, IX, X,
Protein C
Prothombin
thrombin
Thrombin:
Accelerates prothrombinase formation
fibrinogen
fibrin
F XIII
Serum (straw yellow in colour)
=
blood – clotting factors
Clot retraction
+ RBC = clot
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(a) Extrinsic pathway
(b) Intrinsic pathway
Tissue trauma
Blood trauma
Damaged
endothelial cells
expose collagen
fibers
Tissue
factor
(TF)
Damaged
platelets
Activated XII
Activated
platelets
Ca2+
Ca2+
+
Platelet
phospholipids
Activated X
Activated X
V
1
V
Ca2+
Ca2+
+
PROTHROMBINASE
(c) Common
pathway
Ca2+
Prothrombin
(II)
THROMBIN
Ca2+
Fibrinogen
(I)
Loose fibrin
threads
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XIII
Activated XIII
STRENGTHENED
FIBRIN THREADS
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Prothrombinase
(Prothrombin activator)
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Breakup/limitation of clot: Fibrinolysis
Clot contains
Plasminogen
Plasmin
activate
Thrombin,
Active Factor XII
Tissue –plasminogen activator
(pTA)
clot lysis
(clot breaks up
Prostacyclin
Inhibits platelet adhesiveness
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Natural anticoagulants:
A. Endothelial surface factors
1. Smoothness of endothelial wall:
so has glycocalyx which
repels clotting factors
2. Protein C :inactivates
factors V and VIII
3. Thrombomodulin- a protein bound
to endothelium -combines with
thrombin to reduce clotting.
B. Anticoagulants present in the blood which
remove THROMBIN
1. Fibrin itself
2. Antithrombin III
-antithrombin (α globulin)
+ heparin
-removes activated factors
XII, XI, X
-removes thrombin
C. Anticoagulants may be used
In patients:
i. Heparin
ii. Vit K antagonists
4. This combination also
activates Protein C
Outside the body:
i. Heparin
ii. Calcium antagonists
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Normal coagulation time: 3-7 minutes (finger
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Clinical implications
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Coagulation disorders :
prick method)
i. not enough coagulations factors--?
(Hemophilia (factor VIII deficiency)
transmitted by female to males. Only males suffer)
ii. Not enough Vit K 
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Summary
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Steps of hemostasis
Platelets and their role in hemostasis
Normal bleeding time and its abnormality
Coagulation factors
The clotting cascade and formation of prothrombinase
Natural anti-coagulation
Anticoagulation in treatment
Normal clotting time and abnormalities
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