Blood coagulation is an essential process occurring in living body to prevent the loss of blood when there occurs bleeding due to accident,cuts etc.

1. Blood coagulation
Anup Muni Bajracharya

2. Blood clotting
• Blood clotting, or coagulation, is an important process that prevents
excessive bleeding when a blood vessel is injured.
• Platelets (a type of blood cell) and proteins in our plasma (the liquid
part of blood) work together to stop the bleeding by forming a clot
over the injury.
• Hemostasis or haemostasis is a process which causes bleeding to
stop, meaning to keep blood within a damaged blood vessel (the
opposite of hemostasis is hemorrhage).
• It is the first stage of wound healing. This involves coagulation, blood
changing from a liquid to a gel.

3. Steps in Hemostasis
• Hemostasis has three major steps:
• 1) vasoconstriction/ Vascular spasm
• 2) temporary blockage of a break by a platelet plug, and
• 3) blood coagulation, or formation of a fibrin clot. These processes seal the
hole until tissues are repaired.
Basic events in blood clotting:
The basic events of the process of blood clotting or coagulation can be stated as follows
Vascular injury -------> vascular spasm------>Platelet plug formation ------>blood coagulation.

4. Terminology
• Vascular injury: It refers to the injury of the blood vessel leading to
the disruption of the capillary endothelium that causes loos of blood..
• Vascular spasm: It refers to the contraction of the smooth muscle
fibers of the all of the blood vessel (not a capillary essentially) that
reduced the flow of blood to the damaged section of the vessel.
Signals from nocireceptors initiate the process.
• Platelet plug formation: It refers to the action of the platelets to
form a plug at the site of injury involving platelet adhesion,
platelet release reaction where different substances like
prostaglandins, serotonin, ADP etc. are released from platelets. This
results in formation of a platelet plug along with fibrin and some
WBC and a few stranded RBC.
• Blood coagulation :The aforestated processes cause the coagulation
of blood or blood clotting with blood clotting factors.

5. Vascular spasm (Vasoconstriction)
• When a blood vessel is
damaged, there is an immediate
reflex, initiated by local
sympathetic pain receptors,
which helps promote
vasoconstriction.
• The damaged vessels will
constrict (vasoconstriction)
which reduces the amount of
blood flow through the area and
limits the amount of blood loss.

6. Contd…
• Collagen is exposed at the site of injury, the collagen promotes
platelets to adhere to the injury site.
• Platelets release cytoplasmic granules which contain serotonin, ADP
and thromboxane A2, all of which increase the effect of
vasoconstriction.

7. Platelet plug formation
• Platelets adhere to damaged endothelium to
form a platelet plug (primary hemostasis)
and then degranulate.
• Plug formation is activated by
a glycoprotein called Von Willibrand
factor (vWF), which is found in plasma.
• When platelets come across the injured
endothelium cells, they change shape,
release granules and ultimately become
‘sticky’.
• Platelets express certain receptors, some of
which are used for the adhesion of platelets
to collagen.

8. • When platelets are activated, they
express glycoprotein receptors that
interact with other platelets,
producing aggregation and adhesion.
• Platelets release cytoplasmic granules
such as adenosine
diphosphate (ADP), serotonin and thr
omboxane A2.
• Adenosine diphosphate (ADP) attracts
more platelets to the affected area,
serotonin is a vasoconstrictor and
thromboxane A2 assists in platelet
aggregation, vasoconstriction and
degranulation.

9. Clot formation
• Once the platelet plug has been formed by the platelets, the clotting
factors (a dozen proteins that travel along the blood plasma in an
inactive state) are activated in a sequence of events known as
'coagulation cascade' which leads to the formation of fibrin from
inactive fibrinogen plasma protein.
• Thus, a fibrin mesh is produced all around the platelet plug to hold it
in place; this step is called "Secondary Hemostasis".
• During this process some red and white blood cells are trapped in the
mesh which causes the primary hemostasis plug to become harder:
the resultant plug is called as 'thrombus' or 'Clot'.

10. Blood clotting factors
Clotting involves several chemicals
known as as blood clotting factors.
A few platelet coagulation factors
are released by platelets.
Thromboplastin or tissue factor is
found on surface of various body
cells.

11. Blood clotting factors
Coagulation factor synonym
I Fibrinogen
II prothrombin
III Tissue factor (Thromboplastin)
IV Calcium ions
V proaccelerin / labile factor / accelerator globulin
VII Serum prothrombin conversion accelerator(SPCA) /
stable factor / proconvertin
VIII Antihaemophilic factor(AHF)
IX Christmas factor / plasma thromboplastin component ( PTC)/ AHF B
X Stuart factor / Power factor / thrombokinase
XI Plasma thromboplastin antecedent(PTA) / AHFC
XII Hageman factor ? Glass factor/ contact factor
XIII Fibrib stabilizing factor (FSF) / Fibrinase

12. Stages of biochemical pathway of blood clotting
• Stage 1. Formation of prothrombin activator
• Stage 2. Conversion of protthrombin into thrombin by prothrombin
activator
• Stage 3. Conversion of soluble fibrinogen into insoluble fibrin by
thrombin.
• The formation of prothrombin activator is initiated by the interplay of
of two mechanisms - the Extrinsic(tissue factor) pathway that is
extravascular and the Intrinsic (Contact) pathway, which is
intravascular.
• In both the process of initiation of prothrombin activator is different
but the basic role of blood clotting factors is unchanged.