Blood coagulation, or clotting, is a crucial process that prevents excessive bleeding when blood vessels are injured, involving platelets and plasma proteins. Hemostasis consists of three main steps: vasoconstriction, platelet plug formation, and blood coagulation, leading to wound healing. Key components such as platelet activation and clotting factors facilitate the transition from liquid blood to a solid clot, which helps seal the injury until tissue repair occurs.

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.