Hemostasis is the process by which bleeding is stopped. It involves four key mechanisms: 1) Vasoconstriction of blood vessels to reduce blood flow from the injury site. 2) Formation of a platelet plug through adhesion, activation, and aggregation of platelets at the injury. 3) Coagulation of blood to form a fibrin clot from fibrinogen. 4) Later lysis of the clot through fibrinolysis to restore blood flow.

1. HEMOSTASIS

2.  Haemostasis means “arrest of bleeding”.
 During haemostasis several mechanisms
interact to slow blood flow
(vasoconstriction), block the vessel wall
defect with a platelet plug (primary
haemostasis), convert fibrinogen to a
jelly like fibrin clot (coagulation of
blood) and later re-establish the flow of
blood through a mechanism of slow clot
lysis (fibrinolysis)
HEMOSTASIS

5. 1.Vascular spasm
 Damage to blood vessels
stimulates pain receptors
and cause reflex
contraction of smooth
muscle of small blood
vessels.
 Vasoconstriction causes
decreased blood flow from
injured vessel wall.

6. Cont..
 For smaller vessels, platelets release a
vasoconstrictor substance i.e.,
Thromboxane A2.
 The spasm can last for many minutes or
hours, during which time the process of
platelet plugging and blood coagulation
can take place.

7. 2. Platelet plug formation
It occurs in three steps:
1. Adhesion- platelets adhere to the exposed
collagen on the injured endothelium and to
von willebrand factor.
2. Activation- they undergo a shape change
and become activated and release the
content of granules.
3. Aggregation- the released ADP and
thromboxane act to produce more
aggregation of platelets leading to formation
of platelet plug.

10. Primary and Secondary Hemostasis
Injury to blood vessel and damage to
endothelium
Release of
vasoconstrictors
(serotonin)
Aggregation of
platelets
Activation of platelets
Deposition of fibers
in temporary plug
Vasoconstriction
Primary hemostasis
Formation of
temporary
platelet plug
Blood clotting
Secondary hemostasis

12. Coagulation Cascade

13. Clot Resolution
(Tertiary Hemostasis)
 Activated platelets contract their internal
actin and myosin fibrils in their
cytoskeleton, which leads to shrinkage of
the clot volume.
 Plasminogen then activates to plasmin,
which promotes lysis of the fibrin clot.
 This restores the flow of blood in the
damaged/obstructed blood vessels.

14. Review