Platelets are the smallest blood cells that help with hemostasis and clot formation. They are discoid in shape and contain various organelles. Platelets have properties like aggregation, agglutination, and adhesiveness which allow them to stick together and to sites of injury to form a platelet plug. This is the first step in hemostasis. The blood coagulation process involves the intrinsic and extrinsic pathways which ultimately result in the conversion of fibrinogen to fibrin to form a clot. Platelets also assist with clot retraction through contraction.

2. STRUCTURE & COMPOSITION :-
small plate like
thrombo – clot; cytes—cells.
Smallest blood cells ; 2-4 μm
Spherical or oval discoid
5.8 μm^3.
Colorless
Absent

3. • Cell membrane.
• Microtubules.
• Cytoplasm.

5. • Made up of Polymerized
proteins called Tubulins.
• Forms compact bundle
below membrane &
encircle whole cytoplasm.
• Responsible for discoid
shape.

6. Contains – Golgi
apparatus, Endoplasmic
reticulum, Mitochondria,
microtubules, micro
vesicles, filament,
granules, glycogen,
Lysosomes, protein
enzymes & hormonal
substances.

7. • Aggregation – property to stick to each other
-Factors responsible are ADP & Thromboxane A2
• Agglutination – property of clumping together of
platelet
- Its due to platelet Agglutinins.
• Adhesiveness – whenever comes in contact with any
wet or rough surface gets activated & stick to surface.
- Factors responsible are –
collagen, thrombin, ADP,
thromboxane A2, Ca ion & Von-Willebrand Factor

8. • Role in Haemeostasis.
• Role in clot formation.
• Role in clot retraction.
• Role in repair of injured blood
vessels.
• Role in Defense mechanism.
• Transport & storage function.

9. NORMAL COUNT &
VARIATIONS.
150,000 – 400,000
per mm3
2.5 L/mm3
• If the count goes
below 50,000 per
mm3 ,the condition is
called critical count.

10. Less in infant, reach
adult level by 3 months of age
No difference but during
menstruation reduced in
females
Increases after meal
Increases
Increases

11. LIFE SPAN & FATE OF
PLATELETS
• Life span – 8-12 days
• Average – 10 days.
• Fate – Destroyed by tissue
macrophage system in spleen.
• Splenomegaly – reduces platelet
count.
• Splenectomy – increases platelet
count.

14. Haemostasis includes three
basic steps :-

16. Properties of Platelets which help in
platelet Plugging:-

18. Blood Coagulation
• The ultimate step in clot formation is the
conversion of fibrinogen → fibrin.

19. :
• An Intrinsic system.
• An Extrinsic system

20. • The initial reaction is the conversion of
inactive factor XII to active factor XIIa.
• Factor XII is activated in vitro by exposing
blood to foreign surface (glass test tube).
• Activation in vivo occurs when blood is
exposed to collagen fibers underlying the
endothelium in the blood vessels.

22. • Requires contact with tissue factors external
to blood.
• This occurs when there is trauma to the
vascular wall and surrounding tissues.
• The extrinsic system is triggered by the
release of tissue factor (thromboplastin from
damaged tissue), that activates factor VII.
• The tissue thromboplastin and factor VII
activate factor X.

24. • Clot formation is fully developed in
3-6 min
• Contraction of platelets trapped
within the clot shrinks the fibrin
meshwork pulling the edges of the
damaged vessel closer together.
• During clot retraction serum is
squeezed from the clot.
• Normal Clotting Time :- 3 – 6 min.

26. • Blood group antigens are actually sugars
attached to the red blood cell.
• Antigens are “built” onto the red cell.
• Individuals inherit a gene which codes for
specific sugar(s) to be added to the red
cell.
• The type of sugar added determines the
blood group

27. Principles :-
• There are two principles
• 1-almost all normal healthy individuals above 3-6
months of age have “ naturally occurring Abs” to the
ABO Ags that they lack.These Abs termed naturally
occurring because they were thought to arise without
antigenic stimulation.
• 2- These “naturally occurring” Abs are mostly IgM
class. That means that, they are Abs capable of
agglutinating saline/ low protein suspended red cell
without enhancement and may activate complement
cascade.

28. ABO Group Antigen Present Antigen Missing Antibody present
A A B Anti A
B B A Anti B
AB None A and B Anti A & B
O A and B None None

29. Matching-Mismatching during
Transfusion
Donors Group Recipients Group
A B AB O
A Yes No Yes No
B No Yes Yes No
AB No No Yes No
O Yes Yes Yes Yes

30. • Rh factor is an antigen present in red blood cells.
This antigen was first found in Rhesus monkey. So
it was named Rh Factor.
• There are many Rh antigens but only the D is main
antigen,
• The persons having D antigen are called Rh
Positive and those without D antigen are Rh
negative.
• If Rh negative blood is exposed for Rh positive
blood for first time, then Anti-D is formed in that
person.
• On the other hand, Rh positive person can receive
Rh negative blood without any the risk of
developing complications.

31. A+ A- B+ B- AB+ AB- O+ O-
A+ Yes No No No Yes No No No
A- Yes Yes No No Yes Yes No No
B+ No No Yes No Yes No No No
B- No No Yes Yes Yes Yes No No
AB+ No No No No Yes No No No
AB- No No No No Yes Yes No No
O+ No No Yes No Yes No Yes No
O- Yes Yes Yes Yes Yes Yes Yes Yes