2. Hemostasis

5. Platelets or thrombocytes
Fragments of megakaryocytes, a large cell in the bone marrow- 60 micrometer in diameter
One megakaryocyte gives about 1000 platelets
1-4 micrometer diameter
150,000 – 300,000 per cmm of blood
No nuclei
Actin & myosin molecules for contraction
Special contractile protein Thrombosthenin
Thrombopoietin→↑ Megakaryocytes & Platelets production in the bone marrow

6. Megakaryocyte

7. Megakaryocyte

8. Platelets or thrombocytes
Residual ER and Golgi complex synthesize enzymes and store Ca++
Fibrin stabilizing factor (FSF) production
Mitochondria form ADP & ATP
Prostaglandin, Thromboxane A2 -local hormones produced
ADP and Thromboxane A2 activate Platelets
Growth factor for growth & multiplication of
Endothelial cells
Vascular smooth muscle cells
Fibroblasts

9. Platelets or thrombocytes
Glycocalyx on the cell membrane
Repulsive for normal endothelium
Adhesive to damaged endothelial cells and collagen fibers
Cell membrane contains Phospholipid involved in blood coagulation
Half life = days
Removed from circulation by macrophages mostly in spleen and liver

10. Hemostasis Prevention of blood loss Four processes
Vascular constriction
Formation of platelets plug
Formation of blood clot
Fibrous organization of clot

11. Vascular constriction
Local myogenic spasm
due to direct trauma to smooth muscles of the vessels
Local vasoconstrictors
From the traumatized tissue and platelets e.g. thromboxane A2 and Serotinin
Nervous reflexes
Due to pain & other sensory signals from the traumatized tissue
Vascular constriction is very strong when the vessel is severely damaged
Most effective in smaller vessels
Can significantly reduce blood loss for minutes

12. Formation of platelets plug
Contact with damaged vascular cells or Collagen
Swelling
Form spiked processes, Irregular shape Many pseudopodia
Release of sticky substances by contraction
Stick to each other, Collagen,
von Willebrand factor etc.

13. Formation of platelets plug
Contraction and release of active substances
Secretion of ADP & thromboxane A2 (TXA2)
Both activate more and more platelets
A positive feedback cycle is initiated that forms a platelet plug and seals the hole within 1 minute

14. Platelet Plug formation

21. Formation of platelets plug
The platelet plug must be limited in size and extent
Plug should be limited to the damaged area only
Prevention of blocking the whole vessel
Prostacyclin, produced by endothelial cells is a strong inhibitor of platelet plug formation
It stops the +ve feed back cycle of plug formation

22. Formation of platelets plug
Initially loose plug but successfully blocks small openings in the capillary wall
Later on plug is invaded by fibrin meshwork in the clotting process for permanent repair
Important in day to day small damages in the capillary walls
Process may actually result in formation of additional endothelial cell membrane due to fusion of platelets and endothelial cells thereby closing the small holes in endothelial cells .
Failure of plug formation in thrombocytopenia causes Purpura

23. Thrombocytopenic purpura
Very low platelets count
Tendency to bleed more
Many small purplish blotches in the skin
Count may be as low as 50,000/cmm
Count below 10,000/cmm is lethal
Idiopathetic thrombocytopenia
Antibodies formed against platelets
Treatment
Platelets transfusion from a fresh blood
Fresh whole blood

24. Blood coagulation
Conversion of fluid state of blood into semisolid state by activation and interaction of pro-coagulants in the blood
Set in motion with formation of the platelet plug
Many steps involving > 30 plasma factors
Proteins (made in the liver),
Vitamin K necessary for the synthesis of the liver proteins
Ca++

25. Formation of blood clot
Starts earlier (15-20 seconds) if trauma is severe and late (1-2 minutes) if trauma is trivial
The activating factors are
Traumatized vessel wall or surrounding tissue
Platelets
Plasma proteins
Clotting completes in 3-6 minutes
Clot retracts after 20 minutes

26. Blood coagulation Last three phases of critical importance
Prothrombin activator is formed
Which converts prothrombin  thrombin
Which converts fibrinogen  fibrin
Fibrin threads form a meshwork which traps blood cells and seals the hole until permanent repair

27. RBCs Trapped in Clot

29. Fibrous organization of clot
Within a few hours clot is invaded by fibroblasts which form connective tissue all through the clot.
The process is promoted by growth factor secreted by platelets.
Complete fibrous organization of the clot within about 1-2 weeks.

30. Thank-you
Questions ??