L4,5 Hemostasis , physio.


 L4, Hemostasis 1, physio.

L5, Hemostasis 2, Physio.


Shape: The platelets are small, non- nucleated round discs 2-4 μ in diameter.
Formation: They are formed from giant
cells in the bone marrow called megakaryocytes.
Count: The normal platelet count range from 150.000-450.000/mm2 (average 300.000)
Life span: average 8 days.

1-Plasma Membrane It contains:1- A coat of glycoprotein on its surface which helps its adhesion to the injured endothelium, but not to the normal endothelium. 2- Glycoprotein receptors for von-Willebrand factor (VWF), collagen of the vessel wall and fibrinogen. 3- Phospholipids that include platelet factor III which plays an important role in blood clotting. It invaginates into the interior of the platelets to form the open canalicular system which serves as a pathway for the uptake of extra cellular Ca+2 and release of intracellular substances.

2-Platlet cytoplasm It contains 2 types of granules:_ a. Dense granules that contain non-protein substances •ADP •Ca •Serotonin b. Alpha granules that contain the secreted proteins as •fibrin stabilizing factor XIII) •platelet derived growth factor (PDGF) •and platelet activating factor.

▸ Contractile proteins: actin, myosin and thromb-asthenine which are present beneath the plasma membrane and enable activated platelets to contract. • ▸ Glycogen granules for production of energy. • A skeleton of microtubules that keeps the disc shape of platelets. • Residuals of both endoplasmic reticulum and Golgi apparatus where various enzymes are synthesized, and large amounts of calcium ions are stored. • Mitochondria and Lysosomes.

Hemostasis Def: It means stoppage of bleeding from injured vessel. Steps of hemostasis A) Vasoconstriction (V.C) of the injured vessel. B) Formation of platelet plug. C) Formation of blood clot.

D) Repair of the damaged blood vessel

1) Vasoconstriction of the injured vessel
-contraction of vascular smooth muscles. It occurs due to:_
1-Nervous reflexes initiated by the pain of injury & sympathetic stimulation
2-Myogenic contraction of the injured vessel as a direct effect of trauma.
3-Release of serotonin, ADP and thromboxane A from platelets.
• Vasoconstriction is weaker if the vessel is cut longitudinally or irregularly.
• The greater the trauma the stronger the vasoconstriction and vice versa..