1-2 Hemostasis Physiology

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  Blood Physiology   Hemostasis and Blood Coagulation    Events in Hemostasis  The term  hemostasis   means prevention of blood loss  Whenever a vessel is cut or ruptured, hemostasis is achieved by several mechanisms: (1)Vascular constriction (2)Formation of a platelet plug (3)Formation of a blood clot as a result of blood coagulation (4)Growth of fibrous tissue into the blood clot to close the hole in the vessel permanently  1- Vascular Constriction  Immediately after a blood vessel has been cut or ruptured, the smooth muscle in the wall contract  The contraction results from: (1) Local myogenic spasm : vasoconstriction probably results from local myogenic contraction   of the blood vessels initiated by direct damage to the vascular wall,the platelets are responsible for much of the vasoconstriction by releasing a vasoconstrictor substance, thromboxane A 2.   (2) Local autacoid factors: from the traumatized tissues and blood platelets - augmented by the local secretion of factors such as Endothelin (3) Nervous reflexes: the nervous reflexes are initiated by pain nerve   Events in Hemostasis  2- Formation of the Platelet Plug (Primary Hemostasis) If the cut in the blood vessel is very small-indeed, the cut is often sealed by a  platelet plug ,   rather than by a blood clot. Physical and Chemical Characteristics of Platelets  Platelets ( thrombocytes  ) are minute discs 1 to 4 micrometers in diameter.  The megakaryocytes fragment into the minute platelets either in the bone marrow or soon after entering the blood, especially as they squeeze through capillaries.  The normal concentration of platelets in the blood is between 150,000 and 450,000 per microliter   It has a half-life in the blood of 8 to 12 days  It is eliminated from the circulation mainly by the tissue macrophage system. More than one half of the platelets are removed by macrophages in the spleen Events in Hemostasis
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