Platelets

Lec 6 Nov.2015

Platelets (Thrombocytes)

Objectives Structure Normal Count Life span Thrombopoiesis Functions hemostasis

Platelets are referred to as thrombocytes because of their sealing function i.e. they form a thrombus, or plug, when there is a break in the circulatory system.

structure

No nuclei . are discoid-shaped . sponglike appearance. Clotting factors such as factor I (fibrinogen), factor V (labile factor) & factor VIII (antihemophilic factor A) are adhere to these openings


phospholipids [(platelet factor 3(PF3)] cytoplasm ------ ADP, ATP, serotonin, Ca++ & thromboxanes A2, factor XIII , actin & myosin ----clot retraction

5. Thrombopoiesis: is the process of platelet production or development: All platelets are divided from cells in bone marrow called megakaryoblasts, these cells in the circulation called megakaryocytes which actually release platelets from its cytoplasm. The maturation of megakaryocytes is regulated by growth factor called thrombopoietin. .

Function of platelets 1. are related to haemeostasis (prevention of blood loss).

Platelets are required to maintain the structural integrity of the vessel walls Platelets play a role in keeping endothelial cells in good shape.

2.The formation of platelet plugs, that seal holes in damaged blood vessel walls as a result of vessel injury, is known as the primary haemostatic plug ..

3.The formation of clots, which help seal off larger wounds in the vessels. This process of haemostasis is called secondary haemostasis.

Haemostasis

Haemostasis: means arrest of bleeding which is very important to the maintenance of homeostasis. Bleeding------ decreasing blood volume & blood pressure & resulting in death Haemostasis can be divided into two types: Primary haemostasis. 2. Secondary haemostasis

Primary haemostasis

Primary haemostasis 1. Vascular spasm (vasoconstriction). 2. A temporary haemostatic plug of platelets.

Vascular spasm

Local vasoconstriction: contraction of smooth muscle. endothelin released from damaged blood vessels, thromboxane A2 is the most potent vasoconstrictor released by the platelets at the site of injury.

Platelets plug formation: a platelet plug

steps of the primary haemostatic plug Step I :: platelets --- to exposed collagen fibers. This adhesion requires certain essential coagulation factor such as Von willebrand factor (is a part of complex clotting factor VIII ) which is secreted by blood vessel endothelial cells.


Step II: activation of platelets by collagen cause release of ADP, thromboxane A2 ------activated additional plateletsStep III: fibrinogen will bind platelets together resulting in the formation of a platelet plug stops within few 2 – 6 minutes which is called bleeding time. Step IV: Activated platelets express phospholipids (platelet factor 3) & coagulation factor V (labile factor), which are important part of clot formation. The clot (fibrin) plugs the hole in the blood vessel until the endothelial repaired.

Clinical Note: Aspirin. aspirin production of thromboxanes resulting in reduced platelet activation. If an expectant mother ingests large quantities of aspirin near the end of pregnancy, synthesis of thromboxane A2 is inhibited & several effects are possible. The infant, mother, or both may die as a result of hemorrhage. On the other hand in a stroke (cerebral infarction) or heart attack (myocardial infarction), platelet plugs and clots can form in vessels & threaten the life of the individual. Small amounts of aspirin daily can help prevent such vascular problems. Aspirin is a drug that inhibits platelet activity i.e. anti -platelet drug.