Genesis of Platelets The stem cell for platelets

Genesis of Platelets The stem cell for platelets is the • hemocytoblast The sequential developmental pathway is • hemocytoblast, megakaryoblast, promegakaryocyte, and platelets Figure 17. 12

Platelets function in the clotting mechanism by forming a temporary plug that helps seal breaks in blood vessels Active platelets Inactive platelets

Factors Preventing Undesirable Clotting • Unnecessary clotting is prevented by the structural and molecular characteristics of endothelial cells lining the blood vessels • Platelet adhesion is prevented by: – The smooth endothelial lining of blood vessels – Heparin and PGI 2 secreted by endothelial cells – Vitamin E

Hemostasis Disorders: Bleeding Disorders Thrombocytopenia – condition where the number of circulating platelets is reduced Patients have small purple blotches on the skin due to spontaneous, widespread hemorrhage Caused by suppression or destruction of bone marrow (e. g. , malignancy, radiation) Treated with whole blood transfusions or spleenectomy

Hemostasis Disorders: Bleeding Disorders Inability to synthesize procoagulants (factors regulating clotting mechanism) by the liver results in severe bleeding disorders Causes can range from vitamin K deficiency to hepatitis and cirrhosis Vitamin K normally synthesis in large intestine. Inability to absorb fat can lead to vitamin K deficiencies as it is a fat-soluble substance and is absorbed along with fat Liver disease can also prevent the liver from producing bile, which is required for fat and vitamin K absorption

Hemostasis Disorders: Bleeding Disorders Hemophilias – hereditary bleeding disorders caused by lack of clotting factors Hemophilia A – most common type (83% of all cases) # Hemophilia B

Hemostasis Disorders: Thromboembolytic Conditions Thrombus – a clot that develops and persists in an unbroken blood vessel Thrombi can block circulation, resulting in tissue death Coronary thrombosis – thrombus in blood vessel of the heart Myocardial infarction (heart attack)

Embolism Pulmonary embolism 1 st or 2 nd most common cause of unexplained death Usually originates as from deep vein thrombosis that travels into pulmonary artery and blocks artery Risks: - Post surgical patients - Prolonged bed rest - After MI - After heart bypass surgery