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Platelets are cell fragments that function in the clotting system. Thrombopoietin helps control the number of circulating platelets by stimulating the bone marrow to produce megakaryocytes, which in turn shed platelets from their cytoplasm. Thrombopoietin is produced in the liver at a constant rate and its circulating level is determined by the extent to which circulating platelets are cleared, and possibly by bone marrow megakaryocytes. Platelets circulate for 7 to 10 days. About one third are always transiently sequestered in the spleen.
The platelet count is normally 140,000 to 440,000/mcL. However, the count can vary slightly according to menstrual cycle phase, decrease during near-term pregnancy (gestational thrombocytopenia), and increase in response to inflammatory cytokines (secondary, or reactive, thrombocytosis). Platelets are eventually destroyed by apoptosis, a process independent of the spleen.
Platelet disorders include
An abnormal increase in platelets (Essential Thrombocythemia and Reactive Thrombocytosis (Secondary Thrombocythemia) )
A decrease in platelets (thrombocytopenia)
Any of these conditions, even those in which platelets are increased, may cause defective formation of hemostatic plugs and bleeding.
The risk of bleeding is inversely proportional to the platelet count and platelet function (see table Platelet Count and Bleeding Risk). When platelet function is reduced (eg, as a result of uremia or aspirin use), the risk of bleeding increases.
Platelet Count and Bleeding Risk
Risk of Bleeding*
Minor bleeding after trauma
Severe, possibly life-threatening spontaneous bleeding
*Reduced platelet function (eg, due to uremia or aspirin use) adds to risk of bleeding in each platelet count range.
Last modified in- ByDavid J. Kuter