Measurement
Platelet concentration is measured either manually using a hemocytometer, or by placing blood in an automated platelet analyzer using electrical impedance, such as a Coulter counter. The normal range (99% of population analyzed) for platelets in healthy white people is 150,000 to 450,000 per cubic millimeter (a mm3 equals a microliter). or 150–450 × 109 per liter. The normal range has been confirmed to be the same in the elderly and the population of Spain.The number of platelets varies across individuals. The normal physiologic range is 200,000 to 500,000 per microliter of blood. Since they contain receptors for thrombopoietin (the protein that facilitates the maturation of megakaryocytes and release of platelets), a higher number of platelets binds more of the protein. Consequently, there is stimulation for more production of thrombopoietin in the liver and kidneys. This is the basis for the production of more thrombopoietin and, as a result, more platelets in the bloodstream during the blood clotting process. Shape
In a first approximation, the platelet shape can be considered similar to oblate spheroids, with a semiaxis ratio of 2 to 8. This approximation is often used to model the hydrodynamic and optical properties of a platelet population, as well as to restore the geometric parameters of individual measured platelets by flow cytometry. More accurate biophysical models of the platelet surface morphology, which model its shape from first principles, make it possible to obtain a more realistic platelet geometry in a calm and activated state.
These processes may overlap: the spectrum is from a predominantly platelet plug, or "white clot" to a predominantly fibrin, or "red clot" or the more typical mixture. Some would add the subsequent retraction and platelet inhibition as fourth and fifth steps to the completion of the process and still others would add a sixth step, wound repair. Platelets also participate in both innate and adaptive intravascular immune responses. The platelet cell membrane has receptors for collagen. Following the rupture of the blood vessel wall, the platelets are exposed and they adhere to the collagen in the surrounding connective tissue. Low platelet concentration is called thrombocytopenia, and is due to either decreased production or increased destruction. Elevated platelet concentration is called thrombocytosis, and is either congenital, reactive (to cytokines), or due to unregulated production: one of the myeloproliferative neoplasms or certain other myeloid neoplasms. A disorder of platelet function is called a thrombocytopathy or a platelet function disorder. Normal platelets can respond to an abnormality on the vessel wall rather than to hemorrhage, resulting in inappropriate platelet adhesion/activation and thrombosis: the formation of a clot within an intact vessel. This type of thrombosis arises by mechanisms different from those of a normal clot: namely, extending the fibrin of venous thrombosis; extending an unstable or ruptured arterial plaque, causing arterial thrombosis; and microcirculatory thrombosis. An arterial thrombus may partially obstruct blood flow, causing downstream ischemia, or may completely obstruct it, causing downstream tissue death.