Last revised by Daniel J Bell on 15 Nov 2021

Platelets, also known as thrombocytes, are an essential constituent of the cellular component of blood. They play a key role in normal hemostasis. Normal platelet levels in adult patients are 150-400 x 109/L.


Platelets are tiny (2-4 μm) cells that lack nuclei 1-3. They are mass produced (in a continual process called thrombopoiesis) by megakaryocytes located in the bone marrow and lungs and released into the bloodstream. Platelets have a short circulatory lifespan of 7-10 days before they are degraded by the liver and spleen.

Their central physiological role is in hemostasis, which is the body's response to bleeding. The platelets attach to the traumatised wall of the blood vessel and clump together to form the so-called "platelet plug". Coagulation is a separate, yet complementary, physiological process contributing to hemostasis.

It is now known that platelets have other physiological roles in addition to hemostasis, in particular in the immune response 1-3.

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