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16. Origin and function of blood platelets

Thrombocytes

Thrombocytes, also called platelets, are a component of blood whose function is to clump together to form a platelet plug stop bleeding. They have a lifetime of 8 – 12 days in the plasma. The normal number of platelets is 150 000 – 400 000/µL. They are 1 – 3 µm in size.

Platelets that have been in the plasma for longer than 12 days are removed from the circulation by the spleen. If there is a problem with the spleen will the elimination of platelets be affected as well, as we’ll see below.

Their cell membrane is rich in glycoproteins that can bind to collagen and other platelets.

They contain actin and myosin, proteins that are used for contraction and to maintain their shape.

Platelets have two types of granules (small vesicles that contain certain molecules). Alpha granules contain certain clotting factors and growth factors. Dense granules contain ADP, Ca2+ and serotonin. This will be important in haemostasis.

Thrombocytopaenia is the condition where the number of platelets is below 150 000/µL. This can occur in:

  • Viral infection
  • Splenomegaly (enlarged spleen) – an enlarged spleen will remove too many platelets
  • Massive bleeding – because thrombocytes are used up.

Thrombocytosis is the condition where the number of platelets is above 400 000/µL. It can occur in:

  • Myeloproliferative diseases – diseases of the bone marrow where megakaryocytes are over-produced
  • Inflammation
  • Hyposplenism (small spleen)

Synthesis

Thrombocytes aren’t really cells. They are fragments of the cytoplasm of a very large cell type called megakaryocyte, which is found in the bone marrow. These megakaryocytes proliferate and cleave off fragments of their own cytoplasm, which are then released into the circulation as thrombocytes.

Function

When there is damage to the endothelium of a vessel and a bleeding occurs, the body should stop the bleeding. This process is called haemostasis and is a complicated process. A very important part of this process is the formation of the platelet plug, also called primary haemostasis. The process of haemostasis is detailed in topic 18.

The blood parameter bleeding time measures the function of the platelets. The bleeding time should be 2 – 3 minutes.