Thrombocytopenia
(Low Platelet Count)
Medical Author: Siamak Nabili, MD, MPH
Medical Editor: William C. Shiel, Jr., MD, FACP, FACR
What is thrombocytopenia?
Thrombocytopenia is a lower than normal number of
platelets in the blood.
Platelets are one of the components of the blood along with white and red
blood cells. Platelets play an important role in clotting and bleeding.
Platelets are made in the bone marrow similar to other cells in the blood such
as, white blood cells and red blood cells.
Platelets originate from megakaryocytes which are large cells found in the bone
marrow. The fragments of
these megakaryocytes are platelets that are released into the blood stream. The
circulating platelets make up about two third of the platelets that are released
from the bone marrow. The other one third is typically stored (sequestered) in
the spleen.
Platelets, in general, have a brief 7 to 10 days life in
the blood, after which they are removed from the blood circulation. The number of platelets in
the blood is referred to as the platelet count and is
normally between 150,000 to 450,000 per micro liter (one millionth of a liter) of blood. Platelet counts
less than 150,000 are termed thrombocytopenia. Platelet counts greater that
450,000 are called thrombocytosis.
The function of platelets is very important in the
clotting system. Platelets are a part of a very complicated pathway. They
circulate in the blood vessels and become activated if there is any bleeding or
injury in the body.
Certain chemicals are released from the injured blood vessels or other structures
that signal platelets to become activated and join the other components of the
system to stop the bleeding. When activated, the platelets become sticky and
adhere to one another and to the blood vessel wall at the site of the injury to
slow down and stop the bleeding by plugging up the damaged blood vessel or
tissue (hemostasis).
It is important to note that, even though, the platelet numbers are decreased
in thrombocytopenia, their function usually remains completely intact. Other
disorders exist that can cause impaired platelet function despite normal
platelet count.
Low platelet count in severe cases may result in spontaneous bleeding or may
cause delay in the normal process of clotting. In mild thrombocytopenia, there
may be no adverse effects in the clotting or bleeding pathways.
Picture of platelets and a blood clot
Next: What causes thrombocytopenia? »
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