Platelets: Cells That Clot Blood

Activated platelets, artwork
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Platelets, also called thrombocytes, are the smallest cell type in the blood. Other major blood components include plasma, white blood cells, and red blood cells. The primary function of platelets is to aid in the blood clotting process. When activated, these cells adhere to one another to block the flow of blood from damaged blood vessels. Like red blood cells and white blood cells, platelets are produced from bone marrow stem cells. Platelets are so named because unactivated platelets resemble miniature plates when viewed under a microscope.

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Platelet Production

Platelets are derived from bone marrow cells called megakaryocytes. Megakaryocytes are huge cells that break into fragments to form platelets. These cell fragments have no nucleus but do contain structures called granules. The granules house proteins that are necessary for clotting blood and sealing breaks in blood vessels.

A single megakaryocyte can produce anywhere from 1000 to 3000 platelets. Platelets circulate in the bloodstream for about 9 to 10 days. When they become old or damaged, they are removed from circulation by the spleen. Not only does the spleen filter blood of old cells, but it also stores functional red blood cells, platelets, and white blood cells. In instances where extreme bleeding occurs, platelets, red blood cells, and certain white blood cells (macrophages) are released from the spleen. These cells help to clot blood, compensate for blood loss and fight infectious agents such as bacteria and viruses.

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Platelet Function

The role of blood platelets is to clog broken blood vessels to prevent the loss of blood. Under normal conditions, platelets move through blood vessels in an unactivated state. Unactivated platelets have a typical plate-like shape. When there is a break in a blood vessel, platelets become activated by the presence of certain molecules in the blood. These molecules are secreted by blood vessel endothelial cells.

Activated platelets change their shape and become more round with long, finger-like projections extending from the cell. They also become sticky and adhere to one another and to blood vessel surfaces to plug any breaks in the vessel. Activated platelets release chemicals that cause the blood protein fibrinogen to be converted to fibrin. Fibrin is a structural protein that is arranged into long, fibrous chains. As fibrin molecules combine, they form a long, sticky fibrous mesh that traps platelets, red blood cells, and white blood cells. Platelet activation and blood coagulation processes work in conjunction to form a clot. Platelets also release signals that help to summon more platelets to the damaged site, constrict blood vessels, and activate additional clotting factors in blood plasma. 

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Platelet Count

Blood counts measure the number of of red blood cells, white blood cells, and platelets in the blood. A normal platelet count is between 150,000 to 450,000 platelets per microliter of blood. A low platelet count may result from a condition called thrombocytopenia. Thrombocytopenia can occur if the bone marrow does not make enough platelets or if the platelets are destroyed. Platelet counts below 20,000 per microliter of blood are dangerous and may result in uncontrollable bleeding. Thrombocytopenia can be caused by a number of conditions, including kidney disease, cancer, pregnancy, and immune system abnormalities. If a person's bone marrow cells make too many platelets, a condition known as thrombocythemia can develop.

With thrombocythemia, platelet counts may rise above 1,000,000 platelets per microliter of blood for reasons that are unknown. Thrombocythemia is dangerous because the excess platelets may block the blood supply to vital organs such as the heart and brain. When platelet counts are high but not as high as the counts seen with thrombocythemia, another condition called thrombocytosis may develop. Thrombocytosis is not caused by the abnormal bone marrow but by the presence of a disease or another condition, such as cancer, anemia, or an infection. Thrombocytosis is rarely serious and usually improves when the underlying condition subsides.

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  • Dean L. Blood Groups and Red Cell Antigens [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2005. Chapter 1, Blood and the cells it contains. Available from: (
  • Caring for the Patient With Cancer at Home. National Cancer Society. Updated 08/11/11 (
  • What Are Thrombocythemia and Thrombocytosis? National Heart, Lung, and Blood Institute. Updated 07/31/12 (​
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Your Citation
Bailey, Regina. "Platelets: Cells That Clot Blood." ThoughtCo, Apr. 5, 2023, Bailey, Regina. (2023, April 5). Platelets: Cells That Clot Blood. Retrieved from Bailey, Regina. "Platelets: Cells That Clot Blood." ThoughtCo. (accessed May 28, 2023).