What Are Platelets?

Platelets are tiny blood cells that help your body form clots to stop bleeding. If one of your blood vessels gets damaged, it sends out signals that are picked up by platelets. The platelets then rush to the site of damage and form a plug, or clot, to repair the damage.

The process of spreading across the surface of a damaged blood vessel to stop bleeding is called adhesion—when platelets get to the site of the injury, they grow sticky tentacles that help them adhere. They also send out chemical signals to attract more platelets to pile onto the clot in a process called aggregation.

Facts about platelets

Illustration of bone marrow

Platelets are made in your bone marrow along with your white and red blood cells. Your bone marrow is the spongy center inside your bones. Another name for platelets is thrombocytes, and health care providers usually call a clot a thrombus. Once platelets are made and circulated into your bloodstream, they live for eight to 10 days.

Under a microscope, a platelet looks like a tiny plate. Your health care provider may do a blood test called a complete blood count to find out if your bone marrow is making the right number of platelets:

  • A normal platelet count is 150,000 to 450,000 platelets per microliter of blood.

  • Risk for bleeding develops if a platelet count falls below 10,000 to 20,000.

  • Some people make too many platelets and can have platelet counts from 500,000 to more than 1 million.

What happens if your platelet count is high or low

Three medical conditions occur with abnormal platelet counts:

  • Thrombocytopenia. In this condition, your bone marrow makes too few platelets, or your platelets are destroyed. If your platelet count gets too low, bleeding can occur under the skin as bruising, inside the body as internal bleeding, or outside the body through a cut that won't stop bleeding or from a nosebleed. Thrombocytopenia can be caused by many conditions, including several medications, cancer, kidney disease, pregnancy, infections, and an abnormal immune system.

  • Thrombocythemia. In this condition, your bone marrow makes too many platelets. People with thrombocythemia may have platelet counts that exceed 1 million. Symptoms can include blood clots that form and block blood supply to the brain or the heart. The cause of thrombocythemia is unknown.

  • Thrombocytosis. This is another condition caused by too many platelets, but platelet counts do not get as high as in thrombocythemia. Thrombocytosis is more common and is not caused by abnormal bone marrow. Rather, the cause is another disease or condition in the body that stimulates the bone marrow to make more platelets. About a third of people with thrombocytosis have cancer; other causes include infection, inflammation, and reactions to medications. Symptoms are usually not serious, and the platelet count goes back to normal when the underlying condition gets better.

Platelets are tiny but important cells in your blood that help your body control bleeding. If you have symptoms such as easy bruising, a cut that keeps bleeding, or frequent nosebleeds, let your health care provider know. A simple blood test is all you need to find out if your platelet count is normal.