Platelets

Platelets are one of the four main components of blood, along with red blood cells (RBCs), white blood cells (WBCs), and plasma. Platelets are also called thrombocytes (THROM-buh-sites). Platelets are cell-like structures primarily responsible for clotting and preventing blood loss when blood vessels become damaged due to injury. An abnormally high number of platelets in the blood can place a person at risk for clots, while an abnormally low number can increase a person's bleeding risk.

Background

Blood is the liquid that delivers nutrients and oxygen to cells in the body's tissues and organs. Blood gathers waste products from these cells and tissues and transports them away for elimination. Blood moves throughout the body in a series of blood vessels that includes arteries, veins, and capillaries. Arteries generally carry oxygen-rich blood, while veins generally carry oxygen-poor blood. Capillaries, the smallest of the body's blood vessels, serve as locations where cells exchange waste products for nutrients and oxygen.

The four main components of blood are plasma, RBCs, WBCs, and platelets. Plasma is the liquid component of blood that carries RBCs, WBCs, and platelets. About 50 percent of a person's total blood volume is plasma. Plasma is a mixture of water, sugars, proteins, and other materials. Plasma's main role is to carry blood cells and platelets throughout the body along with nutrients, hormones, and other materials that the body needs to function.

RBCs, or erythrocytes, account for another 40 to 45 percent of blood volume. RBCs contain hemoglobin, a protein that helps carry oxygen to cells and carbon dioxide away from cells. Hemoglobin in RBCs gives blood its red color.

WBCs, or leukocytes, are less plentiful that RBCs. The five types of WBCs are neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Each type performs a specific function. WBCs are a core component of the body's immune system, aiding in the fight against diseases and infections. Although a single drop of blood may contain 7,000 to 25,000 white blood cells, WBCs account for just 1 percent of total blood volume.

Platelets, or thrombocytes, are less numerous than RBCs but more numerous than WBCs. The ratio of RBCs to platelets in the blood is about twenty to one. Although platelets are cell-like structures, they are not actually cells. Platelets assist in the clotting of blood, or coagulation, following an injury. Platelets gather at the site of a wound and stick to the lining of a blood vessel to form a clot, or thrombus. The clot prevents blood from leaking from the blood vessel. It becomes the foundation upon which new tissue forms to heal the wound.

Overview

Like RBCs and many WBCs, platelets are produced in bone marrow, the spongy material inside the bones. Cells in the bone marrow called megakaryocytes are responsible for the production of platelets. Each of these cells grows quite large before it breaks apart, releasing more than a thousand fragments that become platelets. Following their creation, platelets circulate in the blood for eight to ten days. A normal platelet count ranges from about 150,000 platelets per microliter of blood to about 450,000 platelets per microliter of blood.

When viewed under a microscope, inactive platelets (i.e., those that have not been activated to help with clotting) resemble very small plates, which is why they are known as "platelets." They have a diameter about one-fifth the size of an RBC. Platelets are the lightest of the blood cells, so as blood flows throughout the various blood vessels of the body, they tend to get pushed toward vessel walls.

When a blood vessel becomes damaged as a result of a wound, platelets are activated to respond to the wound site. Once they become active, their shape begins to change. They form long string-like structures that resemble tentacles, which attach to the vessel wall or to other platelets to begin forming a clot. Adhesion is the process by which platelets adhere, or stick, to the wall of a damaged blood vessel and spread across it to begin forming a clot. As this occurs, platelets also release chemical signals that cause more platelets to gather at the wound site to strengthen the clot. This process is called aggregation.

Three medical conditions associated with abnormal platelet counts are thrombocytosis (throm-buh-sigh-TOW-sus), thrombocythemia (throm-buh-sigh-THEE-me-uh), and thrombocytopenia (throm-buh-sigh-tuh-PEE-nee-uh). Both thrombocytosis and thrombocythemia occur when platelet counts in the blood are too high. Thrombocytopenia occurs when platelet counts in the blood are too low. Physicians can use a test called a complete blood count (CBC) to determine the number of platelets in the blood.

With thrombocythemia, the bone marrow produces too many platelets. Platelet counts may be double or triple the normal range. Thrombocythemia is dangerous because it can cause clots that prevent blood from flowing to the brain or heart. Physicians are unsure what causes thrombocythemia.

Like thrombocythemia, thrombocytosis causes platelet counts in excess of the normal range. It can increase a person's risk of developing blood clots. However, thrombocytosis typically results from an underlying disease or condition that triggers the bone marrow to produce too many platelets. Platelet counts usually do not climb as high with thrombocytosis as they do with thrombocythemia. Once the underlying condition has been remedied, platelet counts typically return to normal levels.

With thrombocytopenia, the bone marrow makes too few platelets, or the platelets are destroyed shortly after they are produced. People with thrombocytopenia are at risk for excessive bleeding, especially if platelet counts drop below 50,000. In some cases of thrombocytopenia, antibodies—proteins in the body that attack foreign materials such as bacteria or viruses—see the body's platelets as foreign invaders and destroy them. In other cases, medications, chemotherapy treatments, certain cancers, kidney disease, pregnancy, and even excessive alcohol consumption can cause thrombocytopenia.

The American Red Cross estimates that someone in the United States needs platelets every thirty seconds. Luckily, it is possible for people to donate platelets to those in need. Because donated platelets must be used within five days, the need for donors is constant. The whole process takes approximately three hours, and people may donate every seven days. They may donate a maximum of twenty-four times in a one-year period.

Bibliography

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