Platelets, along with red cells and plasma, form a major proportion of both human and animal blood. Microscopically, they look like little thorned or spiky ovals, and they can only be viewed microscopically, as the average size is about four hundred thousandths of an inch (1 to 3.5 um). Platelets are actually fragments of the cells in bone marrow, called megakaryocytes. Stimulated by the hormone thrombopoietin, platelets break off the megakaryocytes and enter the blood stream, where they circulate for about 10 days before ending their short lives in the spleen. In the healthy body, thrombopoietin will help to maintain the count of platelets at a normal level, which is approximately 4.2-6.1 million of these tiny cells in two hundred thousandths of a teaspoon (1ul) of blood.

Most are familiar with the blood's ability to coagulate should one receive a cut or bruise. Specifically, platelets provide the necessary hormones and proteins for coagulation. Collagen is released when the lining of a blood vessel is damaged. The platelet recognizes collagen and begins to work on coagulating the blood by forming a kind of stopper, so further damage to the blood vessel is prevented.

A higher than normal count of platelets, known as thrombocytosis, can cause serious health risks. Too much clotting of the blood can lead to formation of blood clots that can cause stroke. Conversely, lower than normal counts can lead to extensive bleeding.

However, in some cases, inducing a lower platelet count is desirable, for instance if a person has susceptibility to strokes or has had extensive heart repair. Platelet counts can be lowered by a daily intake of aspirin or other clot reducing drugs. Additionally, when a patient has an intravenous drip (IV), heparin is used to keep the IV from clotting so fluids can be either taken from or added to the body.

While disease or a genetic disorder can cause a lower number of platelets, other times, they are depleted because of a specific treatment or surgery. Burn victims, organ transplant patients, marrow transplant patients, those undergoing chemotherapy, and those who have undergone heart surgery often require not only blood transfusions but platelet transfusions as well.

Almost anyone who is able to donate blood, and is not taking aspirin or other anti-coagulants, is also eligible for platelet donation, called platepheresis or apheresis. In this case, blood is drawn and placed in a centrifuge, where the platelets are separated from the other blood products. The rest of the blood is returned to the body, instead of being collected as it would be in a regular blood donation. The procedure takes from about 90 minutes to two hours.

Once collected, platelets only have a shelf life of about five days, and one donation provides only a sixth of a platelet transfusion unit. Given that bone marrow transplant patients often require up to 120 units of platelets, it is a foregone conclusion that new platelet donations are required daily. Information about platelet donation is available from local blood banks.