platelet

A platelet is not a whole cell: it is a small fragment that circulates in the blood and whose job is clotting. When a vessel wall is torn, platelets gather at the break, stick to each other and to the wall, and begin the plug that becomes a clot, sealing the break until the vessel wall can be repaired. Platelets are one of the four components of blood beside plasma, red blood cells and white blood cells, and each has one sentence on the exam: plasma carries cells, dissolved substances and the clotting proteins, red cells carry oxygen on hemoglobin, white cells defend the body, and platelets clot. A platelet is a fragment rather than a cell, which is the detail a question uses to separate it from a white blood cell, and it carries no oxygen of its own.