There is another tangible component of blood called platelets. Platelets are much smaller than red blood cells and white blood cells and do not have the necessary structure of a cell, so they cannot be called cells but only platelets. Platelets are closely related to hemostasis and coagulation, and their reduced number or malfunction can cause bleeding in various organs and skin mucous membranes. Platelet function often changes in lupus erythematosus. The number of platelets in normal human blood is about 200,000 per cubic millimeter. The life span of platelets is usually 10 days. Platelets are made by megakaryocytes in the bone marrow.