A major transfusion is defined as more than four units of packed red blood cells in one hour or more than ten units of packed red blood cells in a 24-hour period. This is enough blood to replenish the total blood volume of an average-sized human. The standard dose of blood components is one unit, but it may be given in multiple doses over several hours or days. For example, two units of packed red blood cells are given over eight hours and four units of platelets are given on two consecutive days. When ordering blood products from a laboratory, you will be asked how much product you need per unit. For example, one unit of whole blood requires five ounces of packed red blood cells and three tablespoons of platelets. The lab will also ask what type of donor the blood came from (unrelated, family member) and whether any viruses are present. Your physician may also have some additional questions for you about your health before you get blood products. Your blood volume is the amount of blood that is contained in all of your blood vessels combined. It is usually around 60-70% of your total body weight. Blood is made up of different types of cells that come in different sizes. Smaller cells such as lymphocytes require more blood per cell than larger cells like neutrophils which can survive without food or oxygen for several days.