Why do girls need more blood than boys?

Girls need more iron than boys for this simple fact. Girls lose blood, and therefore need to compensate for blood due to the menstrual cycle. during which in a 3 to 7 day event when the lining in the uterus becomes thick with blood, which then breaks away. As you probably know, this is called a period, and it is when a lot of blood is lost. therefore girls need more iron to create the blood that is lost.