its called menstruation.Menstruation is the shedding of the uterine lining (endometrium). It occurs on a regular basis in reproductive-age females of certain mammal species. Overt menstruation (where there is bleeding from the uterus through the vagina) The females of other placental mammal species have estrous cycles,

note: boys can have peeing blood too. that mean they have cancer.so us girls are very lucky :)