Some one told me... the later you have your period - the heavier it is. is this true? also, do periods help clean/ filter our blood?

Period: The first statement is false. Periods don't filter blood either. The liver and kidneys are the main organs to filter one's blood.

No and no: When a period comes doesn't have much to do with amout of bleeding. If women skip months at a time, then sometimes bleeding is heavier. A menstrual cycles sole purpose is to prepare uterus for possible fertilized egg. In old times, myths about purity or cleansing were/are that..... Myths. Women actually have more menstrual cycles now than in centuries past, due to being able to control fertilit.