Menstruation is the process of the shedding of the uterine lining which is basically the blood lining, on a regular monthly basis.
It begins at puberty when the female individual starts producing ovum. The ovum is released into the fallopian tube and waits till it gets fertilized by a sperm, otherwise it sheds along with the blood vessels during menstruation.
The blood vessels develops in the uterine lining in the form of a cushion to receive a fertilized egg. As a result the inner lining of the uterus becomes thick and is supplied with blood to nourish the growing embryo. If the ovum is not fertilized, then the lining of the uterus breaks down and gets released in the form of blood through the vagina. This lasts for about two to eight days. As the cycle occurs every month, so it is known as the menstrual cycle.