The menstrual cycle includes all the cyclical biological changes that happen in organs considered to be part of the female reproductive system. It is a natural phenomenon seen in people having female reproductive structures like the ovaries, the uterus, the cervix, and the vagina. Every month, the ovaries produce special cells acting as reproductive units called eggs or ova that can fuse with sperms entering inside the female reproductive tract during sexual intercourse.
If an egg and a sperm fuse, they will form a single cell that will become the embryo, stick to the wall of the uterus(womb) and gradually grow into a new individual over the course of the pregnancy, eventually becoming for birth. That is why, every month, in anticipation of the event of an egg-sperm fusion, the uterus prepares itself to become a suitable place where the embryo can develop into a new human being for the next nine months. The wall of the uterus becomes thick and full of blood vessels for the embryo to bind to it and receive nourishment to survive and grow. When the egg- sperm fusion does not occur, the body gets rid of the thickenings on the wall of the uterus in the form of menstrual or period blood.
Thus, period blood usually contains copious amounts of blood, a few blood clots, tissues shed from the uterus wall, and secretions from the cells of the vagina and cervix that consist of water, ions, mucous, and bacteria. The smell of period blood is due to the presence of all the metal ions like iron, copper, and the bacteria that live in a healthy vagina.