1. ANATOMY of the FEMALE REPRODUCTIVE SYSTEM
Here is the female reproductive system as viewed from the front.
Structures again are the fallopian tubes, ovaries, uterus, the lining of the uterus is called the endometrium and myometrium, the cervix, and the vagina.

2. MENSTRUAL PERIOD
Girls have two ovaries, each ovary holds thousands of eggs or ova.
During the menstrual cycle, one of the eggs comes out of an ovary and begins a trip through the fallopian tube, ending up in the uterus (the uterus is also called the womb).

3. Periods & Cycles
A normal menstrual cycle for girls and teens ranges from 21 to 45 days
A normal period lasts from 2 to 7 days.
Before the egg is released from the ovary, the uterus has been building up its lining with extra blood and tissue.
A period usually lasts from 5 to 7 days, and about 2 weeks after the start of the period a new egg is released, which marks the middle of each cycle.

4. Periods & Cycles
For the first year, a girl's periods are often irregular and hard to predict.
It's a good idea to keep track of your period with a calendar.
For older girls and women, their periods pretty much stick to a regular pattern.
About 6 years after a girl's first period, the cycles usually get shorter and more regular.

5. How much blood? Menstrual flow can seem like a lot, but usually isn't.
For most girls, it amounts to about 2 tablespoons (30 milliliters).