Menstruation
While the egg and follicle are developing in the ovary, tissues are building up inside the
uterus. The uterus develops a thick lining that is rich in tiny blood vessels. This prepares
the uterus to receive an egg. If a sperm doesnotunite with the egg in the Fallopian tube, the
lining of the uterus breaks down. Blood and other tissues from the lining are shed from the
uterus. They pass through the vagina and out of the body. This is calledmenstruation.
Menstruation is also called a menstrual period. It lasts about 4 days, on average. When the
menstrual period ends, the cycle repeats.
Some people think that the average length of a menstrual period is the same as the “normal”
length. They assume that shorter or longer menstrual periods are not normal. In fact,
menstrual periods can vary from 1 to 8 days in length. Such variation is usually normal.
The length of the overall menstrual cycle can also vary. The average length of the cycle is
about 28 days, but there is no “normal” cycle length.
Hormones and the Menstrual Cycle
Hormones control the events of the menstrual cycle. The hormones are estrogen, proges-
terone, LH, and FSH. The ovaries secrete estrogen and progesterone. The pituitary gland
secretes LH and FSH (see theControlling the Bodychapter).
The events of the menstrual cycle, including how hormone levels change throughout the
cycle, are shown inFigure 22.5. As the figure indicates, a menstrual cycle begins with
menstruation. When menstruation ends, the ovaries start secreting more estrogen. Estrogen
causes the lining of the uterus to build up, which prepares the uterus to receive an egg.
It also causes the pituitary gland to secrete FSH. FSH, in turn, causes an egg and follicle
to mature in an ovary. The maturing follicle secretes even more estrogen. When estrogen
reaches a certain level, it causes the pituitary gland to release a surge of LH. The LH surge
causes ovulation. It also causes the empty follicle to develop into a corpus luteum. The
corpus luteum secretes progesterone. This hormone maintains the lining of the uterus so it
doesn’t break down.
What happens next depends on whether a sperm unites with the egg. If it does, the egg
secretesahormonethatpreventsthecorpusluteumfrombreakingdown. Thecorpusluteum,
in turn, keeps secreting progesterone. This maintains the lining of the uterus. What happens
if a sperm doesnotunite with the egg is shown inFigure22.5. The corpus luteum breaks
down and stops secreting progesterone. As a result, the lining of the uterus is no longer
maintained. It breaks down and is shed from the uterus. Thus, menstruation begins and
the cycle repeats.
Dr. JoAnne Richards is a world-famous endocrinologist. An endocrinologist is a scientist
that studies hormones. Dr. Richards helped discover how FSH and LH control the growth
of follicles and ovulation. She has received many awards and honors for her work, including