Video Transcript
In this video, we will learn about
the menstrual cycle and how it’s controlled by various hormones, then we’ll try a
practice question, and, finally, we’ll review what we’ve learned. So let’s make like an egg during
ovulation and get started. The purpose of the reproductive
system is to allow us to generate offspring and propagate our species. Here we have a cross sectional
diagram of the female reproductive system. It includes the vagina and its
external features; the cervix, which is the opening between the vagina and the
uterus; the uterus, also commonly referred to as the womb, which possesses thick,
muscular walls; the fallopian tubes, which connect the ovaries to the uterus; and
the ovaries, which produce the female gamete, which is the egg cell.
Within each ovary or follicles,
each follicle contains an immature egg cell. Approximately once a month, in
preparation for possible pregnancy, one of the ovaries will release a mature egg
cell. This egg cell will travel down the
fallopian tube, where if it’s fertilized by a sperm cell, it’ll begin to develop
into a fetus. In preparation for possible
pregnancy, the uterus will develop a thick lining rich in blood vessels. If the egg cell is not fertilized,
within 24 to 48 hours of release, it begins to break down. The unnecessary uterine lining also
breaks down and exits the body through the vagina in a process called
menstruation. Then this cycle begins again. The process we just described is
controlled by hormones that come from your endocrine glands, specifically the
ovaries, which function within the reproductive system and within the endocrine
system, and the pituitary gland, located deep within the brain.
You may recall that hormones are
chemical messengers, which are produced by glands and transported within the
bloodstream to the target cells or organs upon which they act. When discussing the menstrual
cycle, we’ll primarily be concerned with the actions of four hormones: estrogen and
progesterone, which are produced by the ovaries, and follicle-stimulating hormone
and luteinizing hormone, which are produced by the pituitary gland. These hormones tend to act on the
reproductive system, but since they’re carried by your bloodstream, they can affect
other parts of your body as well. Hormones also have the ability to
act on other glands. A hormone stimulates a gland if it
causes the production of more hormones. Another word for stimulate is to
promote. And a hormone is said to inhibit a
gland if it causes the release of less hormones.
Estrogen causes the uterine lining
to thicken. It also inhibits
follicle-stimulating hormone and promotes luteinizing hormone. Progesterone is responsible for
maintaining the uterine lining. It also inhibits the production of
both follicle-stimulating hormone and luteinizing hormone. Follicle-stimulating hormone
stimulates the follicles within the ovaries, which causes the immature egg cells to
mature. Follicle-stimulating hormone also
stimulates the ovaries to produce estrogen. Luteinizing hormone causes the
ovary to release an egg into the fallopian tube, also known as ovulation. So now that we have an idea of what
the menstrual cycle is and the actions of the hormones that control it, let’s take a
closer look at what’s actually occurring within the female reproductive system.
The menstrual cycle describes what
happens within the female reproductive system to prepare for possible pregnancy, and
it occurs about once a month if pregnancy is not achieved. The typical menstrual cycle is
about 28 days long, although they can vary pretty widely. Day one is marked by the first day
of menstruation, also commonly called a period. The uterine lining is being shed
through the vagina, which we often refer to as bleeding. FSH is slowly rising. Follicles containing immature egg
cells develop within the ovaries. Estrogen levels are low, which can
cause feelings of low energy and depressed mood. Also, contractions within the
uterus as the lining is shed can cause painful cramping.
During days six through nine, the
uterine lining begins the process of thickening again. This is caused by the rising
estrogen levels, which also have the effect of boosting your energy and mood. Also, FSH reaches one of its peak
levels, causing a follicle to mature. During days 10 through 13, the
female reproductive system is gearing up for ovulation. Estrogen is rising rapidly, which
causes the uterine lining to continue to thicken and also boosts feelings of
emotional and physical wellbeing. The rising estrogen levels also
inhibit the production of FSH by the pituitary gland, so FSH levels begin to
drop. Ovulation occurs about halfway
through the menstrual cycle, shown here on day 14.
At this time, estrogen levels reach
a peak and then drop sharply, which causes two things to happen. The sharp drop-off in estrogen
levels allows FSH to spike one last time. At the same time, the peak in
estrogen levels causes luteinizing hormone to spike. This series of hormonal events
causes one egg to be released from the ovaries and enter the fallopian tube. We refer to this as ovulation. If the egg cell were fertilized by
a sperm cell during days 15 through 23, it would implant into the thickened uterine
lining and begin to develop into an embryo. At this point, the menstrual cycle
would stop and pregnancy would begin. So we’re gonna assume that hasn’t
happened.
Estrogen is being released in fair
quantities again, which is causing the uterine lining to continue to grow. After ovulation, progesterone is
also being released by the ovaries in fair quantities. Progesterone acts to maintain the
uterine lining. It can also cause oily skin and
bloating. During this time, the high levels
of estrogen and progesterone keep the FSH and LH levels low. After this, since implantation has
not occurred, the body begins to prepare for another menstrual cycle. Estrogen levels drop off suddenly,
which allows FSH to begin to rise again. Progesterone levels also fall,
which allows the uterine lining to begin to break down.
The sudden drop in hormone levels
can cause feelings of anxiety, irritability, and depression in some women. This is called PMS or premenstrual
syndrome, since it occurs in the days before a new menstrual cycle begins. This brings us back to day one, the
first day of menstruation, and the entire cycle begins anew.
Before we move on to our practice
questions, let’s take a moment to examine this graph of the hormonal interactions
during the menstrual cycle that you see pretty commonly. And let’s start down here with the
uterine lining. As we said before, the menstrual
cycle starts with menstruation or the uterine lining being shed. The line on our graph shows the
thickness of the uterine lining changing. From the end of menstruation until
it begins again, the uterine lining rebuilds. At the beginning of menstruation,
estrogen is at its lowest levels and grows slowly. Headed towards ovulation, estrogen
rises rapidly, peaks, and then falls. After ovulation, estrogen rises
again, causing the uterine lining to continue to thicken, and then it drops off
abruptly in preparation for a new menstrual cycle.
Progesterone maintains pretty low
levels until ovulation, after which progesterone levels rise to their highest to
maintain the uterine lining and then drop off suddenly to allow menstruation to
begin. Follicle-stimulating hormone rises
slowly throughout menstruation, and that drives the rise we see in estrogen
levels. However, as estrogen levels rise,
they begin to inhibit the production of follicle-stimulating hormone, which
eventually drops. We see a spike in FSH during
ovulation when estrogen levels drop off. That spike causes estrogen levels
to rise again, and the combination of estrogen and progesterone keeps FSH low until
the beginning of the next menstrual cycle. Luteinizing hormone levels are very
low throughout the entire menstrual cycle, except for the spike that we see which
triggers ovulation. This spike itself was triggered by
the peak in estrogen levels.
Now we’re ready to try a practice
question.
Complete the table to describe the
key events that take place in a typical 28-day menstrual cycle.
In order to answer this question,
first we’ll recall the events that occur during a typical menstrual cycle. And then we’ll use that information
to complete the table provided. The menstrual cycle is a series of
events that occur within the female reproductive system, approximately once every 28
days in preparation for possible pregnancy. Here we’ve drawn a little diagram
of the female reproductive system. It includes the vagina and its
external features; the uterus, also referred to as the womb; the cervix, which is
the opening between the vagina and the uterus; the ovaries, which contain the female
gametes or egg cells; and the fallopian tubes, which connect the ovaries to the
uterus.
About once a month, an egg cell
erupts from the ovary and travels down the fallopian tube. While that’s occurring, a lining
rich in blood vessels is building up within the uterus. If the egg cell is fertilized by a
sperm cell, it will implant into this lining and pregnancy will begin. Otherwise, the egg cell breaks
down. The unnecessary uterine lining also
breaks down, and it is shed through the vagina in a process known as
menstruation. Then the cycle begins all over
again. This is what we call the menstrual
cycle, and it typically takes about 28 days to occur.
The first day of the menstrual
cycle is marked by the first day of menstruation or the shedding of the uterine
lining through the vagina. Typically called a period,
menstruation lasts about five days. In the days after menstruation, the
uterus lining begins to build up again in preparation for possible pregnancy. About halfway through the menstrual
cycle, ovulation occurs and the egg cell is released from the ovary into the
fallopian tube. After ovulation, the uterus lining
is maintained. If the egg cell has been
fertilized, it will implant into the uterus lining during this time. If not, the uterus lining will be
shed during menstruation and the entire cycle will begin again.
So now we’re ready to complete our
table. The event at the beginning of the
menstrual cycle is menstruation. The event about halfway through on
day 14 is ovulation. And between menstruation and
ovulation, the uterus lining is building up.
Let’s wrap our lesson up by
reviewing what we’ve learned. In this video, we learned about the
key events of the menstrual cycle, including menstruation and ovulation. We also learned about the different
hormones that influence the menstrual cycle, their effects, and their
interactions.
