Lesson Video: The Menstrual Cycle Biology

In this video, we will learn how to describe the roles of hormones in controlling the menstrual cycle, and explain how they interact with each other.


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.

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