Understanding the Menstrual Cycle

Summary

This video provides a clear and straightforward explanation of the menstrual cycle, focusing on the roles of four main hormones: FSH, LH, estrogen, and progesterone. It describes the two phases of the cycle—follicular and luteal—highlighting how hormone levels fluctuate to regulate ovulation, menstruation, and the endometrial lining. The speaker also explains the role of key structures like the hypothalamus, pituitary gland, and ovaries, while offering insights into how pregnancy affects hormone production. The video serves as a helpful resource for medical students and anyone interested in understanding the menstrual cycle.

Takeaways

• 🧠 The menstrual cycle is regulated by four main hormones: FSH (Follicle Stimulating Hormone), LH (Luteinizing Hormone), estrogen, and progesterone.
• 🗓️ The menstrual cycle typically lasts 28 days and consists of two phases: the follicular phase (first 14 days) and the luteal phase (last 14 days).
• 🥚 The follicular phase involves the development of an egg within a follicle, while the luteal phase begins after ovulation, when the egg has exited the ovary and the corpus luteum forms.
• 📏 The length of the follicular phase can vary, affecting the overall length of the cycle, but the luteal phase is consistently 14 days long.
• 🧬 The hypothalamus and anterior pituitary gland play key roles in initiating the cycle by releasing hormones that stimulate the ovaries.
• 🔄 Estrogen, released by developing follicles, promotes the thickening of the uterine lining and prepares the body for possible pregnancy.
• 📈 LH spikes right before ovulation, causing the release of the egg from the ovary around day 14 of the cycle.
• 💊 After ovulation, the corpus luteum produces progesterone, which helps maintain the uterine lining and supports early pregnancy if fertilization occurs.
• ❌ If fertilization doesn't happen, the corpus luteum breaks down, causing a drop in estrogen and progesterone, leading to the shedding of the uterine lining (menstruation).
• 🔁 The cycle restarts as FSH levels rise again after menstruation, stimulating the next batch of follicles.

Q & A

What are the four main hormones involved in the menstrual cycle?

The four main hormones involved in the menstrual cycle are follicle stimulating hormone (FSH), luteinizing hormone (LH), estrogen, and progesterone.

How long is a typical menstrual cycle and how are its phases divided?

A typical menstrual cycle is 28 days long, divided into two phases of 14 days each. The first phase is the follicular phase, and the second phase is the luteal phase.

Why is the luteal phase always 14 days long regardless of the cycle length?

The luteal phase is always 14 days long because it is determined by the lifespan of the corpus luteum, which remains constant. If the cycle is shorter, the follicular phase is shorter, and if it's longer, the follicular phase is longer.

What are the key structures in the body that release hormones related to the menstrual cycle?

The key structures are the hypothalamus and the pituitary gland in the brain, and the pelvic organs including the ovaries, which release hormones that regulate the menstrual cycle.

What is the role of the endometrium in the menstrual cycle?

The endometrium is the lining of the uterus that provides a supportive environment for a fertilized egg to implant. It breaks down and bleeds every month during menstruation to be renewed for a new cycle.

How does the hypothalamus initiate the menstrual cycle?

The hypothalamus initiates the menstrual cycle by releasing gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary gland to release FSH and LH.

What happens to the follicles during the follicular phase?

During the follicular phase, FSH stimulates the development of 15 to 20 primordial follicles within the ovaries, and as they develop, they secrete increasing amounts of estrogen.

What causes the surge in luteinizing hormone (LH) that leads to ovulation?

A dip in estrogen levels just before ovulation triggers a surge in LH, which causes one of the follicles to release the ovum or unfertilized egg.

What is the function of the corpus luteum after ovulation?

After ovulation, the follicle that released the ovum collapses and becomes the corpus luteum, which secretes high levels of progesterone and some estrogen to maintain and thicken the endometrial lining.

What happens if the ovum is not fertilized?

If the ovum is not fertilized, the corpus luteum degenerates, stopping the production of progesterone and estrogen. This drop in hormone levels leads to the breakdown of the endometrium and the start of menstruation.

How does menstruation occur and what is its duration?

Menstruation occurs when the levels of estrogen and progesterone drop, causing the endometrium to break down. The tissue is shed through the cervix and vagina, causing bleeding that lasts between one and eight days.

Outlines

00:00

🩺 Understanding the Menstrual Cycle and its Hormones

The video starts by explaining the purpose of discussing the menstrual cycle in a simple and clear way. It introduces the four main hormones involved: Follicle Stimulating Hormone (FSH), Luteinizing Hormone (LH), estrogen, and progesterone. The menstrual cycle is typically 28 days, split into two phases: the follicular phase and the luteal phase. The luteal phase is always 14 days long, and variations in the length of the cycle are due to changes in the follicular phase. The pituitary gland and hypothalamus play a key role in regulating these hormones, while the ovaries and other reproductive organs are responsible for egg development and fertilization.

05:01

🧬 The Role of Hormones and Structures in Ovulation

The video details the process of how hormones are released and their functions. The hypothalamus releases gonadotropin-releasing hormone (GnRH), prompting the anterior pituitary gland to release FSH and LH. FSH stimulates follicle development, and LH triggers ovulation. Each ovary contains many immature eggs, and FSH stimulates the growth of 15-20 follicles, with estrogen production increasing as they develop. Estrogen plays several roles, such as preparing the body for ovulation and fertilization, promoting female characteristics, and regulating blood vessels and the uterine lining. Rising estrogen levels create negative feedback that limits further release of FSH and LH.

🌸 Ovulation and the Luteal Phase

Ovulation occurs on day 14 of a 28-day cycle. During the luteal phase, the follicle that released the egg becomes the corpus luteum, which secretes progesterone and some estrogen. Progesterone thickens and maintains the endometrial lining, preventing anything from entering the uterus. If pregnancy occurs, the embryo secretes human chorionic gonadotropin (HCG), which keeps the corpus luteum functioning. If fertilization does not occur, the corpus luteum degenerates, causing progesterone and estrogen levels to drop, triggering the breakdown of the endometrium and leading to menstruation.

🔄 Menstruation and Hormonal Cycle Reset

The menstrual cycle resets when estrogen and progesterone levels drop, leading to the breakdown of the endometrium. Menstruation, or the period, is the shedding of the superficial layers of the endometrial lining, lasting between 1-8 days. The video recaps the hormone process, highlighting the roles of GnRH, FSH, LH, estrogen, and progesterone in regulating the cycle. Estrogen and progesterone levels fluctuate throughout the cycle, controlling the thickening of the endometrium and preventing or allowing hormone release from the hypothalamus and pituitary gland. After menstruation, the cycle starts again.

📊 Hormonal Changes Throughout the Cycle

The video goes through a diagram illustrating the hormonal changes throughout the menstrual cycle. FSH levels rise at the start of the cycle, promoting follicle development, and slightly spike before ovulation. Estrogen levels increase as follicles mature, dip before ovulation, and rise again during the luteal phase. LH spikes just before ovulation, initiating the release of the egg. Progesterone levels are low during the follicular phase but rise during the luteal phase to maintain the endometrial lining. If fertilization does not occur, the drop in estrogen and progesterone triggers menstruation, and the cycle repeats.

🌐 Additional Resources for Learning

The video concludes with a mention of additional resources available on the Zero to Finals website, such as notes, illustrations, and a test section for students preparing for exams. The presenter also offers career tips for medical students and invites viewers to engage with the YouTube channel by leaving comments, subscribing, and liking the video.

Keywords

💡Menstrual Cycle

The menstrual cycle is a recurring physiological process that occurs in females of reproductive age, typically characterized by the preparation of the female reproductive system for potential pregnancy. In the video, the menstrual cycle is described as having two phases, each lasting approximately 14 days, with a total cycle length of about 28 days. The cycle involves a complex interplay of hormones that regulate the development and release of an egg, as well as the preparation and shedding of the uterine lining.

💡Follicle Stimulating Hormone (FSH)

FSH is a hormone produced by the anterior pituitary gland that plays a crucial role in the development of ovarian follicles in the menstrual cycle. As explained in the video, FSH stimulates the growth of several follicles within the ovaries, each containing an immature egg. The hormone is key to the follicular phase of the menstrual cycle, where it initiates the process that eventually leads to ovulation.

💡Luteinizing Hormone (LH)

LH is another hormone secreted by the anterior pituitary gland and is essential for triggering ovulation. The video describes a surge in LH levels just before ovulation, which causes the mature follicle to rupture and release its egg. LH is also involved in the formation of the corpus luteum, which is critical for the production of progesterone during the luteal phase of the menstrual cycle.

💡Estrogen

Estrogen is a primary female sex hormone that is produced by the developing follicles in the ovaries. The video explains that estrogen has multiple functions, including the stimulation of female secondary sexual characteristics and the preparation of the uterine lining (endometrium) for potential implantation of a fertilized egg. It also plays a role in the cervical mucus thinning, facilitating sperm movement towards the egg.

💡Progesterone

Progesterone is a hormone produced by the corpus luteum after ovulation. As detailed in the video, progesterone is responsible for maintaining and thickening the endometrial lining to support a fertilized egg. It also contributes to the changes in cervical mucus and body temperature, and if pregnancy occurs, it helps maintain the uterine environment for the developing embryo.

💡Ovulation

Ovulation refers to the release of a mature egg from the ovary, typically around day 14 of a 28-day menstrual cycle. The video describes ovulation as a critical event in the menstrual cycle, where the surge in LH levels triggers the follicle to rupture, allowing the egg to be released. This event is the climax of the follicular phase and marks the beginning of the luteal phase.

💡Corpus Luteum

The corpus luteum is a temporary endocrine structure in the ovary that forms after ovulation from the remnants of the ruptured follicle. As explained in the video, it is responsible for secreting progesterone and some estrogen, which are essential for preparing the uterine lining for implantation. If fertilization does not occur, the corpus luteum degenerates, leading to a drop in hormone levels and the onset of menstruation.

💡Endometrium

The endometrium is the inner lining of the uterus, rich in blood vessels and glands. The video describes how the endometrium thickens in response to estrogen and progesterone, creating a supportive environment for a potential fertilized egg. If implantation does not occur, the endometrium breaks down and is shed during menstruation.

💡Menstruation

Menstruation is the shedding of the uterine lining, or endometrium, which results in bleeding from the uterus. The video explains that menstruation marks the start of a new menstrual cycle, typically on day 1, and involves the breakdown and expulsion of the endometrial tissue. This process is a natural part of the female reproductive cycle when pregnancy does not occur.

💡Hypothalamus and Pituitary Gland

The hypothalamus and pituitary gland are key structures in the brain that regulate the menstrual cycle through the release of hormones. The video describes how the hypothalamus releases gonadotropin-releasing hormone (GnRH), which in turn stimulates the anterior pituitary to release FSH and LH. These hormones are essential for controlling the development of follicles and the timing of ovulation.

💡Human Chorionic Gonadotropin (hCG)

hCG is a hormone produced by the developing placenta after fertilization and implantation of a fertilized egg. The video mentions that if the egg is fertilized, the resulting embryo secretes hCG, which maintains the corpus luteum and prevents it from degrading, thus sustaining the production of progesterone necessary for early pregnancy.

Highlights

Introduction to the four main hormones involved in the menstrual cycle: FSH, LH, estrogen, and progesterone.

Explanation of the two phases of the menstrual cycle: the follicular phase and the luteal phase.

Overview of the typical 28-day menstrual cycle and variations in length (21-35 days).

Role of the hypothalamus and pituitary gland in regulating hormone release, particularly FSH and LH.

FSH stimulates the development of follicles, and LH triggers ovulation on day 14 of a 28-day cycle.

Explanation of how follicles release estrogen, which promotes secondary sexual characteristics and prepares the uterus for pregnancy.

The luteal phase begins after ovulation, where the corpus luteum secretes high levels of progesterone to maintain the uterine lining.

Role of human chorionic gonadotropin (HCG) in maintaining the corpus luteum if pregnancy occurs, and its detection in pregnancy tests.

If fertilization does not occur, the corpus luteum degenerates, leading to a drop in estrogen and progesterone, triggering menstruation.

Description of how the endometrium breaks down during menstruation, and how FSH levels begin to rise again, restarting the cycle.

Summary of hormone levels throughout the cycle, including how estrogen rises, dips before ovulation, and progesterone spikes after ovulation.

Negative feedback mechanism of estrogen and progesterone on FSH and LH production during the luteal phase.

Detailed breakdown of menstruation, where the endometrial lining is shed if fertilization does not occur.

Diagram explanation of hormone levels throughout the menstrual cycle, including FSH, LH, estrogen, and progesterone.

Closing remarks encouraging viewers to explore additional resources, test questions, and tips for medical school success on the Zero to Finals website.

Transcripts

hi this is tom from zero finals comm i

wanted to make a video explaining the

menstrual cycle in a way that's really

straightforward so that you can really

understand how all of these different

structures produce hormones that

regulate this complicated cycle the

first thing to understand is the four

main hormones involved in the menstrual

cycle follicle stimulating hormone which

we abbreviate to FSH luteinizing hormone

or LH estrogen and progesterone the

second thing to understand is that

there's two phases of the menstrual

cycle

the typical menstrual cycle is 28 days

long and each phase is 14 days long the

first phase is the follicular phase and

is called this because the egg which we

also call the ovum is inside a follicle

that's developing and the second phase

is called the luteal phase because the

egg has exited the ovary during

ovulation and what's left inside the

ovary is called the corpus luteum

in many women the cycle is different

lengths for example 21 days or 35 days

and the luteal phase or the second phase

is always 14 days long so when the cycle

is shorter then it means the follicular

phase is shorter and when it's longer

the follicular phase is longer and the

luteal phase always remains 14 days long

you need to be aware of some key

structures in the body that released the

hormones firstly in the middle of the

brain you have the hypothalamus and the

pituitary gland the pituitary is split

into two parts the anterior part at the

front and the posterior part at the back

and these are responsible for regulating

a large number of hormones in the body

then you have the pelvic organs so

there's the vagina the uterus where the

babies grow the fallopian tubes and the

ovaries the ovaries are whether eggs

develop and then when they're released

they get fertilized in the tubes and

they travel down the fallopian tubes and

settle in the uterus and that's where

the babies grow the lining of the uterus

is called the endometrium it's like a

soft cushiony tissue that the fertilized

egg will implant into and it has a great

blood supply and is a great environment

to support the growing baby and this is

the area that breaks down and bleeds

every month so that it can be renewed

and refreshed for a fresh attempt at

becoming pregnant

so let's start day 1 of the menstrual

cycle at the very top the hypothalamus

kicks everything off by releasing gana

detroy and releasing hormone this

hormone is pretty self-explanatory it

does what it says on the tin it causes

the release of ganado try fans or

follicle stimulating hormone and

luteinizing hormone from the anterior

pituitary gland the main job of FSH is

to stimulate the development of the

follicles as the name would suggest and

the main job of LH is to cause ovulation

each ovary has a finite number of

immature eggs that are called primordial

follicles and there's about 200,000 of

these in each ovary they lie waiting for

the woman's whole life from birth until

the month that they become active so

once a month at the start of the

menstrual cycle follicle stimulating

hormone stimulates 15 or 20 of these

follicles to start developing within the

ovaries and as the follicles develop the

cells surrounding them called the

granulosa cells secrete increasing

amounts of estrogen

estrogen is a steroid sex hormone that

acts on tissues with estrogen receptors

to promote female secondary sexual

characteristics they stimulate the

development of breast tissue and other

female sex organs around puberty such as

the vulva vagina and uterus they also

stimulate the blood vessels in the

uterus and the development of the

endometrium and they cause the mucus in

the cervix to become thinner so that the

sperm are able to penetrate it around

the time of ovulation so that they can

get into the uterus and get to the egg

to fertilize it the east region also

causes a negative feedback on the

anterior pituitary and hypothalamus

so as the level of estrogen rises it

suppresses the release of LH and FSH

just before ovulation there's a bit of a

dip in estrogen as the follicles are

getting ready to release the egg there's

a spike in luteinizing hormone that

causes one of the follicles to reach the

surface of the ovary and release the

ovum or the unfertilized egg ovulation

happens at day 14 of a 28-day cycle

at the luteal phase of the menstrual

cycle the follicle that released the

ovum collapses and becomes the corpus

luteum the corpus luteum secretes high

levels of progesterone and progesterone

is a steroid sex hormone as well it's

produced by the corpus luteum after

ovulation and if pregnancy occurs then

the placenta will take over the

production of progesterone from around

five to ten weeks of pregnancy

progesterone acts on the tissues that

have previously been stimulated by

estrogen so for example they cause

thickening and maintaining of the

endometrial lining they thicken the

cervical mucus to prevent things going

in and out of the uterus and they also

cause a slight rise in the body

temperature the corpus luteum also

secretes some estrogen if the egg is

fertilized then the embryo secretes

something called human chorionic

gonadotropin or HCG and that keeps the

corpus luteum alive

hey TG is the hormone that we check in

the pregnancy test but if the ovum is

not fertilized and no hate CG is

produced the corpus luteum will

degenerate and it stops producing

progesterone and estrogen and this drop

in East Yujin and progesterone removes

the negative feedback to the

hypothalamus and pituitary gland and the

levels of FSH begin to rise again and

the cycle is restarted from the starts

the drop in East region and progesterone

also causes the endometrium to break

down and menstruation to occur our

menstruation starts from day one of the

cycle her menstruation is where the

superficial and middle layers of the

endometrium separate from the basal

layer of the endometrium and this tissue

is broken down inside the uterus and

it's released through the cervix and

vagina and this causes fluid containing

blood to be released from the vagina and

that normally lasts between one and

eight days and this is what we call a

period

to recap whether hormones come from the

hypothalamus releases

ganado Traven releasing hormone the

anterior pituitary releases FSH and LH

the developing follicles and the ovary

release East region and the corpus

luteum releases progesterone and

estrogen and if the egg is fertilized

that embryo will release human chorionic

gonadotropin or HCG

so now I'm going to go through the

levels of the hormone in the menstrual

cycle on this diagram FSH is released

from the anterior pituitary at the start

of the menstrual cycle and this causes

development at the follicles FSH

slightly spikes prior to ovulation

estrogen is released by those developing

follicles and it gradually rises as they

develop and then Falls just before

ovulation as the follicle prepares to

release the egg luteinizing hormone

doesn't really change much until it

spikes just before ovulation and

stimulates that ovulation to happen

there's only a very low level of

progesterone during the follicular phase

as it doesn't really have a role during

this phase progesterone and to a lesser

extent estrogen are increasingly

produced by the corpus luteum these

hormones help to thicken and maintain

the endometrium unless fertilization

occurs the corpus luteum degenerates and

the progesterone and estrogen levels

drop this drop in their levels results

in the endometrial lining breaking down

and menstruation occurring the levels of

progesterone and estrogen high during

this phase this causes negative feedback

that prevents FSH and LH from being

secreted but as they drop at the end of

this phase this removes the negative

feedback on the hypothalamus and

pituitary and FSH starts to be secreted

and this is where the cycle restarts all

over again

so thanks for watching I hope you found

this video helpful if you did don't

forget there's plenty of other resources

on the zero to finals website including

loads and loads of notes on various

different topics that you might cover in

medical school with specially made

illustrations there's also a whole test

section where you can find loads of

questions to test your knowledge and see

where you're up to in preparation for

your exams

there's also a blog where I share a lot

of my ideas about a career in medicine

and tips on how to have success as a

doctor and if you want to help me out on

YouTube you can always leave me a thumbs

up give me a comment or even subscribe

to the channel so that you can find out

when the next videos are coming out so

I'll see you again soon