Understanding the Menstrual Cycle
Summary
TLDRThis 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
🩺 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.
🧬 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.
Mindmap
Keywords
💡Menstrual Cycle
💡Follicle Stimulating Hormone (FSH)
💡Luteinizing Hormone (LH)
💡Estrogen
💡Progesterone
💡Ovulation
💡Corpus Luteum
💡Endometrium
💡Menstruation
💡Hypothalamus and Pituitary Gland
💡Human Chorionic Gonadotropin (hCG)
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
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