Understanding the Menstrual Cycle

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

Channel: Zero To Finals

Views: 499,154

Rating: 4.9318895 out of 5

Keywords: medical, education, medicine, doctor, gynaecology, physiology, menstrual cycle, ovulation, fertilisation, medical student, oestrogen, progesterone, luteinising hormone, follicle stimulating hormone, hypothalamus pituitary gonadal axis, menstruation

Id: 3Lt9I5LrWZw

Length: 9min 53sec (593 seconds)

Published: Sun Jul 15 2018