Menstrual Cycle

• Hormones are released by glands into the bloodstream, travel in the bloodstream and trigger effects in specific target organs, this is known as the endocrine system.
• During puberty, reproductive hormones cause secondary sexual characteristics to develop, for example, pubic hair in men and breasts in women.
• In humans, the ovaries produce the hormone estrogen and the testes produce the hormone testosterone.
• Every 28 days, an egg is released from the ovary, this is called ovulation.
• During the menstrual cycle, the uterus lining becomes thick and spongy in preparation for ovulation.
• If sperm is present, the egg can be fertilized and if this happens, it can implant into the uterus wall and develop into a baby.
• If the egg does not get fertilized, both the egg and the uterus lining are released and recorded as a period.
• Follicle stimulating hormone or FSH causes an egg to mature in the ovary, luteinizing hormone or LH causes this egg to be released, and estrogen and progesterone are involved in maintaining the uterus lining in case the egg fertilizes.
• In the first stage of the menstrual cycle, follicle stimulating hormone or FSH is released by the pituitary gland, FSH travels in the blood to the ovaries where it causes an egg to mature and triggers the ovaries to make estrogen.
• Luteinizing hormone or LH triggers ovulation, causing the mature egg to be released.
• Once the ovary has released its egg, it produces the hormone progesterone, which stops the pituitary gland from releasing any more FSH and LH and keeps the lining of the uterus thick in case a fertilized egg implants.
• If fertilization does not take place, the level of progesterone falls and the uterus lining and the egg are now released and the woman has a period.
• In the exam, you could be asked to interpret graphs of these hormones during the menstrual cycle.
• The level of FSH rises, triggers the ovaries to release estrogen, and inhibits the production of FSH, causing the level of FSH to fall and triggering the release of LH.
• LH triggers ovulation, causing the mature egg to be released.
• Puberty is the period during which adolescents start to develop secondary sexual characteristics, such as facial hair in men and breasts in women, triggered by reproductive hormones.
• The menstrual cycle is a four-stage process involving the interaction of four hormones: estrogen, progesterone, luteinizing hormone (LH), and follicle stimulating hormone (FSH).
• The average length of a menstrual cycle is around 28 days but it varies a lot between different people and that's perfectly healthy.
• Stage one of the menstrual cycle is known as menstruation, which is the period of bleeding that normally lasts about four days and is due to the breakdown of the uterus lining.
• Stage two of the menstrual cycle is when the uterus lining starts to build up again and becomes a thick spongy layer with lots of blood vessels in it, lasting around ten days up to day 14 and preparing the uterus lining for a fertilized egg.
• Stage three of the menstrual cycle is ovulation, which takes place in a single day and involves the egg being released from one of the ovaries.
• Stage four of the menstrual cycle, which stretches all the way to day 28, involves maintaining the lining of the uterus once we get to the end of the cycle.
• If no fertilized egg has made it to the uterus by the end of the cycle, the uterus lining would start to break down and the cycle would repeat.
• If a fertilized egg does make it to the uterus, the cycle would stop because the woman is pregnant.
• The hormones responsible for the menstrual cycle are estrogen, which is produced in the ovaries and stimulates the uterus lining to grow, progesterone, which is also produced in the ovaries and maintains the lining of the uterus, luteinizing hormone (LH), which is produced in the pituitary gland and stimulates the release of an egg on day 14, and follicle stimulating hormone (FSH), which is also produced in the pituitary gland and stimulates the maturation of an egg in one of the ovaries.
• FSH stimulates the ovaries to produce estrogen, which is why we see higher levels of FSH just before estrogen starts to increase.
• LH stimulates the release of an egg on day 14, which is known as ovulation.
• Progesterone inhibits both LH and FSH, which is why we see higher levels of progesterone when FSH levels get high.
• Progesterone also inhibits LH, which is why we see a LH spike when progesterone levels get high.
• Negative feedback loops are used to regulate temperature, blood glucose levels, water balance, pH level, and oxygen concentration.
• The body has negative feedback systems to maintain homeostasis, which means that if there's an imbalance or change, it will be corrected by the opposite effect.
• Homeostasis is the maintenance of a stable internal environment.