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Menstrual Cycle

Menstrual Cycle. I.    The menstrual cycle is part of the reproductive cycle of the human female. Some examples of events in the reproductive cycle include the maturation and release of 1 egg per month and the periodic shedding of the uterine lining. Menstrual Cycle

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Menstrual Cycle

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  1. Menstrual Cycle I.   The menstrual cycle is part of the reproductive cycle of the human female. Some examples of events in the reproductive cycle include the maturation and release of 1 egg per month and the periodic shedding of the uterine lining.

  2. Menstrual Cycle A.    Overall picture: Once a cycle one mature egg leaves the ovary. At the same time the wall of the uterus thickens with blood vessels which will provide nourishment for the embryo if fertilization has occurred. If fertilization has not occurred, the built up portions of the uterine wall breaks down and is sloughed off along with the unfertilized egg and blood. This exits the body as the menstrual flow.

  3. The structures which undergo cyclic changes are: 1.      Ovary 2.      Uterus • The hormones which control these changes are: 1.      Estrogen, made in the ovary 2.      Progesterone, made in the ovary 3.      Follicle stimulating hormone (FSH), made in the anterior pituitary 4.      Lutenizing hormone (LH), made in the anterior pituitary

  4. Remember the release of the pituitary hormones, LH and FSH are controlled by releasing or inhibiting factors made in the hypothalamus. • The menstrual cycle lasts approximately 28 days. • The cycle is divided into 4 stages with menstruation “period” as the last stage.

  5. Follicle Stage A.    This stage starts at the end of a period of bleeding, so the uterine lining is low since it has just been shed. There are also no mature eggs in the ovary. B.     Events of the follicle stage: 1.      FSH is secreted by the pituitary. 2.      FSH causes several follicles to start growing, however, only 1 makes it to maturation. 3.      The maturating follicle secretes estrogen. 4.      Estrogen stimulates the lining of the uterus to thicken. C.    The follicle stage lasts approximately 10-14 days.

  6. Ovulation A.    This stage occurs when estrogen levels reach a critical level. B.     Events occurring at ovulation: 1.      The hypothalamus “senses” high estrogen levels in the blood, and releases an FSH inhibiting factor to stop the pituitary from releasing more FSH. As a consequence estrogen levels drop. This is an example of negative feedback. 2.      The hypothalamus tells the pituitary to instead secrete LH. 3.      High levels of LH trigger ovulation. C.    Ovulation usually occurs in the middle of the cycle, day 14.

  7. Corpus Luteum or Luteal Stage A.    This stage follows ovulation. B.     Events in the luteal stage: 1.      LH changes the old follicle minus the egg into a yellow mass of cells called a corpus luteum. 2.      The corpus luteum secretes some estrogen but mostly progesterone. 3.      Progesterone’s job is to build up and maintain the uterine lining. Progesterone is often called the hormone of pregnancy. 4.      Progesterone also inhibits FSH secretion so no more follicles will grow. The menstrual cycle stops during pregnancy.

  8. The luteal stage lasts 14-15 days. D.    Birth control pills contain synthetic progesterone, since progesterone inhibits FSH secretion, follicles will not mature, and without an egg you cannot get fertilization.

  9. Menstruation A.    This stage occurs if fertilization does not occur. B.     Events in menstruation: 1.      If there is no fertilization the corpus luteum atrophies (dies). 2.      Progesterone levels drop. 3.      The uterine lining cannot be maintained and is shed with the unfertilized egg and some blood. 4.      Low progesterone levels allow FSH to be secreted and a new cycle starts. C.    Menstruation lasts for 3-5 days.

  10. When does the menstrual cycle start, stop, and finally end? A.    The menstrual cycle begins at puberty between the ages of 10-14. B.     The menstrual cycle can temporarily stop or never begin due to: 1.      Pregnancy (high progesterone levels) 2.      Physical abnormalities 3.      Strenuous exercise 4.    Hormonal imbalance 5.      Emotional stress 6.      Side effects of medication 7.      Genetic disorders (Turner’s Syndrome) 8.      Diet

  11. Permanent cessation of the menstrual cycle is called menopause which occurs at 45-50 years old. During menopause women experience many physiological and psychological disturbances, these symptoms may be due to fluctuating hormone levels. Symptoms exist until a new balance is reached. • Premenstrual Syndrome (PMS) occurs 2-3 days before a period begins. It is at this point in the cycle where hormone levels are at their combined lowest.

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