Q3 WK2 G10 science

• Menstruation is also called a menstrual period
• Females who have gone through puberty will experience menstrual bleeding
• Menstrual bleeding occurs every month from the first onset, which could happen when a female is between 10 to 13 years old
• Bleeding usually lasts for 3 - 8 days
• A menstrual cycle is the monthly hormonal cycle a female’s body goes through to prepare for pregnancy
• The cycle is counted from the first day of the period up to the first day of the next period
• The hormone levels (estrogen and progesterone) change throughout the menstrual cycle and can cause menstrual symptoms
• The average length of a menstrual cycle is 28 days, but it varies greatly among women (ranging from 21 to 35 days)
• The events of the menstrual cycle:
• The first day of menstrual bleeding is considered Day 1 of the cycle
• The pituitary gland controls and starts the cycle
• The pituitary gland releases hormones that cause the egg in the ovary to mature
• The ovary releases estrogen, which causes the uterine lining to increase in thickness
• An egg is released from one of the ovaries around day 14 and moves to the uterus
• If the egg is not fertilized, hormonal changes signal the uterus to shed its lining, and the egg breaks down and is shed along with the lining
• Negative feedback mechanism:
• Involves a response that is the reverse of the change detected, functioning to reduce the change
• A change is detected by a receptor, and an effector is activated to induce an opposite effect, promoting equilibrium
• Positive feedback mechanism:
• Involves a response that reinforces the change detected, functioning to amplify the change
• A change is detected by a receptor, and an effector is activated to induce the same effect, promoting further change
• Four key events of a typical menstrual cycle:
• Follicular phase, ovulation, luteal phase, and menstruation
• Changes to hormonal levels, follicular development, and the status of the endometrium distinguish these events
• Follicular Phase:

  • FSH stimulates the growth of ovarian follicles
  • Dominant follicle produces estrogen, inhibiting FSH secretion to prevent other follicles from growing
  • Estrogen stimulates the thickening of the endometrial layer
• Ovulation:

  • Estrogen stimulates the anterior pituitary to secrete hormones, resulting in a surge of LH and FSH
  • LH causes the dominant follicle to rupture and release an egg (ovulation)
• Luteal Phase:

  • The ruptured follicle develops into a corpus luteum
  • The corpus luteum secretes high levels of progesterone and lower levels of estrogen
  • Estrogen and progesterone act on the uterus to thicken the endometrial lining in preparation for pregnancy
• Estrogen and progesterone act on the uterus to thicken the endometrial lining in preparation for pregnancy
• Estrogen and progesterone inhibit secretion of FSH and LH, preventing any follicles from developing
• If fertilization occurs, the developing embryo will implant in the endometrium and release hormones to sustain the corpus luteum
• If fertilization does not occur, the corpus luteum eventually degenerates, forming a corpus albicans after ~2 weeks
• When the corpus luteum degenerates, estrogen and progesterone levels drop, and the endometrium can no longer be maintained
• The endometrial layer is sloughed away and eliminated from the body as menstrual blood (a woman's period)
• As estrogen and progesterone levels are too low to inhibit the anterior pituitary, the cycle can now begin again
• The loss of cells from the uterine lining, blood, and egg is called menstruation
• The menstrual cycle is a series of changes in the reproductive system of mature females that repeats every month
• Ovulation is the release of an egg from the ovary, which occurs at about the midpoint of the cycle, around day 14 of a 28-day cycle
• The ovary releases estrogen, which causes the uterine lining to increase in thickness for potential egg attachment
• If the egg is not fertilized or implantation does not occur, the thick uterine lining breaks off and leaves the vagina, starting the cycle again
• In negative feedback, rising hormone levels decrease hormone production in the hypothalamus and pituitary gland; in positive feedback, rising hormone levels increase hormone production