menstrual cycle

• Puberty is the period in which adolescents start to develop secondary sexual characteristics.

  • For example, facial hair in men and breasts in women.
• Puberty is triggered by reproductive hormones.

  • In women, oestrogen which is produced by the ovaries.
  • In men, testosterone which is produced in the testes and stimulates sperm production.
• The average length of a menstrual cycle is 28 days.
• On a menstrual graph, the red stuff represents the uterus lining.

  • The y-axis tells us the thickness of the uterus wall.
  • The x-axis tells us the day number.
• Stage 1 is known as menstruation:

  • This is the period of bleeding that normally lasts about 4 days.

  The bleeding is due to the breakdown of the uterus lining.

  • The thickness of the lining decreases.
• Stage 2 is when the uterus lining begins to build up again.

  • The lining becomes a thick, spongy layer with lots of blood vessels in it.

  This lasts about 10 days, up to day 14.
• Stage 2's role is to prepare the uterus lining for a fertilized egg.

  • This is because the fertilized egg implants into the uterus lining.
• Stage 3 is 'ovulation'.

  This stage takes place in a single day.

  • This involves the egg being released from one of the ovaries.
• Stage four stretches from day 14 to day 28.

  • This stage 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;

  • The uterus wall will start to break down so we will be back to stage 1 and the whole cycle repeats.
• If there was a fertilized egg, it would implant into the uterus lining and slowly develop into a foetus.

  • The cycle does not repeat - it will stop.
• Oestrogen stimulates the uterus lining to grow.

  • We see levels of oestrogen increase in stage 2 as the uterus lining develops and fall once the lining has grown (stage 3).
• Progesterone maintains the lining of the uterus, so levels of progesterone increase in stage 4.

  • If progesterone levels drop, the uterus lining breaks down - this is what restarts the cycle.
• Progesterone is produced by an empty follicle in the ovaries.
• LH and FSH are produced in the pituitary gland.
• FSH stimulates the maturation of an egg in one of the ovaries.
• LH stimulates the release of the egg on day 14, which we call 'ovulation'.
• How hormones interact:

  1. 'FSH' stimulates the ovaries to produce 'Oestrogen'.

  This is why we see higher levels of 'FSH' just before 'oestrogen' starts to increase.
• How hormones interact:

  2. As the 'Oestrogen' levels begin to increase, they

  start to inhibit 'FSH'.

  This is an example of negative feedback.
• How hormones interact:

  3. When 'Oestrogen' levels get high, they stimulate

  the release of LH.

  This is what causes the LH spike on day 14 and results in ovulation.
• How hormones interact:

  4. 'Progesterone' inhibits both 'LH' and 'FSH'.
• Oestrogen is produced in the ovaries.