15.1. Endocrine System + Menstrual Cycle

Question 1

Endocrine Glands

Answer 1

organs containing cells that secrete hormones

Question 2

How do hormones that are secreted travel around the body?

Answer 2

• transported in solution in the blood plasma all over the body
• hormones reach target cells that have specific receptors for these hormones

Question 3

What is the Menstrual Cycle

Answer 3

When the ovaries and the uterus go through a series of changes that recur approximately every 28 days - after puberty
- controlled by 4 hormones

Question 4

4 hormones that control the Menstrual Cycle

Answer 4

1) gonadotropic peptide hormone - FSH
2) gonadotropic peptide hormone - LH
3) steroid hormone - oestrogen
4) steroid hormone - progesterone
alphabetical order?

Question 5

Oestrogen

Answer 5

• produced by ovaries

- thickens uterus lining

Question 6

Progesterone

Answer 6

• produced by corpus luteum and placenta

- maintains uterus lining

Question 7

FSH (Follicle Stimulating Hormone)

Answer 7

• produced by anterior pituitary gland

- causes maturation of oocyte within follicle

Question 8

LH (Luteinising Hormone)

Answer 8

• produced by anterior pituitary gland

- stimulates ovulation

Question 9

First Half of the Menstrual Cycle

Answer 9

• Begins with the onset of menstruation which lasts for around 4 - 8 days
• During this time, the anterior pituitary gland secretes LH and FSH, and their concentrations increase over the next few days.
• In the ovary, one follicle becomes the ‘dominant’ one.
• The presence of LH and FSH stimulates the secretion of oestrogen from the cells surrounding the follicle.
• The presence of oestrogen in the blood has a negative feedback effect on the production of LH and FSH, so
  the concentrations of these two hormones decrease.
• The oestrogen stimulates the endometrium (lining of uterus) to grow, thicken and develop numerous blood capillaries.

Question 10

Mid Point of the Menstrual Cycle

Answer 10

• At day 13 or 14, the oestrogen concentration in blood would have reached a level of around 2 to 4 times its level originally
• There is a surge in LH, and to a lesser extent of FSH
• Surge of LH causes the primary oocyte in a single ovarian follicle to complete meiosis I and continue to metaphase of meiosis II
• it also causes the follicle to burst and shed the secondary oocyte into the oviduct
• This is ovulation

Question 11

Second Half of the Menstrual Cycle

Answer 11

• The follicle then collapses to form the corpus luteum (yellow body), which secretes progesterone and some oestrogen
• Together these two hormones maintain the lining of the uterus, making it ready to receive the embryo if fertilisation occurs.
• Progesterone also inhibits the anterior pituitary from
  secreting FSH so no more follicles develop.
• High levels of oestrogen and progesterone in the
  second half of the cycle inhibit the secretion of FSH and
  LH by the anterior pituitary.
• This means that there is less stimulation of the corpus luteum so that it begins to degenerate and secrete less oestrogen and progesterone.
• As the concentrations of these two hormones decrease, the endometrium is not maintained and menstruation begins.
• The decrease also releases the anterior pituitary from
  inhibition, so FSH is secreted to begin another cycle.

Question 12

Negative Feedback in the Menstrual Cycle

Answer 12

• High levels of oestrogen inhibits the secretion of FSH and LH by the anterior pituitary gland (like during the first half of the menstrual cycle, causing levels of FSH and LH to fall)
• However, when oestrogen levels are very high, a surge of LH occurs, which brings about ovulation
• Towards the end of the cycle, levels of oestrogen and progesterone fall, so the inhibition of LH and FSH is lifted. Concentration of these 2 hormones increase and it leads to the start of a new cycle

Question 13

Birth Control

Answer 13

• taking control over if and when a couple have a child

- may be contraceptives, IUDs, hormone methods

Question 14

The Birth Control Pill

Answer 14

• Only for women
• Pill contains steroid hormones that suppress
  ovulation.
• Usually, synthetic hormones rather than natural ones are used, because they are not broken down so rapidly
  in the body and therefore act for longer.
• Some forms of the pill contain progesterone only, but most contain both progesterone and oestrogen, and are known as ‘combined oral contraceptives’.
• Both oestrogen and progesterone suppress the secretion of FSH and LH from the anterior pituitary gland, which is negative feedback
• There is therefore no development of secondary follicles in the ovary and no ovulation

Question 15

The Morning-after Pill

Answer 15

• intended to be taken after a woman has had unprotected sexual intercourse and thinks that she might be pregnant.
• works for up to 72 hours after intercourse, not just the ‘morning after’.
• The pill contains a synthetic progesterone-like hormone.
• If taken early enough, it reduces the chances of a sperm reaching and fertilising an egg.
• However, in most cases, it probably prevents a pregnancy by stopping the embryo implanting into the uterus.