Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
352
The birth control pill
At the moment, a birth control pill is available only
for women, although considerable research is going
into producing a pill that men could use as a
contraceptive measure.
The ‘pill’ was developed in the 1960s, and its
introduction had a huge impact on the freedom of women
to have sexual intercourse without running the risk of
becoming pregnant. While most would consider that this
has been a great advance, it has also contributed to the rise
in the incidence of sexually transmitted diseases, including
HIV/AIDS, because more people have had unprotected sex
with more than one partner.
The 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’. There are many different types, with
slightly different ratios of these hormones, because women
are not all alike in the way their bodies respond to the pill.
With most types of oral contraceptive, the woman takes
one pill daily for 21 days and then stops for seven days,
during which time menstruation occurs. For some types, she
continues to take a different coloured, inactive, pill for these
seven days.
Both oestrogen and progesterone suppress the secretion
of FSH and LH from the anterior pituitary gland. This is
an example of negative feedback (page 301). Look again at
Figure 15.32. You will see that, in the menstrual cycle, the
highest concentrations of FSH and LH are produced when
the concentration of oestrogen starts to fall and when
progesterone concentration has only just started to rise.
Taking the pill daily, starting at the end of menstruation,
keeps oestrogen and progesterone concentrations high.
This suppresses the secretion of FSH and LH, and prevents
their concentrations from reaching the levels that would
stimulate ovulation. This essentially mimics the natural
situation during the second half of the menstrual cycle.
Stopping taking the pill after 21 days allows the
concentrations of oestrogen and progesterone to fall to the
point at which the uterine lining is no longer maintained.
Menstruation occurs, and this reassures the woman taking
the pill that she is not pregnant.
This combined oral contraceptive is very effective at
preventing conception, but the women taking it have to
be careful not to miss even a single day’s pill as this might
allow ovulation to take place, and lead to fertilisation if she
has unprotected sex at that time.
The combination of oestrogen and progesterone can
also be given by means of a skin patch from which the
hormones are absorbed through the skin, by injection or
by inserting an implant under the skin that is effective
for several months. Obviously, with these methods, no
menstruation takes place.
Pills containing only progesterone may allow ovulation
to occur. They seem to work as contraceptives by reducing
the ability of sperm to fertilise the egg and by making the
mucus secreted by the cervix more viscous and so less
easily penetrated by sperm.
QUESTION
15.11 The graphs in Figure 15.34 show part of a woman’s
28-day oral contraceptive cycle. The top row
shows the days on which she took a combined
progesterone and oestrogen pill. The part of
the graph below this illustrates the changes in
concentrations of progesterone and oestrogen
(steroids) in her blood. The bottom graph shows the
activity of the follicles in her ovaries.
Concentration of pill steroids
in the blood
Follicular activity
pill taking
menstruation
pill taking
14 16 18 20 22 24 26 28 2 4 6 8
Time / day number of the oral contraceptive cycle
Figure 15.34
a How many days of the cycle are shown in these
graphs?
b Describe the patterns shown by the concentration
of steroids in the woman’s blood, and relate these to
her pill-taking schedule.
c Describe the patterns shown by the level of follicular
activity. Explain how the concentrations of steroids in
the blood can cause the patterns you describe.