The Human Body Background notes 13
13. The reproductive system: making babies
Our reproductive system, unlike our other body systems, does very little for the first 10 to 15 years of life.
Although it is not essential for our individual survival, it ensures that we as a species continue to live on.
Human reproduction involves both women and men. The male and female reproductive systems must unite
in order for a new life to form.
Puberty: preparing to reproduce
Changes begin to take place in our bodies when we reach puberty
– some time between the ages of ten and fifteen. During puberty,
specialised glands in the brain (the hypothalamus and the pituitary
glands) become active. These glands produce chemical
messages (hormones), which travel around the body in the blood
and cause changes to cells and tissues in the body.
Male puberty
Changes occur in both males and females
when they reach puberty.
Hormones released from a man’s brain at puberty causes the
testes to start producing a very important male hormone called
testosterone.
Testosterone travels around the body and is responsible for the
development of male characteristics.
• Body, facial and pubic hair grows
• The penis and testicles grow
• The voice breaks and deepens
• Muscles develop
• Body shape changes
• Wet dreams – involuntary emissions of semen during the
night - occur.
Sperm cells. Source: Monash IVF, Epworth
Hospital
Follicle stimulating hormone (FSH) is also released from the brain. It triggers the testicles to start
producing about 50,000 sperm cells per minute. Sperm are so small that it would take about 25 million to
cover a pin-head. Most men will continue to produce sperm and be fertile until they reach the age of 70.
Men have a linear (non-cyclic) reproductive pattern.
Female puberty
Hormones released from the female brain at puberty cause a cyclic reproductive pattern to begin, called
the menstrual cycle. These hormones cause the ovaries to begin to produce two very important female
hormones – oestrogen and progesterone. These two hormones increase and decrease at different times
in the cycle and cause continuous changes in the female body.
Oestrogen is responsible for the development of female
characteristics at puberty.
• Body and pubic hair grows
• Breasts develop.
• Body shape changes and distribution of body fat changes
– women become curvy.
Most females continue the reproductive cycle from puberty until
their supply of premature eggs runs out, usually somewhere
between 45 – 55 years of age. The menstrual cycle gradually
changes and eventually stops over a period of time, which is often
referred to as menopause.
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Mature egg follicle. Source: Monash
Medical Centre
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The Human Body Background notes 13
A female makes egg cells
Follicle stimulating hormone (FSH) is released from the brain every month. It causes the ovaries to make
oestrogen and ripen one or two ova (egg) cells.
Oestrogen causes cervical mucus to become thin and watery (so that it supports and protects sperm) and
the lining of the womb to begin to thicken.
When oestrogen levels rise to a certain point, the pituitary gland releases a sharp burst of hormone called
luteinising hormone (LH), which causes the ovary to release its ripest egg. This is called ovulation. Eruption
of the egg leaves a small crater on the ovary, called the corpus luteum. As it slowly heals, this crater
secretes the hormone called progesterone.
Progesterone prepares the woman’s body for the attachment of a fertilised egg (if there is one) to the womb.
It causes the cervical mucus to become thick, which prevents entry of any additional sperm during this time.
It also causes the lining of the womb to become even thicker with blood and nutrients for implantation of the
embryo.
If pregnancy does occur progesterone continues to rise until birth, after which it drops suddenly. High levels
of progesterone stop the ovaries from releasing new eggs during pregnancy. If pregnancy does not occur
progesterone levels begin to fall as the corpus luteum heals. Without progesterone the lining of the womb
begins to break down and it sheds, along with blood, during menstruation. The brain registers the drop in
progesterone and responds by releasing more follicle stimulating hormone (FSH) which starts the whole
cycle over again.
(Left to right) : Ovary with mature egg follicle. Source: University of Melbourne; the menstrual cycle.
A male makes sperm
The testes produce sperm. Sperm cannot survive if they get too hot so the scrotum, a pouch of tissue that
holds the testes, hangs outside of the body to keep them cool. The scrotum and testes are sometimes pulled
back into the body when it gets too cold outside.
Once sperm are produced they travel to a store-house called the epididymis where they mature. On sexual
arousal blood is pumped into the penis and it becomes hard and erect. Sperm begin to travel through the
contracting vas deferens and into the seminal vesicles. Here they are mixed with seminal fluid, which nourish
and gives energy to continue their journey. The sperm continue along the vas deferens to the prostate gland,
which produces a second fluid that enables them to start propelling themselves forward. The resulting
mixture of sperm and fluid is called semen.
Sperm travel down the ejaculation duct and into the urethra, which is also the same tube that empties urine
from the bladder. However, during sexual arousal special glands in the urethra produce a third fluid that
neutralises the urine and allows the sperm to survive.
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The Human Body Background notes 13
Male reproductive system
Female reproductive system
Fertilisation: sperm meets egg
In order for reproduction to occur, sperm and egg must meet. Sexual intercourse provides the natural means
for human reproduction. A man and woman’s reproductive organs are designed to work together to achieve
this. A man releases sperm from his penis into a woman’s vagina. Stimulation of his erect penis results in it
becoming erect and the ejaculation of semen and sperm may occur.
When a woman becomes aroused her clitoris also becomes erect and her vagina becomes lubricated with
secretions.
During this process both the man and woman may experience pleasant sensations and moments of intense
pleasure called an orgasm.
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The Human Body Background notes 13
Two cells make one: a fertilised egg
A newly released egg must be fertilised within 24 hours. During intercourse, only a few sperm survive the
swim to the egg in the fallopian tube. If a sperm cell reaches an egg in the uterine tube, it attaches to it and
releases a substance that dissolves a path through the egg’s outer shell. Only a single sperm cell is able to
enter the egg. The fertilising sperm makes the egg impenetrable to other sperm. If sperm enters the egg,
fertilisation has occurred. The new cell begins to divide into a cluster of cells. Conception has begun.
(Left to right) : Human egg cell surrounded by sperm. Source: National University Hospital of Singapore; electron micrograph of sperm
penetrating egg; electron micrograph of sperm cells trying to penetrate an egg cell. Source: Visual Atlas of Human Sperm Structure and
Function, Sathananthan et al, 1996.
The fetus: from one cell to billions
The fertilised egg drifts into the uterus. There the
egg attaches to the uterine lining, which provides
nourishment. The cells of the fertilised egg
continue to divide. Within about two weeks the
cluster of cells starts to look like an embryo. The
embryo floats in a protective sac of amniotic fluid.
At eight weeks the embryo has developed into a
fetus. All the organs have formed and the heart is
beating. Nutrients circulate from the mother’s
blood, via the placenta and umbilical cord, to the
fetus.
By forty weeks, the baby is about eight times
bigger than it was at twelve weeks and is ready
to be born.
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Ultrasound image of a fetus in the uterus at 27 weeks.
Source: Monash Ultrasound.
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The Human Body Background notes 13
Pregnancy: a heavy load
Once fertilisation occurs a woman’s uterus provides a protective environment in which the embryo develops
until birth. It takes an embryo approximately 280 days to develop from conception to birth. A pregnant
woman’s body goes through many changes to accommodate a growing baby.
The woman’s uterus must eventually expand to hold a 40cm fetus weighing about 3kg. Her digestive tract
and chest are squashed, causing constipation and breathing difficulties. Her kidneys produce more urine
than normal, and she may experience morning sickness in reaction to high levels of hormones in her body.
6 weeks
19 weeks
27weeks
40 weeks.
Anatomy of pregnant woman.
A tight squeeze: childbirth
At birth, the baby is pushed from its mother’s uterus, through her pelvis and out through her vagina. In the
first 12-14 hours of labour, the cervix softens and gradually widens. Eventually, the baby’s head passes
through the cervix as the contractions of the uterus become stronger and more frequent. The mother begins
to push and eventually the baby’s head appears at the vaginal opening. Once the baby’s head is delivered,
the rest of its body usually follows quickly. Once the baby is born the placenta (afterbirth) is delivered.
Giving birth follows a particular pattern, but is a different experience for each woman and every birth.
Once the baby is born the placenta (afterbirth) is delivered.
The infant: into the world
The baby gasps air into its lungs for the first time. This takes some effort because its lungs have never held
air before. The umbilical cord stops circulating blood from the placenta and can be cut. This leaves the baby
with a scar called a belly button.
Mothers can provide nourishment for their babies in the form of breast milk. When the baby is placed at its
mother’s breast, it sucks instinctively at the nipple. Breast milk is rich in antibodies to help protect the baby
from infection. It also contains nutrients that help the brain to develop and grow.
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As we grow
Body proportions
The proportions of a child change dramatically as
it grows. The proportions stop changing when the
individual reaches its full height at adolescence.
Once a child develops past puberty the lifecycle
can start all over again.
newborn,
2 years,
5 years
adolescence
Preventing pregnancy
When a man and a woman do not want to have a baby they may use a contraceptive. There are many
different forms of contraceptive methods.
Vasectomy and tubal ligation (tying tubes) involves cutting and sealing the male vas deferens and the
fallopian tubes respectively. These procedures stop the sperm and egg from meeting during intercourse.
The contraceptive pill is hormone therapy that stops ovulation and the release of eggs occurring in
females.
A diaphragm is a cap that is inserted into the vagina over the cervix and it blocks the passage of sperm into
the uterus.
Condoms are rubber sheaths that catch semen as it is ejected from the penis of the male.
The rhythm method relies on a woman being aware of her menstrual and ovulation cycle, changing mucous
patterns and resting body temperature. Based on these body signs and the knowledge that female eggs
survives up to two days after ovulation and sperm survives up to three days after it enters the vagina, the
couple prevent pregnancy by avoiding sexual intercourse for several days before ovulation and for about four
days after it. This ensures that sperm and egg do not meet and fertilisation does not occur.
Menopause
Menopause occurs when a woman’s ovaries run out of eggs and stop making the hormone oestrogen. In
some women changes in oestrogen levels during this time may cause hot flushes, mood changes and brittle
bones. Some women are treated with hormone replacement therapy, which involves regular replacement of
oestrogens. This type of treatment may help prevent menopause symptoms however, it may increase the
risk of breast cancer in some women. Some women are able to effectively reduce menopause symptoms by
modifying their diet and lifestyle habits. Other women do not experience any changes or symptoms during
menopause.
Sexually transmitted diseases
Infections spread by sexual intercourse are called sexually transmitted diseases or STDs. STDs tend to be
more common among young people and those who have multiple sexual partners. Some STDs may be
treated while others may cause serious long term problems.
STDs can be prevented by limiting the number of sexual partners that one has and by participating in safe
sexual practices. Although condoms are not fool-proof they are still one of the most reliable ways of limiting
the transfer of STDs. Careful and well considered choices about sexual activity can generally prevent the
uncomfortable and longer term consequences of STDs.
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