Human reproduction A: The female
system
Specialised reproductive systems provide maximum chance
for fertilisation of the sex cells (gametes) as well as protection
of the developing young to ensure continuity of the species through
time.
Anatomy
The female reproductive organs are protected by the
bones of the pelvis.
Ovulation
The gametes (sex cells) are produced in the ovaries.
From the onset of sexual maturity (puberty) when there is increased
secretion of the female sex hormone oestrogen, one egg cell (ovum)
matures about every 28 days, and is released into the oviduct (Fallopian
tube). This is called ovulation.
The
menstrual cycle
Hormones control the maturation of the egg cells
(ovulation) in parallel with the cyclic maturation of the uterus lining
to ensure that if the egg is fertilised the uterine wall will be ready
to receive and nourish it.
The contraceptive pill contains hormones which over-ride this
normal cycle, preventing ovulation.
If the egg is not fertilised, it continues down
the oviduct to the uterus, and the uterus lining of blood-rich tissue
breaks away from the wall and passes out through the vagina as the
monthly menstrual flow.
Fertilisation
If the ovum encounters a sperm cell in the oviduct, fertilisation
can occur forming the first cell of the new individual (zygote). Many
sperm will reach the ovum, but only one can penetrate the surface
and fertilise it.
The
zygote then moves down into the womb (uterus) and embeds in the blood-rich
lining. The fertilised egg cell divides into two, four and eight cells,
and these cells continue to divide to make up the billions of cells
in the baby at birth.
Hormones now produced prevent the breakdown of the uterus lining
so menstruation (monthly period) no longer occurs
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The embryo continues to grow and the cells differentiate
into different specialised tissues, as the foetus develops.
Placenta
The placenta is an organ attached to the lining of the uterus where
the mother's blood vessels exchange nutrients and wastes with the
blood of the foetal circulatory system. The umbilical cord connects
the placenta to the baby, joining at the navel.
Birth (parturition)
At full term (gestation in humans is approximately 40 weeks from the
last menstrual period), the strong muscle walls of the uterus contract,
pushing the baby's head through the neck of the womb (cervix) and
down the birth canal (vagina).
The
umbilical cord is cut and tied by the doctor, and the baby's first
cry draws air into its lungs.
The placenta is then delivered (afterbirth), with the
remainder of the uterus lining being expelled over the next few days.
Milk production (lactation)
The mammary glands (breasts) first produce a clear liquid containing
antibodies (colostrum), and later produce milk to nourish the baby.
The hormones controlling breast feeding usually prevent further ovulation,
ensuring more attention for the baby and greater chance of survival
as a consequence.
