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Human reproduction

Reproductive organs   Development of the embryo   The placenta    Contraception

Humans, along with all other mammals, reproduce by sexual reproduction.

This means that an egg (from the mother) needs to be fertilized a single sperm (from the father).

Fertilization is internal which means that the egg is fertilized inside the mothers body and not outside (as with a fish, for example).

The sperm (spermatozoa) are made in the testis. They leave the man's body through his penis. They are much smaller than the egg and can move by thrashing their tail.

The eggs (ova, singular ovum) are made in the ovary and one egg is released every 28 days.

Human egg cell x 60	Human sperm x 1000
Actual size of egg = ·	The sperm is too small to see without a microscope

The sperm enter the mothers body through the vagina.

The egg is fertilized in the Fallopian tube. The fertilized egg is known as a zygote.

After fertilization the egg cell starts to divide and an embryo begins to form. The embryo develops in the uterus (womb).

Male reproductive organs

Female reproductive organs

Development of the embryo

The embryo obtains food and nourishment through its umbilical cord which is attached to the mother at the placenta.  The umbilical cord, placenta, heart and brain are the first organs to form.  When the embryo is recognisable as a baby (after about 2 months of development) it is known as a fetus. After about 4 months the embryo is nearly completely developed, but is still small (about 15cm).  .

In about another 5 months the baby is ready to be born and the body will have turned so that the head faces downwards towards the vagina (birth canal)

The total time from fertilization (conception) to birth is known as the gestation period which in humans is
9 months.

A new born baby weighs about 4kg. It's head is quit large because the brain needs to be quite well developed.

At the age of about 13 a boy reaches puberty. This is when his sexual organs become mature and his body is able to produce sperm. Girls reach puberty slightly younger than boys.

The placenta

The placenta is an organ which attaches the embryo to the mother via the umbilical cord. Food and oxygen pass from the mother's blood to that of the embryo. Waste, mostly carbon dioxide and urea, pass back from the embryo to the mother. Other dangerous chemicals, such as alcohol or medicines, can also pass from the mother to the embryo and effect its growth. Note that the mother's blood cells do NOT pass across the placenta. If they did it could seriously harm the baby and the mother.

Contraception
Contraception is any way of preventing a sperm from meeting an egg so that the mother does not get pregnant. Contraceptives include the condom, worn by the man and the contraceptive pill, which is taken by the mother. The use of contraceptives is important to prevent families from getting too large.

WORDS TO KNOW

KIDNEY i) Removes waste (urea) from the blood.

ii) Removes water from the blood and so controls the blood concentration. The urea + water form urine. The urine passes down the URETER to the bladder.

BLADDER Stores urine. Urine leaves the bladder through a tube called the URETHRA. In males the urethra leaves the body through the penis.

TESTIS Where sperms (male gamete) are manufactured.

SPERM DUCT Carry sperm from the testis to the urethra.

(vas deferens)

PROSTATE GLAND Produces a liquid to help nourish the sperm.

PENIS Contains the urethra. Where urine and sperm leave the body.

OVARY Where eggs/ova (female gamete) are manufactured.

FALLOPIAN TUBE Carries the eggs from the ovary to the uterus.

(egg duct) Where an egg is fertilized. The eggs take about seven days to travel down the Fallopian tube.

UTERUS Organ where the embryo/foetus develops.

EMBRYO The young child inside the uterus. While the child is only partly formed it is often called a FETUS.

AMNION A water filled sac (containing AMNIOTIC FLUID) that helps support and protect the developing embryo.

PLACENTA An organ that supplies the developing embryo with food and oxygen from the mother's blood. Waste (carbon dioxide and urea) travels back through the placenta from the embryo to the mother. Other substances (alcohol, nicotine, drugs) can also travel through the placenta and these can effect the development of the embryo. Blood cells do NOT pass through the placenta.

UMBILICAL CORD An artery and vein from the embryo that connects the embryo's blood circulatory system to the placenta.

GAMETE Sex cell. One of the cells that join during sexual reproduction. Contains only half the number of chromosomes in the nucleus compared to a normal cell.

In animals the male gamete is in the sperm and the female gamete is in the ovum.

FERTILIZATION The fusion (joining) of the male and female gamete.

ZYGOTE The cell formed by the fertilization of a male and female gamete. ie a gamete is a fertilized egg.

GESTATION PERIOD

The time taken for the embryo to form in the uterus (ie the time from fertilization to birth) In a human this is 9 months.

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