Reproduction in Human Beings - class 8 science revision notes

• Fertilization
  • Types of Fertilization
  • Foetal Development.
• Excretion
  • Excretory Organs (SKIL)

Fertilization

Fertilization is the fusion or union of the nuclei of the ovum and the sperm in the
oviduct/fallopian tube.

Types of Fertilization.

There are two types of Fertilization that takes place in the female's body. Namely;

• Internal fertilization- Which takes place inside the female's body, mainly in human beings,
  birds and reptiles.
• External fertization - Which takes place outside the female's body, mainly amphibians and
  fish.

Process of Fertilization in human beings

During sexual intercourse/coitus/copulation, sperms are introduced in the vagina by the penis.

Sperms swim in the semen using their tails until they reach the oviduct where they meet a
mature ovum.

Many sperms surrounds the ovum but only one penetrates the ovum, the nuclei of the sperm
and the ovum fuse to form a Zygote

After fertilization a woman becomes pregnant and this is called conception. NB: The sperms
which do not penetrate the ovum are killed.

The zygote then undergo cell division, travels down to uterus and attaches itself on the uterine walls, this is called implantation

After implantation, the zygote is called embryo 

Foetal Development.

After implantation, the embryo develops a disc-shaped organ called placenta which joins the
embryo to the mother.

At 6weeks the embryo has formed most of the important body organs such as the
lips, nose, arms, ears, eyes, feet and toes.

At 8weeks(2months), the embryo is called foetus. The foetus is surrounded by the amnion which
contains a fluid called amniotic fluid.

Functions of the placenta

• Exchange of carbon dioxide and oxygen between the mother and the foetus.
• Supply food from the mother's blood to the foetus.
• Production of hormones that maintains pregnancy.
• Facilitates transfer of waste productsfrom the foetus to the mother's blood.

Functions of the umbilical cord.

• Joins the foetus to the placenta.
• It is a passage for oxygen, food and carbon dioxide.

Functions of the amniotic fluid.

• It acts as a shock absorber to protect the foetus against physical injuries.
• It keeps the foetus warm.
• It prevents the foetus from drying.
• It lubricates the foetus for easy movement.

NB: Gestation period takes nine months or 36 to 40weeks or 280days in human beings, this is
the period between conception and birth.

Signs of pregnancy.

• Menstruation stops.
• The breast becomes larger, tender and sensitive.
• Increased frequency of urination.
• Craving for some foods.
• In later stage, expectant mothers experience some slight backaches.
• Heartburns.
• The abdomen enlarges.
• Breathlessness,tiredness and increased pulse rate.
• Movement of the foetus is felt.
• Morning sickness or nausea.

Birth process.

It is also called parturition.

When the foetus is fully developed, the head faces downwards and is positioned directly above the
cervix.

Stages in the birth process.

• The uterus walls contract causing labor pain.
• The cervix widens.
• Amnion burst and amniotic fluid flows out of the vagina.
• The baby is pushed out through the vagina with the head first.
• Breathing is induced by pinching of patting the baby.
• The umbilical cord is tied and cut, this prevents loss of blood from the mother and the baby also separates the baby from the mother.
• The placenta is expelled from the body. The expelled placenta is called afterbirth.

Excretion

This is the removal of waste productsfrom the body.

Excretory Organs (SKIL)

There are 3 main excretory organs, namely;

• Skin
• Lungs
• Kidneys.

Waste products removed by these organs are;

• Excess water.
• Excess salt.
• Urea.
• Carbon dioxide.
• Lactic acid.

Water, salt, urea and lactic acid forms sweat

Water, urea and salt forms urine.

Order of urine.

Kidney----->Ureter----->Bladder----->Urethra.