Ch. 20.3

The Human Life Cycle

The egg is fertilized during the first few days after ovulation.

When sperm is deposited into the vagina, the sperm move through the uterus and into the fallopian tubes. If the sperm fertilizes an egg, pregnancy can occur.

Fertilization

A Zygote – a fertilized egg – is no larger than the period at the end of a sentence.

The zygote develops fist into an embryo and then into a fetus.

Development Before Birth

Zygote – 1 cell human (sperm + egg)

Embryo – 2 cell through week 8

Fetus – week 9 - birth

After the egg and sperm join, the zygote moves down the fallopian tube toward the uterus.

During this trip, which takes about 4 days, the zygote begins to divide.

The original cell divides to make 2 cells.

Then these 2 cells each divide for a total of 4 cells.

This continues over and over again until it forma a hollow ball.

Zygote and Embryo

This hollow ball of human cells then attaches to the lining of the uterus.

From the 2 cell stage through week 8, the developing human is called an embryo.

Embryo

The developing human is called a Fetus from weeks 9 – birth.

The fetus starts out the size of a walnut and by the end it will look more like a baby.

Fetus

Internal organs develop.

The head is about half the size of the body’s total size.

Fetus’s brain is developing rapidly.

Dark eye patches, fingers and toes also develop.

By the end of the 3rd month, the fetus is about 9 cm long and 26 g.

Months 2-3

Bones become distinct.

Heartbeat can be heard with a stethoscope.

Soft hair grown on skin.

Arms and legs develop more completely.

Fetus starts to move and kick – a sign muscles are growing.

Fetus is 700 g and 30 cm long.

Months 4-6

The fetus prepares to survive outside the mother’s body.

The brain surface develops grooves and ridges.

The lungs become ready to carry out the exchange of oxygen and carbon dioxide.

The eyelids can open.

The fetus doubles in length to 60cm.

Mass is 3000g (3kg) or larger.

Months 7-9

A growing baby needs nourishment and protection.

After the embryo attaches to the uterus wall, many changes take place.

The hollow ball of cells grown inward.

New membranes form.

The membranes and other structures that form during development protect and nourish the developing embryo, and later fetus.

Protection and Nourishment

One membrane surrounds the embryo and develops into a fluid-filled sac called the amniotic sac.

The fluid in the amniotic sac cushions and protects the developing baby like bubble rap.

Amniotic Sac

The placenta is another membrane.

It is the link between the embryo and the mother.

Blood vessels are located next to the mother’s blood vessels.

The blood DOES NOT MIX between the mother and the child.

But they do EXCHANGE substances like oxygen, food molecules, etc.

Placenta

A ropelike structure forms between the fetus and the placenta.

It contains the blood vessels that link the fetus to the mother.

This prevents blood born diseases from being passed onto the baby.

BUT! Other things can pass between them such as: alcohol, chemicals in tobacco (cigarettes) and other drugs.

This is why pregnant ladies aren’t supposed to drink, smoke or do drugs.

Umbilical Cord

After about 9 months, the baby is ready to be born.

The birth of a baby takes place in 3 stages:•Labor•Delivery•afterbirth

Birth

Strong muscular contractions in the uterus begin.

Contractions are called labor.

The cervix enlarges to a size for a baby to come out.

Labor can be from 2 – 20 hours.

Labor

The 2nd stage of birth.

With a normal delivery, the baby is pushed completely out of the uterus through the vagina.

The baby usually comes out head first.

At this time the baby is still connected to the umbilical cord and the placenta.

The delivery usually takes less time than labor. (5 min – 2 hours)

Delivery

After the baby is born, the doctors clamp the umbilical cord about 5 cm away from the baby’s future belly button.

When it heals, this turns into the belly button.

Stage 3 of birth process.

About 15 minutes after the baby is born, contractions in the uterus push the placenta and anything else left in the uterus out of the mother’s body.

This is called the afterbirth and takes less than an hour.

Afterbirth

This is a very stressful process physically for the mother and baby.

The response to these changes causes the baby’s endocrine system to release adrenaline.

The baby’s heart rate increases.

A few seconds after delivery, the baby begins to breath and cry or cough.• This helps the baby get used to living outside the mother’s

body by allowing oxygen to get into the lungs.

Birth and the Baby

If a mother gives birth to more than one baby at a time, this is called a multiple birth.

In the US, about 1 in 30 births are born with a twin.

Triplets and Quadruplets occur less frequently.

There are 2 types of twins:• Identical• Fraternal

Multiple Births

1 egg and 1 sperm.

As the zygote divides and turns into an embryo, some of the cells divide completely away from the rest starting 2 embryos that are genetically identical.

Identical Twins

2 DIFFERENT sperm and 2 DIFFERENT eggs.

They develop separately.

This is how they can be girl and boy twins.

They are no more related to each other than regular brothers and sisters.

Fraternal Twins

What actions can a newborn baby do?• Crying

• Sucking

• Yawning

• Blinking

The changes that take place between infancy and adulthood include physical changes.• Increase in size and coordination

• Mental changes (ability to communicate)

• Solve complex problems

Growth and Development

Babies go through many changes.

Physically, the baby’s head is 1/4th the size of its whole size when its born.

As it grows, the legs and arms grow faster and by the end the head looks more normal in size.

After 3 months, the baby can hold its own head up.

7 months – most babies can crawl.

10-16 months – babies begin to walk by themselves.

Infancy (age 0-2)

Between years 1 and 3 babies learn to talk.

By the end of year 2, many children can do things for themselves:•Understand simple directions• Feed themselves• Play with toys

Children continue to grow.

They become taller and heavier as their bones and muscles increase in size.

They become more coordinated as they practice walking and coloring.

They start to show curiosity and increased mental abilities.

Language skills improve rapidly

Learn to read and make friends.

Childhood (years 2-8)

Adolescence is the stage of development during which children become adults physically and mentally.

They slowly become able to think like adults.

They start to take on adult responsibilities.

Between ages 10-14, they body goes through puberty (when girls become women and boys become men physically).

Adolescence (years 9 – 18)

Adults continue to grow mentally, but not usually physically.

After about age 30, the aging process begins.

As people age, the skin becomes wrinkled and muscle strength decreases.

Eyes sometimes begin to lose focus on close objects.

Hair loses its coloring.

Adulthood (18- death)

Men usually continue to produce sperm their entire lives.

Women stop ovulating around age 50.

The effects of aging can be slowed through:• Healthy diets• Good exercise plans• Taking care of your body and mind