The baby starts smiling, paying attention to other people, and grabbing toys.
The baby begins making babbling sounds. By the end of the first year, the baby is starting to say a few words, such as “Mama” and “Dada.”
The baby learns to sit, crawl, and stand. By the end of the first year, the baby may be starting to walk.
Figure 22.8
This baby is six months old, and his baby teeth have started to come in. Babies often chew on toys or other objects when they are getting new teeth. They may even chew on their toes, as this baby is doing. Putting things in their mouth also helps them learn about the world. What do you think this baby might be learning by putting his toes in his mouth?
Childhood begins after the baby’s first birthday and continues until the teens. Between one and three years of age, a child is called a toddler. During the toddler stage, growth is still rapid, but not as fast as it was during infancy. A toddler learns many new words. The child even starts putting together words in simple sentences. Motor skills also develop quickly during this stage. By age three, most children can run and climb steps. They can hold crayons and scribble with them. They can also feed themselves. Most children are toilet trained by age three, as well.
From age three until the teens, growth is slower. The body also changes shape. The arms and legs get longer relative to the trunk. Children continue to develop new motor skills. For example, many young children learn how to ride a tricycle and then a bicycle. Most also learn how to play games and sports (Figure below). By age six, children start losing their baby teeth. Their permanent teeth begin coming in to replace them. They also start school and learn how to read and write. They develop friendships and become less dependent on their parents.
Figure 22.9
Children develop better motor skills as they get older. What motor skills is this child demonstrating by playing soccer?
Puberty and Adolescence
The reproductive organs are present at birth. However, they grow very little during childhood. They do not mature and start functioning until puberty.
Puberty
Puberty is the stage of life when a child becomes sexually mature. Puberty lasts from about 12 to 18 years of age in boys and from about 10 to 16 years of age in girls.
The age when puberty begins varies from one child to another. Children that begin puberty much earlier or later than their peers may feel self-conscious. They may also worry that something is wrong with them. Usually, an early or late puberty is perfectly normal. If you have concerns about puberty, tell a parent. Your doctor can check to make sure you are developing normally.
In boys, puberty begins when LH from the pituitary gland triggers the testes to secrete testosterone. Testosterone causes the penis and testes to grow. Along with FSH from the pituitary gland, testosterone also causes the testes to start making sperm. Testosterone leads to the growth of pubic and facial hair, as well. In addition, it causes the shoulders to broaden and the voice to deepen.
In girls, puberty begins when LH from the pituitary gland triggers the ovaries to secrete estrogen. Estrogen causes the uterus and ovaries to grow. Along with FSH from the pituitary gland, estrogen also causes the ovaries to start releasing eggs. Estrogen causes the menstrual cycle to begin, as well. In addition, it leads to the growth of pubic hair. Estrogen also causes the hips to widen and the breasts to develop.
Teen girls that are athletic may develop a condition called the female athlete triad. It occurs when very active girls eat too few calories to provide all the energy they need. Lack of energy leads to low levels of estrogen. As a result, girls do not begin menstruating or their menstrual periods stop. They also develop osteoporosis. This is a serious disorder in which bones lose minerals and can break easily. The female athlete triad may have lifelong effects on health. It can even be fatal. It requires medical treatment and an increase in calories in the diet.
Boys and girls are close to the same height during childhood. In both boys and girls, growth in height and weight is very fast during puberty. However, boys grow faster than girls during puberty. Their period of rapid growth also lasts longer. As a result, by the end of puberty, boys are an average of 10 centimeters taller than girls.
Adolescence
Adolescence is the period of life between the start of puberty and the beginning of adulthood. Adolescence includes the physical changes of puberty. It also includes may other changes. During adolescence,
teens develop new thinking abilities. For example, they can think about abstract ideas such as freedom. They are also better at thinking logically. They are usually better at solving problems, as well.
teens try to establish a sense of who they are as individuals. They may try to become more independent from their parents. Most teens also have emotional ups and downs. This is partly due to changing hormone levels.
teens usually spend much more time with peers than family members (Figure below). The opinions of their peers are also very important to them. Most teens feel pressured to dress and act as their peers do in order to be accepted.
Figure 22.10
These teens are good friends. Like most teens, they spend more time with one another than they do with family members. These teens are volunteering at their local library. What do you enjoy doing with your friends?
Adulthood
When is a person considered an adult? That depends. Most teens become physically mature by the age of 16 or so. However, they are not adults in a legal sense until they are older. For example, in the U.S., you must be 18 to vote. You must be 21 to sign legal contracts. Once adulthood begins, it can be divided into three stages. The stages are early, middle, and late adulthood.
Early Adulthood
Early adulthood starts at age 18 or 21. It continues until the mid-30s. During early adulthood, people are at their physical peak. They are also usually in good health. The ability to reproduce is greatest during early adulthood, as well. This is the stage of life when most people complete their education. They are likely to begin a career or take a full time job. Many people also marry and start a family during early adulthood.
Middle Adulthood
Middle adulthood begins in the mid-30s. It continues until the mid-60s. During middle adulthood, people start to show physical signs of aging. Their hair gradually turns gray. Their skin develops wrinkles. The risk of health problems also increases during middle adulthood. For example, heart disease, cancer, and diabetes become more common during this time. This is the stage of life when people are most likely to achieve career goals. Their children also grow up and may leave home during this stage.
Late Adulthood
Late adulthood begins in the mid-60s. It continues until death. This is the stage of life when most people retire from work. They are also likely to reflect on their life. They may focus on their grandchildren. During late adulthood, people decline in physical abilities. For example, they usually have less muscle tone and slower reflexes. Their immune system also doesn’t work as well as it did. As a result, they have a harder time fighting diseases like flu. The risk of developing diseases such as heart disease and cancer continues to rise. Another health problem that is common in late adulthood is osteoporosis. Arthritis is also common. In arthritis, joints wear out and become stiff and painful. As many as one in four late adults may develop Alzheimer’s disease. In this disease, brain changes cause mental abilities to decline steadily. Exercising the body and brain, and maintaining social connections can alleviate some of these effects. The various stages of adulthood discussed are pictured in Figure below.
Figure 22.11
This family picture shows women in each of the three stages of adulthood. Which stage does each woman represent? What might you infer about each woman from her stage of adulthood?
Despite problems such as these, many people remain healthy and active into their 80s or even 90s. Do you want to be one of them? Then adopt a healthy lifestyle now and follow it for life. Doing so will increase your ch
ances of staying healthy and active to an old age.
Lesson Summary
Fertilization occurs when an egg unites with a sperm to form a zygote.
A zygote develops into an embryo and then a fetus. This occurs as cells divide, different types of cells develop, and organs form.
An individual grows rapidly and develops new abilities during infancy and childhood.
A child becomes sexually mature and changes in many other ways during adolescence.
Adulthood is divided into the stages of early, middle, and late adulthood. Each stage is associated with different traits and concerns.
Review Questions
What is fertilization?
Define embryo and fetus.
At about how many weeks after fertilization is a fetus usually ready to be born?
Describe an embryo at the end of the 8th week after fertilization.
How does a fetus change during the last two months before birth?
Explain the role of the amniotic sac and placenta during fetal development.
Why doesn’t a doctor wait for a newborn baby to breathe on its own before cutting the umbilical cord?
Explain how pituitary hormones control puberty in boys and girls.
Compare and contrast puberty and adolescence.
Why is it difficult to say when adulthood begins?
Further Reading / Supplemental Links
CK–12.org, High School Biology, Chapter 40, Lesson 3.
Anita Ganeri. Human Life Cycles. Heinemann Library, 2006.
DK Publishing. Human Body (DK/Google E. Guides). DK Children, 2005.
Jennifer Prior. The Human Life Cycle. Teacher Created Materials, 2008.
Karen Bledsoe. Human Reproduction, Growth, and Development. Perfection Learning, 2007.
Michaela Miller. Exploring the Human Body: Reproduction and Growth. KidHaven Press, 2005.
http://estrellamountain.edu/faculty/farabee/biobk/BioBookREPROD.html
http://runews.rockefeller.edu/index.php?page=engine&id=31
http://www.keepkidshealthy.com/growthcharts
http://www.merck.com/mmhe/sec22/ch260/ch260b.html
http://www.merck.com/mmhe/sec22/ch260/ch260c.html
http://www.pregnancy.org/pregnancy/fetaldevelopment1.php
http://www.pregnancy.org/pregnancy/fetaldevelopment2.php
http://en.wikipedia.org/wiki
Vocabulary
adolescence
Period of life between the start of puberty and the beginning of adulthood.
amniotic sac
Fluid-filled membrane that surrounds and protects a fetus within the uterus.
childbirth
Process through which a baby passes from the uterus, through the vagina, and out of the mother’s body.
childhood
Period between a baby’s first birthday and puberty.
embryo
Stage of a developing baby between implantation and the end of the 8th week after fertilization.
fertilization
Union of a sperm and egg; occurs in a fallopian tube.
fetus
Stage of a developing baby between the end of the 8th week after fertilization and birth.
implantation
Process in which the ball of cells that will become an embryo embeds in the lining of the uterus.
infancy
First year of life after birth.
placenta
Spongy mass of blood vessels from the mother and fetus that allows substances to pass back and forth between the mother’s blood and the fetus’s blood.
puberty
Stage of life when a child becomes sexually mature.
umbilical cord
Tube containing blood vessels that connects a fetus to the placenta.
zygote
Cell that forms when a sperm and egg unite; the first cell of a new organism.
Points to Consider
By early adulthood, most people have become sexually active. Sexual activity puts people at risk of certain diseases. Do you know what the diseases are? Do you know how they can be prevented? What are other ways of keeping the reproductive system healthy?
Lesson 22.4: Reproductive System Health
Lesson Objectives
Describe common sexually transmitted diseases.
Identify other reproductive system disorders.
List ways to keep the reproductive system healthy.
Check Your Understanding
What is a pathogen?
What types of organisms can cause disease?
What is cancer?
Introduction
A healthy reproductive system is important for two reasons. It is important for overall good health. It is also important for reproduction. If the reproductive system is not healthy, a person may be unable to have children. Many health problems can affect the reproductive system. They include sexually transmitted diseases and cancers. The good news is that many reproductive health problems can be prevented or cured.
Sexually Transmitted Diseases
A sexually transmitted disease (STD) is a disease that spreads through sexual contact. STDs are caused by pathogens. The pathogens enter the body through the reproductive organs. Many STDs also spread through body fluids such as blood. For example, a shared tattoo needle is one way an STD could spread. Some STDs can also spread from a mother to her baby during childbirth.
STDs are more common in teens and young adults than in older people. One reason is that young people are more likely to take risks. They often have the attitude, “It can’t happen to me.” They also may not know how STDs spread. They are likely to believe myths about STDs, like the myths in Table (below) .
Myths and Facts about STDs Myth Fact
If you are sexually active with just one person, you can’t get STDs. The only way to avoid the risk of STDs is to practice abstinence from sexual activity.
If you don’t have any symptoms, then you don’t have an STD. Many STDs do not cause symptoms, especially in females.
Getting STDs is no big deal, because STDs can be cured with medicine. Only some STDs can be cured with medicine; other STDs cannot be cured.
(Source: http://womenshealth.about.com/od/stds/a/stdmythsvsfacts.htm)
Most STDs are caused by bacteria or viruses. STDs caused by bacteria usually can be cured with drugs called antibiotics. However, antibiotics are not effective against viruses. Therefore, viral STDs are not treated with antibiotics. Other drugs may be used to help control the symptoms of viral STDs, but they cannot be cured. Once you are have a viral STD, you are usually infected for life.
Bacterial STDs
In the U.S., Chlamydia is the most common STD caused by bacteria. Females are more likely than males to develop the disease. Rates of Chlamydia among U.S. females in 2006 is given in Figure below. Rates were much higher in teens and young women than in other age groups. Chlamydia may cause a burning feeling during urination. It may also cause a discharge from the vagina or penis. However, in many cases, it causes no symptoms. As a result, people do not know they are infected. Therefore, they don’t go to the doctor for help. If Chlamydia goes untreated, it may cause more serious problems in females. It may cause infections of the uterus, Fallopian tubes, or ovaries. These infections may leave a woman unable to have children.
Figure 22.12
This graph shows data on the number of cases of Chlamydia in U.S. females in 2006. Which two age groups had the highest rates of Chlamydia? Why do you think rates were highest in these age groups?
Gonorrhea is another common STD. Gonorrhea may cause pain during urination. It may also cause a discharge from the vagina or penis. However, some people with gonorrhea have no symptoms. As a result, they don’t seek treatment. Without treatment, gonorrhea may lead to infection of other reproductive organs. This can happen in males as well as females.
Syphilis is a very serious STD. Luckily, it is less common than Chlamydia or gonorrhea. Syphilis usually begins with a s
mall sore on the genitals. This is followed a few months later by a rash and flu-like symptoms. If syphilis is not treated, it may damage the heart, brain, and other organs. It can even cause death.
Viral STDs
Genital warts are an STD caused by human papilloma virus, or HPV. They are one of the most common STDs in teens. HPV infections cannot be cured. However, a new vaccine called Gardasil® can prevent most HPV infections in females. Many doctors recommend that females between the ages of 9 and 26 years receive the vaccine. Preventing HPV infections in females is important, because HPV can also cause cancer of the cervix.
Genital herpes is an STD caused by a virus called herpes. It is another very common STD. You can see how genital herpes is spread at http://www.sexualhealthissues.com/ms/ animations/21/main.html. A related herpes virus causes cold sores on the lips (Figure below). Both viruses cause painful blisters. In the case of genital herpes, the blisters are on the penis or membranes around the vaginal opening. The blisters go away on their own. However, the virus remains in the body. It may cause repeated outbreaks of blisters. The outbreaks are more likely when a person is under stress. There is no cure for genital herpes. However, drugs can help prevent or shorten outbreaks. Researchers are trying to find a vaccine to prevent genital herpes.
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