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# 3.1: Growth of the Body

Difficulty Level: At Grade Created by: CK-12

What changes of puberty do both boys and girls experience?

Puberty involves important changes in all parts of the body. In this section you will learn about growth in height, weight, muscles, and a few other related changes.

Height

Height refers to how tall you are. Several factors influence how tall you will be as an adult. Adult males are generally somewhat taller than adult females. Nutrition and general health will also make some difference. Most importantly, you inherit your height from your biological parents. Genes determine heredity, which means that how tall you will be is already more or less decided by the time you are born. You can't do much about your height, except to realize that it is normal to be shorter or taller than others are.

Did You Know?

At age 10, boys are 78% of their adult height; girls are 84% of their adult height.

Figure 2.1 During puberty bodies of the same height can have very different proportions, as shown in the charts.

Figure 2.2

## Activity 2-1: How Tall?

Introduction

In this activity you learn how tall and how fast boys and girls grow until they reach their adult height. You look at average heights and average rates of growth, and it is important to know that not everyone fits this pattern. For example, if you are 13, you might be 4-feet tall, or 5-feet tall, or 6-feet tall. The average height for your age might be five feet, but the other heights are just as “normal.” Some people follow a slower, and others a faster, rate of growth. There is some chance that you will continue growing taller until you are 20 years old.

Materials

• One metric ruler per student or group
• Activity Report

Procedure

Step 1 Examine the graph in Figure 2.3. The vertical axis shows the average heights for males and females at 10-centimeter intervals. The horizontal line shows the ages in 1-year intervals.

Step 2 Answer the questions on your Activity Report that relate to this chart.

Step 3 Figure 2.3 has shown you how tall average boys and girls are at each age, but growth does not always occur at the same rate over time. There are times in your life when you will grow very quickly for a while, and other times when you will grow more slowly. The next graph shows you how the average rate of growth changes at different ages.

Figure 2.3 Average heights for boys and girls.

Figure 2.4 Growth spurts: rate of growth for boys and girls.

Step 4 Look closely at the graph in Figure 2.4. The vertical axis shows the height gained (growth) during each year. It does not show the height itself.

Step 5 Answer the questions on your Activity Report that relate to this graph.

Step 6 Note that the growth rate for both boys and girls is very high during the first two years of life, but then it slows down. At age 11 for girls and age 13 for boys it shoots up again, only to go down again after a few years. This burst of growth during puberty is called a growth spurt. By the end of this growth spurt, females, at about age 15, have reached 99% of their adult height. Boys do the same by the age of 17.

Did You Know?

How did we learn about growth and development?

• Around 200 B.C., people thought children had higher concentrations of blood and that growth resulted from thinning out the blood; so once blood reached an equilibrium, growth stopped.
• In the 18th century, people thought blood vessels grew smaller and smaller the farther away from the heart. Growth happened until blood vessels got too small to sustain tissue around them.
• Now we think growth stops when cells reach capacity.

Weight

Both boys and girls also show a spurt in weight gain during adolescence. In fact, more non-skeletal growth occurs in puberty than skeletal growth. This growth comes primarily from an increase in muscle tissue and fat. Boys generally develop broader shoulders than girls do, and girls develop generally wider hips (skeletal growth) than boys do. Both boys and girls develop stronger muscles, but on average, boys' muscles develop stronger than girls' muscles do. Both boys and girls put on fat, but on average, girls put on somewhat more than boys do. This fat is largely deposited under the skin, especially in the buttocks, hips, breast area, and back of the arms (see Figure 2.5). This distribution gives girls past puberty more rounded features and boys more angular features. It also allows the muscles of boys to show more clearly.

Figure 2.5 Girls store energy as fat in the hips, breasts, and arms.

On average, adult females have a higher percentage of body fat than men have because of their reproductive functions. To be pregnant for nine months and breast feed her baby, a woman must have extra sources of nutrition if food is scarce. Her body stores this extra supply as fat.

Weight generally varies more than height. Weight depends in part on heredity, but weight is mainly a function of what you eat and how much you exercise.

As you grow older, your body needs more calories (units used to measure energy in food). But the amount of calories needed per pound of body weight actually decreases. For example, babies need about 50 calories of food per pound of body weight. Adolescents aged 13-15 need about 27 calories per pound of body weight. Adults need only 18 calories per pound of body weight.

Muscles

Both boys and girls greatly increase their muscle size and strength during puberty. Before puberty, there is little difference in strength and physical capacity between boys and girls. During puberty, the number of muscle cells in boys increases 14 times, in girls only 10 times. On average, boys also develop larger hearts, lungs, and a greater number of red blood cells. So, although both girls and boys get stronger as they go through puberty, these real and universal differences give males a physiological advantage in terms of the potential for overall strength and capacity for physical exertion.

Did You Know?

Olympic records are only one measure of physical fitness and physical capacity. Longevity, or how long one lives, is also a measure of fitness. Women live, on average, six years longer than men live.

One muscle that especially grows larger at puberty is the heart. A large, strong heart can pump blood throughout the body faster than a weak heart. Because blood carries the oxygen needed by all parts of the body, including muscles, you can exercise more efficiently as your heart grows larger. In addition, the lungs (an organ, not a muscle) become larger and more efficient at puberty, making it possible for you to breathe in larger amounts of air. Working together, the heart and lungs take in oxygen and distribute it to every cell in the body. This makes it possible for those past puberty to exercise harder and for a longer time and to recover from the effects of exercise quickly. Sports requiring endurance, such as cross-country running, are generally possible only after puberty.

Year Men Women
1948 \begin{align*}25'8''\end{align*} \begin{align*}18'8.25''\end{align*}
1952 \begin{align*}24'10''\end{align*} \begin{align*}20'5.75''\end{align*}
1956 \begin{align*}25'8.25''\end{align*} \begin{align*}20'9.75''\end{align*}
1960 \begin{align*}26'7.75''\end{align*} \begin{align*}20'10.75''\end{align*}
1964 \begin{align*}26'5.75''\end{align*} \begin{align*}22'4.5''\end{align*}
1968 \begin{align*}29'2.5''\end{align*} \begin{align*}22'4.5''\end{align*}
1972 \begin{align*}27'0.5''\end{align*} \begin{align*}22'3''\end{align*}
1976 \begin{align*}27'4.5''\end{align*} \begin{align*}22'0.75''\end{align*}
1980 \begin{align*}28'0.25''\end{align*} \begin{align*}23'2''\end{align*}
1984 \begin{align*}28'0.25''\end{align*} \begin{align*}22'10''\end{align*}
1988 \begin{align*}28'7.25''\end{align*} \begin{align*}24''3.5''\end{align*}
1992 \begin{align*}28'5.5''\end{align*} \begin{align*}23'5''\end{align*}
1996 \begin{align*}27'10.75''\end{align*} \begin{align*}23'4''\end{align*}

Figure 2.6 Olympic long-jump winning distances.

Although boys, on average, have a physiological advantage, exercise and fitness greatly affect strength and endurance. Thus, a girl who exercises regularly is likely to outperform a boy who is not physically fit. The differences we see between males and females in physical activity and ability are therefore partly due to the biological changes of puberty and partly due to lifestyle choices.

If women and men continue to improve at the rates shown in Figure 2.6, will women ever jump farther than men? How likely is this to occur? What will it depend on?

Look at the table above showing Olympic performances in the long-jump event. At what rate are the men's and women's records changing?

Figure 2.7 Scoliosis-curvature of the spine.

Health Concerns in Puberty

The most common adolescent health problems are not related to puberty but are due to accidents, violence, alcohol or drug use, excessive dieting, pregnancy, and sexually transmitted diseases. Most teenagers experience no serious health problems due to puberty itself. Many adolescents experience some degree of acne. Although not typically serious, it can be an annoying condition. Acne is usually most severe between the ages of 14 and 17 for girls and between 16 and 19 for boys. Since male sex hormones (a product of various glands, also present in girls but in lesser amounts) bring on the changes in skin glands, boys are more likely to get pimples.

About 6% of adolescents suffer from more chronic, or long lasting, illnesses. Most often these are psychological problems, diseases of the respiratory system (such as asthma), and conditions affecting muscles and bones. One such condition is scoliosis, or abnormal curving of the spine. It is usually first noticed after age 10, when the adolescent has started growing very rapidly. Scoliosis more commonly affects girls than boys. The spine bends sideways in a curve and makes one part of the back higher and fuller than the other side. An early diagnosis can improve chances for successful treatment by using braces or surgery.

Figure 2.8 Acne comes from plugged ducts that are infected with bacteria.

Acne and “Growing Pains”

Decide if these statements are true or false.

• Soap can clean out blockage causing acne.
• Certain foods cause acne.

Acne is one of the most common health concerns among adolescents. What causes acne? Glands in the skin next to hair follicles grow and secrete oil faster than the oil can come out (see Figure 2.8). When the duct of the gland becomes plugged, its secretions accumulate. Sometimes bacteria infect these glands, pus forms, and you get a pimple. If a blockage widens a pore enough to let air in, the waxy plug darkens and you have a “blackhead.”

How do you get rid of acne? Popping a pimple spreads bacteria around and causes more pimples. Continual scrubbing causes the skin to produce even more oil. Because acne in adolescence is typically caused by hormones, the best you can do is clean your skin properly two times a day and be patient! A doctor can prescribe medicine to help clear up your skin if needed.

“Fatty chocolate, greasy pizza, and oily potato chips don't affect the amount of oil your skin makes or the number of bacteria enjoying it.”

-The Body Book

Sara Stein

Another ailment seen in adolescents affects their knees. As young teenagers become more active with sports, their upper thigh muscles may become tender, and their knees may also become sore and painful. Although not usually serious, this condition can come and go for a few years. Rest and reduced physical activity can alleviate these growth pains.

What Do You Think?

On average, since adult males have the potential to develop stronger muscles than adult females, should adult females be excluded from jobs that involve heavy physical labor (for example, firefighting)?

These conditions are just two examples of health concerns commonly seen among adolescents. They occur because of the rapid growth of bones and muscles during puberty. The earlier health concerns are identified, the better chance a physician has of treating them successfully. Because your body is changing so quickly, yearly physical exams by a physician or nurse are important.

What are some ways you mark your growth and development? Do your friends or family celebrate or recognize milestones (accomplishments, growth) in any way? How? How might you recognize those milestones important to you, but not currently celebrated, and share them with someone?

## Review Questions

1. What factors influence how tall you will be?
2. What factors affect your weight?
3. How does puberty affect exercise and endurance?
4. What causes acne, and what can you do about it?
5. Name three other health concerns common to adolescents and describe them.

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