Sunshine StateSTANDARDSSC.F.1.3.1: The studentunderstands that livingthings are composed of major systems that function in reproduction, growth,maintenance, and regulation.SC.F.1.3.7: The studentknows that behavior isa response to the envi-ronment and influencesgrowth, development,maintenance, andreproduction.

584 Unit 5: Human Biology

VOCABULARYmuscular system p. 585skeletal muscle p. 586voluntary muscle p. 586smooth muscle p. 586involuntary muscle p. 586cardiac muscle p. 586

BEFORE, you learned

• There are different types ofbone tissue

• The human skeleton has twoseparate divisions

• Joints function in several different ways

NOW, you will learn

• About the functions of muscles• About the different types of

muscles and how they work• How muscles grow and heal

KEY CONCEPT

The muscular system makes movement possible.

Muscles perform important functions.Every movement of your body—from the beating of your heart, to themovement of food down your throat, to the blinking of your eyes—occurs because of muscles. Some movements are under your control,and other movements seem to happen automatically. However, mus-cles do more than produce movement. They perform other functionsas well. Keeping body temperature stable and maintaining posture aretwo additional functions of muscles.

Check Your Reading What are three functions that muscles perform?

EXPLORE Muscles

How do muscles change as you move?

PROCEDURE

Sit on a chair with your feet on the floor.

Place your hand around your leg. Straighten one leg as shown in the photograph.

Repeat step 2 several times.

WHAT DO YOU THINK?• How did your muscles change during

the activity?• Record your observations.• What questions do you have about the

muscular system?

3

2

1

MAIN IDEA WEBMake a web for the mainidea: Muscles performimportant functions.

Chapter 16: Systems, Support, and Movement 585

MovementThe works with the skeletal system to allow move-ment. Like all muscles, the muscles that produce movement are made upof individual cells called muscle fibers. These fibers contract and relax.

Most of the muscles involved in moving the body work in pairs. Asthey contract, muscles shorten, pulling against bones. It may surpriseyou to know that muscles do not push. Rather, a muscle on one sideof a bone pulls in one direction, while another muscle relaxes. Musclesare attached to bones by stretchy connective tissue.

Maintaining Body TemperatureEarlier you read that processes within the body requirecertain conditions, such as temperature and the rightamount of water and other materials. The balance of con-ditions is called homeostasis. One of the functions of themuscular system is related to homeostasis. Muscles func-tion to maintain body temperature.

When muscles contract, they release heat. Without thisheat from muscle contraction, the body could not main-tain its normal temperature. You may have observed theway your muscles affect your body temperature when youshiver. The quick muscle contractions that occur when youshiver release heat and raise your body temperature.

Check Your Reading How do muscles help maintain homeostasis?

Maintaining PostureHave you ever noticed that you stand up straight withoutthinking about it, even though gravity is pulling your bodydown? Most muscles in your body are always a little bitcontracted. This tension, or muscle tone, is present evenwhen you are sleeping. The muscles that maintain posture relax completely only when you are unconscious.

Try standing on the balls of your feet for a few moments, or onone leg. When you are trying to balance or hold one position for anylength of time, you can feel different muscles contracting and relaxing.Your muscles make constant adjustments to keep you sitting or stand-ing upright. You don’t have to think about these tiny adjustments;they happen automatically.

muscular system

Muscles contract duringshivering, raising bodytemperature.

RESOURCE CENTERCLASSZONE.COM

Discover more aboutmuscles.

586 Unit 5: Human Biology

Your body has different types of muscle.Your body has three types of muscle. All three types of muscle tissueshare certain characteristics. For example, each type of muscle con-tracts and relaxes. Yet all three muscle types have different functions,and different types of muscle are found in different locations.

Skeletal MuscleThe muscles that are attached to your skeleton are called

Skeletal muscle performs voluntary movement—that is, movement that you choose to make. Because they are involvedin voluntary movement, skeletal muscles are also called

Skeletal muscle, like all muscle, is made of long fibers. The fibersare made up of many smaller bundles, as a piece of yarn is made up ofstrands of wool. One type of bundle allows your muscles to moveslowly. Those muscles are called slow-twitch muscles. Another type ofbundle allows your muscles to move quickly. These are called fast-twitch muscles. If you were a sprinter, you would want to developyour fast-twitch muscles. If you were a long distance runner, youwould develop your slow-twitch muscles.

Check Your Reading What does it mean that skeletal muscles are voluntary muscles?

Smooth Muscleis found inside some organs, such as the intestines

and the stomach. Smooth muscles perform automatic movement andare called In other words, smooth muscles workwithout your knowing it. You have no control over their movement.For example, smooth muscles line your stomach wall and push foodthrough your digestive system. Smooth muscle fibers are not as longas skeletal muscle fibers. Also, unlike skeletal muscles, smooth musclesare not fast-twitch. Smooth muscles contract slowly.

Cardiac MuscleYour heart is made of Like smooth muscle, cardiacmuscle moves without conscious control. Each cardiac muscle cell hasa branched shape. The cells of the heart connect in a chain. Thesechains form webs of layered tissue that allow cardiac cells to contracttogether and make the heart beat. Just like the smooth muscle cells,the cardiac muscle cells contract slowly, except in emergencies.

Check Your Reading Compare and contrast the three types of muscle described: skeletal, smooth, and cardiac.

cardiac muscle.

involuntary muscles.

Smooth muscle

voluntary muscles.

skeletal muscles.

VOCABULARYRemember to add foursquares for involuntarymuscles and voluntary mus-cles to your notebook. Note differences in the twodiagrams.

reading tip

The root of the word volun-tary comes from the Latinroot vol-, meaning “wish.”In the word involuntary theprefix in- suggests themeaning “unwished for.”Involuntary movementmeans movement you can’tcontrol.

Chapter 16: Systems, Support, and Movement 587

The marchers in this band are using all three different typesof muscle tissue.

Muscle Tissue

Skeletal muscle movesthe legs of these marchers.

Smooth muscle in theair passages of the lungsallows the band membersto breathe as they play their instruments.

Cardiac muscle allows thehearts of the band membersto pump blood as they marchto the beat of the music.

Which movements of these band members are voluntary, and which are involuntary?

250�

360�

150�

588 Unit 5: Human Biology

Skeletal muscles and tendons allow bonesto move.

Skeletal muscles are attached to your bones by strong tissues calledtendons. The tendons on the end of the muscle attach firmly to thebone. As the fibers in a muscle contract, they shorten and pull the ten-don. The tendon, in turn, pulls the bone and makes it move.

You can feel your muscles moving your bones. Place your left arm,stretched out flat, in front of you on a table. Place the fingers of yourright hand just above your left elbow. Bend your elbow and raise andlower your left arm. You are contracting your biceps. Can you feel themuscle pull on the tendon?

The dancers in the photograph are using many sets of muscles.The diagrams show how muscles and tendons work together to movebones. Muscles are shown in red. Notice how each muscle crosses ajoint. Most skeletal muscles do. One end of the muscle attaches to onebone, crosses a joint, then attaches to a second bone. As the musclecontracts, it pulls on both bones. This pulling produces movement—in the case of these dancers, very exciting movement.

musclecontracts

tendon

musclerelaxes

bones tendons

Skeletal muscles contract by shorten-ing and pulling the tendon.

musclerelaxes

tendon

musclecontracts

bones tendons

Muscle fibers can return to their origi-nal length after being shortened.

Muscles grow and heal.Developing Muscles An infant’s muscles cannot do very much. Ababy cannot lift its head, because the neck muscles are not strongenough to support it. For the first few months of life, a baby needsextra support, until the neck muscles grow strong and can hold up thebaby’s head.

The rest of the skeletal muscles also have to develop and strengthen.During infancy and childhood and into adolescence, humans developmuscular coordination and become more graceful in their movements.Coordination reaches its natural peak in adolescence but can be furtherimproved by additional training.

Exercise and Muscles When you exercise regularly, your muscles mayget bigger. Muscles increase in size with some types of exercise,because their cells reproduce more rapidly in response to theincreased activity. Exercise also stimulates growth of individual musclecells, making them larger.

You may have experienced soremuscles during or after exercising.During exercise, chemicals can build upin the muscles and make them crampor ache. The muscle soreness you feel aday or so after exercise is caused bydamage to the muscle fibers. The mus-cle fibers have been overstretched ortorn. Such injuries take time to heal,because the body must remove injuredcells, and new ones must form.

KEY CONCEPTS1. What are the three main func-

tions of the muscular system?

2. Make a rough outline of ahuman body and label placeswhere you could find each ofthe three types of muscles.

3. Explain why you may be soreafter exercise.

CRITICAL THINKING4. Apply You are exercising and

you begin to feel hot. Explainwhat is happening in yourmuscles.

5. Analyze Describe what hap-pens in your neck muscleswhen you nod your head.

CHALLENGE6. Infer The digestive system

breaks down food and trans-ports materials. How are theshort length and slow move-ment of smooth muscle tissuesin the stomach and intestinesrelated to the functions ofthese organs?

Chapter 16: Systems, Support, and Movement 589

Stretching your musclesbefore exercise helps prevent injury.