Objectives• Relate homeostasis to the internal

environment of the body.• Explain how negative and

positive feedback loops maintain homeostasis.

Section ResourcesUnit Resource Book

Study Guide pp. 5–6Power Notes p. 7Reinforcement p. 8Pre-AP Activity pp. 17–18

Interactive Reader Chapter 28Spanish Study Guide pp. 285–286

Biology Toolkit pp. C18, C39, D8

TechnologyPower Presentation 28.2Media Gallery DVDOnline Quiz 28.2

Activate Prior Knowledge Have students think about what it would be like to live in an environment that constantly hovers at 38°C (100°F). Ask, If you had to live your life constantly at one temperature setting, what would it be? Answers will vary, but most students will probably choose moderate temperatures. Discuss that the normal core body temperature is 37°C (98.6°F).

ONLINE BIOLOGY Have students look at how a runner maintains homeo-stasis. See the simulation in Options for Inquiry on page 867.

VocabularyAcademic Vocabulary Students may wonder if the word homeopathy relates to homeostasis. Homeopathy is a nonmedical approach to treating illness by using minute amounts of the samesubstance believed to be the cause. The principle is to “cure like with like” by stimulating the body’s defenses. There are no conclusive scientific studies as to its effectiveness.

Plan and PreparePlan and Prepare

TeachTeach

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VOCABULARYThe word homeostasis is formed from two Greek words: homos, meaning “similar,” and stasis, meaning “standing” or “stopping.”

28.2 Mechanisms of HomeostasisKEY CONCEPT Homeostasis is the regulation and maintenance of the internal environment.

MAIN IDEAS• Conditions within the body must

remain within a narrow range.

• Negative feedback loops are necessary for homeostasis.

VOCABULARYhomeostasis,homeostasis, p. 858

feedback,feedback, p. 859

negative feedback,negative feedback, p. 860

positive feedback,positive feedback, p. 861

Reviewtissue, organ, organ system

Connect The complex tissues, organs, and organ systems in your body must respond to a wide variety of conditions. For instance, during the summer, you might walk out of a cold, air-conditioned store into a stifling hot summer day. Your body temperature has to remain the same under both conditions in order for you to survive. In fact, your life depends on your body’s ability to maintain the delicate balance of your internal chemistry.

MAIN IDEA

Conditions within the body must remain within a narrow range.

During every moment of your life, trillions of chemical reactions are taking place in your body. The enzymes that control these reactions work best within a narrow range of conditions. One of these conditions is your internal body temperature, which should remain between 36.7°C and 37.1°C (98.2°F and 98.8°F). If it rises only a few degrees, you could easily die from overheating. At temperatures over 41°C (106°F), many enzymes stop functioning. If your internal temperature falls below 27°C (80°F), your heart may fail.

Likewise, the levels of trace minerals in your body must stay within strict limits. For instance, if calcium levels are too high, you can slip into a coma. If they are too low, your heartbeat becomes irregular.

You live in a constantly changing environment. Your body must cope not only with temperature changes but also with pollution, infection, stress, and many other conditions. Every change is a challenge to your body. What keeps the human body from breaking down every time the internal or external environment changes?

Homeostasis and the Internal EnvironmentFortunately, the body has many control systems that keep its internal environ-ment stable. Together, these control systems are responsible for maintaining homeostasis. Homeostasis Homeostasis (HO-mee-oh-STAY-sihs) is the regulation and maintenance of the internal environment—temperature, fluids, salts, pH, nutrients, and gases—within the narrow ranges that support human life. Your internal control systems respond quickly to change, whether from outside conditions or internal ones, as shown in FIGURE 28.5.

858 Unit 9: Human Biology

4.c Describe and differentiate among the organizational levels of organisms (e.g., cells, tissues, organs, systems, types of tissue).

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Differentiated Instruction

SECTION 28.2

ENGLISH LEARNERSCheck on students’ comprehension as you go through the material describing various control systems in the body. You can use simple signals, such as thumbs up or thumbs down for understanding. Or ask questions in which students write responses on a sheet of paper or an index card.

Biology Toolkit, Signals, p. C18; Card Responses, p. C18

858 Unit 9: Human Biology

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FIGURE 28.5 Explain the figure in terms of the four body control systems. Tell students that the control center for regulating body temperature is located in the hypothalamus region of the brain. Ask

• What sensors are involved? tempera-ture sensors in the skin

• What are the targets? sweat glands, blood vessels, and muscles in the skin

• What kinds of messages are involved in this communication system? nerve impulses and hormones

AnswersA Apply Her skin would respond the

way it does in hot temperatures: pores would open, and sweat glands would release water to cool the body.

B Draw Conclusions The processes necessary for life work best within a narrow range of conditions. To maintain these ranges, the internal environment of the body must remain stable.

VocabularyAcademic Vocabulary Point out to students how the words associated with a mechanical system are applied to a living system.mechanism a system of parts that operate or interact like those of a machinecontrol an instrument used to operate, regulate, or guide a machinesensor a device that receives and responds to a signalset point the point at which a circuit is either activated or deactivatedtarget something aimed or fired atfeedback return of a portion of the output of a system to the input, used to maintain performance or controlloop a closed circuit

TEACH FROM VISUALSTEACH FROM VISUALS

homeostasis,feedback,negative feedback,positive feedback,

Homeostasis

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Apply If the girl in cold temperature starts jogging, how would the control mechanisms in her skin respond as she runs?

FIGURE 28.5 Homeostasis and Change Control systems in the skin help reduce or conserve body heat.

Control Systems in the BodyInternal control systems requiresensors, a control center, communica-tion systems, and targets.

Sensors Sensors, also called receptors,gather information about conditionsinside and outside of the body. In coldor hot weather, for instance, sensors inyour skin and nasal passages gatherdata about air temperatures. The bodyhas thousands of internal sensors andother specialized sensors that detectchanges in the outside world.

Control center A control center, oftenthe brain, receives information fromthe sensors. It then compares thisinformation to the set points, or idealvalues, at which the body functionsbest. When conditions move above orbelow a set point, the control centerresponds by sending messagesthrough a communication system.

Communication systems Communi-cation is controlled by the nervoussystem and the endocrine system,which carry messages to all parts ofthe body. These messages, in the formof nerve impulses or hormones, telltargets in the body how to respond tointernal or external changes.

Targets A target is any organ, tissue, or cell that changes its level of activity inresponse to a message. For instance, in a cold environment, a message mightcause the muscles to start shivering to generate more body heat.

Draw Conclusions Why is it so important to maintain homeostasis within the body?

MAIN IDEA

Negative feedback loops are necessary for homeostasis.

Sensors, control centers, communication systems, and targets work together inwhat is known as a feedback loop. Feedback Feedback is information from sensors thatallows a control center to compare current conditions to a set of ideal values.In a feedback loop, information moves continuously among sensors, a controlcenter, and a target. Most functions in the body are regulated by negativefeedback loops.

cold temperature

hot temperature

normal tem

perature

sweat gland

pore

hair follicle muscle

goosebump

Blood flow to the skin increases. Tiny muscles expand the pores. Sweat glands release water to cool the body.

Blood flow to the skin decreases.Tiny muscles contract the pores and the skin around body hairs to conserve heat.

Pores and muscles are relaxed. Blood flow to the skin is normal. Sweat glands are not active.

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BELOW LEVELHave students relate homeostasis to feedback loops by using a graphic organizer, such as a concept definition map. A concept definition map starts with a general category, in this case, how a body regulates its internal environment. A particular concept is identified, either negative or positive feedback. Then students include separate boxes to list properties and examples of each concept. Students should also compare one to the other.

Biology Toolkit, Concept Definition Map, p. D8 Chapter 28: Human Systems and Homeostasis 859

B

A

B

Biochemistry Oxygen is used by the cell in the breakdown of glucose and the accompanying release of energy in the form of ATP. A lack of oxygen can lead to cell death because ATP production stops, and as a result, there is no energy for cell activities.

History of ScienceThe physiologist Walter Bradford Cannon coined the term homeostasis, which he described for the general public in 1932. In his book The Wisdom of the Body, he proposed these four general features of homeostasis:

1. The body, being an open system and subject to change, requires mecha-nisms to maintain constancy and a steady state.

2. Steady-state conditions require that any tendency toward change be met with resistance.

3. Homeostasis must be maintained by a regulating system that consists of a number of cooperating mechanisms acting simultaneously or in sequence.

4. Homeostasis does not occur by chance but is the result of organized self-government.

AnswersA Infer Sensors would detect too

much oxygen and too little carbon dioxide in the blood. Your breathing would stop or slow down for a short time until the gases returned to their set points, then normal breathing would resume.

Teach continuedTeach continued

Connecting CONCEPTSConnecting CONCEPTS

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FIGURE 28.6 Negative Feedback Loop

Infer If you continued to breathe rapidly and deeply for too long in step 4, how would this affect the negative feedback loop?

Negative feedback counteracts any change in the body that moves conditions away from a set point.

1

4

3

2

5

Connecting CONCEPTS

Biochemistry As you read in Chapter 4, cells require a con-stant supply of oxygen to main-tain cell metabolism. Oxygen is not stored in the human body in any great amounts. Once oxygen reserves have been used up, the body must have a fresh supply of oxygen to prevent cell death.

Negative FeedbackIn negative feedback,negative feedback, a control system counteracts any change in the bodythat moves conditions above or below a set point. Negative feedback loopshelp keep the internal environment stable. A thermostat is a good example ofhow a negative feedback loop works. A sensor in the thermostat continuouslymeasures air temperature in a room. A control mechanism then compares thecurrent room temperature to a set point, say 21°C. When the temperature fallsbelow 21°C, the thermostat sends an electronic message that turns on thefurnace. When the sensor indicates the air temperature is at or just above21°C, the thermostat sends another message that turns off the furnace. As aresult, the room always stays within a few degrees of the desired temperature.

Negative feedback loops in the body work in a similar way. They are thereason why you cannot hold your breath for a long time. The control systemsinvolved in this feedback loop are shown in FIGURE 28.6. As you hold yourbreath, sensors in the circulatory and respiratory systems send information tothe brain stem, the body’s respiratory control center. Sensors signal a gradualincrease in carbon dioxide (CO2) and a decrease in oxygen (O2). The controlcenter compares this information with the set points for these gases. When thechange becomes too great, the control center takes steps to counteract it.Messages are sent to the muscles of the diaphragm and the rib cage to relaxand then contract, forcing you to exhale and then inhale deeply. At this point,you cannot stop these muscles from moving. You will continue to breatherapidly and deeply until the gas levels return to their set points.

You inhale and hold your breath. The O2 levels in the blood begin to decline and CO2 levels begin to rise.

You continue to inhale and exhale more deeply and rapidly than nor-mal until O2 /CO2

levels return to their set points.

The muscles of the diaphragm and the rib cage relax, forcing you to exhale. As the muscles contract, you inhale deeply.

Sensors alert the brain stem as O2 /CO2 levels move too far from the set points. Mes-sages are sent through the nervous and endocrine sys-tems to the muscles of the diaphragm and the rib cage.

When O2 /CO2 levels are restored, normal

breathing resumes.

860 Unit 9: Human Biology

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positive feedback,

homeostasis

negative feedbackpositive feedback

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Differentiated Instruction

ENGLISH LEARNERSHave students work with the analogy used in the text, comparing a negative feedback system in the body to a thermostat. Have students read the text and create two cycle diagrams. The first should show how a thermostat regulates temperature. The second can detail the negative feedback loop shown in FIGURE 28.6. Tell students to incorporate the terms control system, sensor, set point, target, communicate, and feedback.

Biology Toolkit, Cycle Diagram, p. C39

PRE-APHave students set up a cycle diagram that shows a fever as a combination of positive and negative feedback mechanisms. Tell students that the body raises its temperature in response to pathogens invading the body. Once the patho-gens have been destroyed by heat, negative feedback brings the body temperature back down. Have students think about the experience of having a fever and relate physiological responses, such as shivering and sweating, to what is happening in the bloodstream.

Biology Toolkit, Cycle Diagram, p. C39860 Unit 9: Human Biology

00.2 ASSESSMENT

B

A

B

Chapter 00: Chapter Name 861

Time 5 minutes

Lab Binder Human Bio, p. 8

Purpose Model a negative feedback loop.

LAB MANAGEMENTUse books that are not too heavy.

AnswersAnalyze and Conclude 1. As sensors in the scalp detect changes

in the book’s position, they send messages to control sensors in the brain, which then send messages to the muscles. Students may have adjusted their speed, shifted their head or body, or grabbed the book with their hands. When students closed their eyes, they lost sensory information about body position relative to their surroundings, which may have made the balancing task more difficult.

2. Other feedback systems include cooling systems in a car engine; thermostats that control air condition-ers or furnaces; and the body’s signals for hunger and thirst. All these feedback loops involve preset values and information about changing conditions that are relayed to control centers, which then send messages to targets to counteract changes and restore conditions to the preset values.

AnswersA Infer Negative feedback is the

means by which the body maintains homeostasis. Positive feedback occurs only in situations in which a change away from set values is needed.

Assess Use the Online Quiz or Section Quiz (Assessment Book, p. 554).Reteach Write the following headings on the board: Sensors, Control Center, Communication Systems, Targets. Have students tell you what to list beneath the headings for the respiratory control system shown in FIGURE 28.6.

QUICK LAB QUICK LAB

Assess and ReteachAssess and Reteach

28.2 ASSESSMENT

Connecting CONCEPTS

ONLINE QUIZClassZone.com

Q U I C K L A B MO D E LI N G

Negative Feedback LoopYou can experience a negative feedback loop by doing a simple demonstration.

PROBLEM How does a negative feedback loop work?

PROCEDURE 1. Balance the hardcover book on your head.

2. Walk 3 meters forward and backward—once with eyes open, then with eyes closed.

ANALYZE AND CONCLUDE 1. Analyze Describe the negative feedback loop that helped keep the book balanced

on your head. How did closing your eyes affect your ability to balance the book?

2. Connect Think of another example of a negative feedback loop that you might observe in your everyday life. Explain how you think this loop works.

MATERIALShardcover book at least 6" � 9"

To find out more about homeostasis, go to scilinks.org.Keycode: MLB028

Positive FeedbackNegative feedback loops maintain homeostasis by counteracting, or reversing, change to return conditions to their set points. In some cases, however, the body actually needs change to accomplish a specific task. In positive feedback, positive feedback, a control center uses information from sensors to increase the rate of change away from the set points. Though not as common in the body, this type of feedback is important whenever rapid change is needed.

For example, if you cut your finger, positive feedback mechanisms increase the rate of change in clotting factors in the blood until the wound is sealed. Once the injury heals, another positive feedback loop occurs as chemicals are released to dissolve the clot. Positive feedback also occurs in the release of certain growth hormones during puberty. Your body needs higher levels of these hormones to accomplish all of the changes that take place at this time.

Infer Why are most of the functions of the body regulated by negative, rather than by positive, feedback mechanisms?

REVIEWING MAIN IDEAS

1. A system to maintain homeostasishomeostasis must have at least four parts that function together. Name these parts and briefly explain what each one does.

2. What is the main difference between the way negative feedbacknegative feedback and positive feedbackpositive feedback mechanisms regulate change in the body?

CRITICAL THINKING

3. Predict When a newborn baby nurses, the mother’s body is stimu-lated to produce milk. What would happen to the milk supply if the mother chose to bottle feed rather than breast feed? Why?

4. Sequence Suppose you go on a long hike in hot weather. Describe a possible negative feedback loop that would keep your body from overheating.

5. Zoology Reptiles regulate their body temperature by changing their environment. A snake, for instance, must lie in sunlight to warm its body. Mammals, on the other hand, can regulate their internal environment to gain or lose heat. How might this ability give mammals an advantage over reptiles?

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28.2 ASSESSMENT

A

1. Sensors gather information. A control center analyzes and compares the infor-mation to the desired values. Communica-tion systems send messages from the control center to regulate the change. Targets receive and respond to the messages.

2. Negative feedback loops counteract change to return to a set point, while positive feedback loops accelerate change away from a set point.

3. A baby nursing creates a positive feedback loop that causes the mother’s body to lactate. Bottle feeding eliminates the stimulus.

4. Sensors would detect a rise in body temperature, increasing blood flow to the skin, activating sweat glands, and increasing heart and breathing rates.

5. Mammals can live in a wider range of habitats and tolerate rapid changes in external conditions.

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