HUMAN BIOLOGY

10 Chapter 1

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each chapter of this book includes information about

diseases and disorders. regardless of whether it’s a case of

the sniffles, an injury, or a life-threatening cancer, illness is a

sign of disturbed homeostasis. it means that affected cells

aren’t able to perform their normal functions.

From the beginning of human history, people have always

lived with countless health threats. today, however, some of

the most pressing health problems, such as certain cancers,

obesity, and type 2 diabetes, are related to lifestyle factors

such as smoking, poor diet, and a sedentary lifestyle. We

are also locked in an escalating global battle with infectious

diseases caused by harmful bacteria, viruses, and parasites.

most of these foes are invisible to the naked eye. Figure 1.11

gives you an idea of what bacteria and viruses may look like

under the microscope.

today health officials worry especially about emerging

diseases. these diseases are caused by pathogens that

until recently did not infect humans or did so only in limited

areas. many are caused by viruses. this group includes

several diseases you’ve probably heard about, such as

the encephalitis caused by West nile virus and the severe

respiratory disease caused by the SarS virus (Figure 1.11b).

you have probably also heard of—or even come down

Living in a World

of Disease threats

homeostasis 1.8

Section 1.1 introduced the concept of homeostasis—the state of

chemical and physical stability inside the body that must exist if

cells, and the whole body, are to stay alive. Homeostasis is one

of the most important concepts in this textbook. Figure 1.10 is a

visual summary of the main ideas, using the muscular system as

an example. Each body system you will study during your human

biology course performs functions that contribute to homeostasis in

other systems. Those chapters conclude with a Connections section

that summarizes each system’s key contributions to homeostasis.

Figure 1.10 The body’s survival depends on mechanisms

that maintain internal homeostasis.

muscle cells

muscular system

Stable conditions allow

cells to survive

Body system

functions to help

maintain homeostasis

Cells make up

body systems

Ed Reschke/Peter Arnold

ConnECtions

© Cengage Learning

1.7

FOCUS ON HealtH

Figure 1.11 A wide variety of pathogens may live on or in the

human body. A Bacteria on the tip of a pin. B The SARS virus,

which causes an emerging respiratory disease. C Ad from a 1944

issue of Life magazine. Penicillin helped many soldiers survive

what might otherwise have been deadly battlefield infections.

A B

(A: Dr. Tony Brain & David Parker/Science Source; B: Sercomi/Science Source)

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