Biology 12

CHAPTER

4

Reflecting

Questions

Homeostatic Mechanisms

106

As shown in the photograph on the

opposite page, some people live in

extreme conditions: the dry heat of

the Sahara desert, the humid heat of

the Amazon jungle, or the freezing

cold of the Canadian Arctic in winter.

Humans manage to survive freezing

cold and blistering hot temperatures

— not a bad claim for a thin-skinned,

warm-blooded, relatively hairless

species that is composed of about

70 percent water!

The ability of humans to survive

almost anywhere on the planet is one

of the things that make us successful

as a species. In spite of the external

environment, the human body strives

to keep its internal environment

relatively constant — a tendency

known as homeostasis. This

tendency, coupled with the aid of

appropriate food, clothing, and

shelter, helps us to survive even

under extreme conditions.

Healthy people around the world

share certain bodily constants:

a blood glucose concentration that

remains at about 100 mg/mL,

a blood pH near 7.4,

a blood pressure of about

160/106 KPa (120/80 mm Hg), and

a body temperature of

approximately 37°C.

Of course, there are slight

differences. A normal body

temperature for one person in one

situation is not necessarily normal

for all people in all situations.

Metabolism varies from person to

person, and therefore so does

temperature. Body temperature can

also vary with time, place, and

activity level, as shown below, and

still be considered normal.

“Normal” blood pressure also

differs. Children tend to have lower

blood pressure than adults, due to the

flexibility of their arteries. As people

age, their arteries lose some of this

flexibility, and their blood pressure

increases.

In this chapter, you will see how

the human body adapts to changing

internal and external environments,

and examine some of the organs and

systems that help us maintain

homeostasis. In Chapters 5 and 6,

you will explore the role played by

the nervous and endocrine systems.

Normal body temperature fluctuates

slightly according to a variety of

factors.

How do our bodies

maintain a stable internal

physiological state despite

constant changes in our

external environment?

How do kidneys help keep

our blood in a healthy

state?

How does the body

recover from illnesses?

Why do some people

develop serious allergic

reactions to environmental

substances?

Why do people with

diabetes have to control

the amount of sugar they

consume each day?

Prerequisite

Concepts

and Skills

Before you begin this chapter,
review the following concepts
and skills:

describing the unique

composition and properties

of macromolecules found

in cells and tissue fluids

(Chapter 1, section 1.2),

identifying and explaining

how some substances move

across cell membranes by

passive or active transport

(Chapter 1, section 1.5;

Chapter 2, section 2.3),

describing how buffers

maintain stable pH in

solutions (Chapter 1,

section 1.3).

35 °C

36 °C

37 °C

38 °C

39 °C

40 °C

during hard exercise

normal body temperature

during moderate exercise;

a few adults; many active children

usual range for normal

early morning; cold weather