HUMAN BIOLOGY

80 Chapter 4

homeostasis: the body in balance

homeostasis requires the interaction

of sensors, integrators, and effectors

Three “partners” must interact to maintain homeostasis.

They are sensory receptors, integrators, and effectors (Fig-

ure 4.13). Sensory receptors are cells or cell parts that can

detect a stimulus—a specific change in the environment.

For a simple example, if someone taps you on the shoul-

der, there is a change in pressure on your skin. Receptors

in the skin translate the stimulus into a signal, which can

be sent to the brain. Your brain is an integrator, a con-

trol center where different bits of information are pulled

together in the selection of a response. It can send signals

to muscles, glands, or both. Your muscles and glands are

effectors—they carry out the response, which in this

case might include turning your head to see if someone is

there. Of course, you cannot keep your head turned indefi-

nitely, because eventually you must eat, use the bathroom,

and perform other tasks that maintain body operating

conditions.

How does the brain deal with physiological change?

Receptors inform it about how things are operating, but the

brain also maintains information about how things should

be operating—that is, information from “set points.” When

some condition in the body shifts sharply from a set point,

the brain brings it back within proper range. It does this by

sending signals that cause specific muscles and glands to

step up or reduce their activity. Set points are important in

a great many physiological mechanisms, including those

that influence eating, breathing, thirst, and urination, to

name a few.

n    Cells and more complex body parts function properly only

when conditions inside the body are stable.

n    Links to Life’s characteristics 1.1, Acid–base balance 2.7

the internal environment is

a pool of extracellular fluid

The trillions of cells in your body all are bathed in fluid—

about 15 liters, or a little less than 4 gallons. This fluid,

called extracellular (“outside the cell”) fluid, is what we

mean by the “internal environment.” Much of the extracel-

lular fluid is interstitial, meaning that it fills spaces between

cells and tissues. The rest is blood plasma, the fluid portion

of blood. Substances constantly enter and leave interstitial

fluid as cells draw nutrients from it and expel metabolic

waste products into it. Those substances can include

ions, compounds such

as water, and other

materials.

All this chemical

traffic means that the

chemical makeup and

volume of extracellu-

lar fluid change from

moment to moment.

If the changes are

drastic, they can

have drastic effects

on cell activities. The

number and type of

ions in extracellu-

lar fluid (such as H^1 )

are especially crucial,

because they must be

kept at levels that allow metabolism to continue normally.

As you read in Chapter 1, homeostasis means “staying

the same.” We use this term because the mechanisms of

homeostasis maintain stability in the chemical makeup

and volume of extracellular fluid.

In maintaining homeostasis, all components of the body

work together in the following general way:

• Each cell engages in metabolic activities that ensure
  its own survival.


• Tissues, which consist of cells, perform one or more
  activities that contribute to the survival of the whole
  body.


• Together, the operations of individual cells, tissues,
  organs, and organ systems help keep the extracellular
  fluid in a stable state—a state of homeostasis that
  allows cells to survive.

Cell Interstitial fluid Plasma

Extracellular fluid

Blood

vessel

F i g u r e 4.13 Different body structures function as sensors,

integrators, and effectors. In humans the brain and spinal

cord are integrators. Organs of the endocrine system and

muscles are effectors. (© Cengage Learning)

Sensors: Cells in

the eyes, ears, skin,

and elsewhere

Integrator:

The brain

Effectors: Muscles

and glands

Jupiter Images

4.10

© Cengage Learning

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