HUMAN BIOLOGY

tIssUes, orGans, and orGan systeMs 81

What are the controls of homeostasis?

• Homeostatic controls are mechanisms that maintain the
  characteristics of the internal environment within ranges
  that allow cells to function properly.


• Negative feedback is the main control operating to maintain
  homeostasis.


• Body parts that serve as sensory receptors, integrators, and
  effectors monitor conditions in the body and carry out negative
  feedback.

taKe-hoMe Message

effector Tissue, gland, or

other body part that carries

out a response ordered by

an integrator.

extracellular fluid Blood

plasma and tissue fluid.

integrator Control center,

such as the brain, that

compares a detected envi-

ronmental change with a

set point and activates a

response.

negative feedback

Control mechanism of

homeostasis that reverses

change to body system if it

exceeds a set point.

sensory receptor Cell or

cell part that can detect

some type of stimulus—a

change in the environment.

the fetus, which exerts more pres-

sure on the uterine wall, and so on

until the fetus is expelled.

As the body monitors and

responds to information about the

external world and the internal envi-

ronment, its organ systems must

operate in a coordinated fashion. In

upcoming chapters we will be ask-

ing four important questions about

how organ systems function:

1. What physical or chemical aspect
   of the internal environment is
   each organ system working to
   maintain as conditions change?


2. How is each organ system kept
   informed of changes?


3. How does each system process
   incoming information?


4. What are the responses?
   As you’ll see, all organ systems operate under precise
   controls of the nervous system and the endocrine system.

negative feedback is the main control
mechanism of homeostasis

Mechanisms for feedback help keep physi cal and chemical
aspects of the body within tolerable ranges. In negative
feedback, an activity alters a condition in the external or
internal environment—that is, extracellular fluid. Sensory
receptors detect the change and send this information to
an integrator. The integrator (often, the brain) triggers a
response that reverses the altered condition (Figure 4.14).
Negative feedback is the main mechanism of control for
maintaining homeostasis.
As an analogy, a thermostat-controlled heating system
works by negative feedback. The thermostat senses the air
temperature and mechanically compares it to a preset point
on a thermometer built into the furnace controls. When the
temperature falls below the preset point, the thermostat
signals a switch that turns on the heating unit. When the
air warms enough to match the preset level, the thermostat
signals the switch to shut off the heating unit. In Section
4.11 you will learn how negative feedback helps regulate
body temperature.

positive feedback plays a role outside
of homeostasis

In a few situations positive feedback operates. In this type
of mechanism, a chain of events intensify a change from
an original condition. Eventually, though, the intensifying
feedback reverses the change. There are not many exam-
ples of positive feedback in body functions that affect the
makeup of extracellular fluid, so positive feedback does not
have a major role in homeostasis. Positive feedback does
occur in the body, however. A familiar example is child-
birth. During labor a fetus exerts pressure on the walls of
its mother’s uterus. The pressure stimulates the produc-
tion and secretion of a hormone (oxytocin) that causes the
mother’s uterine muscles to contract and exert pressure on

F i g u r e 4.14 Animated! negative feedback is the main mechanism for maintaining homeostasis. (© Cengage Learning)

Sensor

(for example,

nerve ending

in the skin)

Effector

(a muscle

or a gland)

Stimulus

(change in the

environment)

Integrator Response

(such as

the brain)

In negative feedback, the response of

the system cancels or counteracts the

effect of the original change.

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