Biology 12

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effectors

(sweat; vasodilation)

receptor

(skin)

integrator

(hypothalamus of

the brain)

effectors

(goose bumps;

shivering;

vasoconstriction)

heat lost

heat

gained

heat lost

heat stored

Negative Feedback Loop for Internal Body Temperature

110 MHR • Unit 2 Homeostasis

Feedback Loops

The body’s reactions to increased or decreased

temperature are examples of negative feedback

loops. A negative feedback loopis a process that

detects and reverses deviations from normal body

constants. This process involves three parts: a

receptor, an integrator, and an effector.

Sensory receptors are found throughout every

body organ and tissue. The function of the sensory

receptorsis to send nerve impulses (stimuli) to the

brain in response to environmental information.

They monitor the body’s internal conditions, such

as temperature, blood pH, blood sugar, and blood

pressure, on a continual basis. For instance, when

the body’s temperature begins to drop or rise,

specialized receptors in the skin detect the

change and signal the hypothalamus (a part of

the brain) accordingly.

The brain, similar to the thermostat in a house,

is an integrator— it sends messages to effectors

(analogous to the furnace or air conditioner in a

house). Effectors, in turn, cause a change in

internal conditions. In this case, the brain sends

messages to various tissues and organs that cause

the body to either generate or conserve heat (see

Figure 4.5). These messages can be transmitted by

the nervous system (the topic of Chapter 5) or by

chemical messengers known as hormones, which

you will study in greater depth in Chapter 6.

Negative feedback loops exist throughout the

body to maintain homeostasis. These systems

prevent blood sugar, blood pressure, temperature,

and other body constants from becoming too high

or too low (see Figure 4.6).

Positive feedback loops also exist, but they are

usually associated with disease or change (for

example, drug addiction) and are therefore rare

in healthy bodies. Another example of a positive

feedback loop is high blood pressure. Damage to

arteries due to high blood pressure results in the

formation of scar tissue. This scar tissue traps

cholesterol, which impedes the flow of blood

through the arteries and thereby increases blood

pressure even more.

Positive feedback loops act to increase the

strength of the stimuli, whereas negative feedback

loops decrease or reverse effects of the stimuli. A

negative feedback loop moves a system toward

balance and stability, while a positive feedback

loop pushes a system away from balance and

Figure 4.6Negative feedback

loops prevent internal body

temperature from becoming

too high or too low.

Figure 4.5When you exercise, sensory receptors detect

your body’s rise in internal temperature and send a message

to the brain (the integrator). The brain then sends a message

to the blood vessels (the effectors) to dilate. The dilation

causes more blood to course through the blood vessels.

The double lines in the diagram indicate that once the

temperature returns to normal, the effectors’ response stops.

Sensory receptors detect rise

in internal temperature.

Message is sent to integrator.

Integrator sends message to

effectors.

Effectors respond (blood vessels

dilate).

Once the temperature returns

to normal, the response of the

effectors ceases.

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