Fundamentals of Anatomy and Physiology

The Human Body

HOMEOSTASIS

Homeostasis (hom-ee-oh-STAY-sis) is the maintenance
(within varying narrow limits) of the internal environ-ment
of the body. One of the first scientists to discuss the
significance of homeostasis to the survival of an organ-ism
was French scientist Claude Bernard (1813–1878).
Homeostasis is essential to survival; hence, many of the
body’s systems are concerned with maintaining this in-
ternal environment. Some examples of homeostasis are
blood sugar levels, body temperature, heart rate, and the
fluid environment of cells. When homeostasis is main-
tained, the body is healthy. This is the reason your doc-tor
takes your temperature and blood pressure as part of a
routine examination.
We shall examine two examples of maintaining
-homeostasis. After ingesting a meal, which is predomi-
nately carbohydrates (salad, vegetables, bread, and perhaps
fruit), the blood glucose level increases dramati-cally due
to the breakdown of the complex carbohydrates by the
digestive system into sugars such as glucose. Cells take in
the glucose they need from the blood, but so much -glucose
is in the blood that now the pancreas -secretes in-sulin,
which moves the excess blood glucose into the liver where
it is stored as glycogen, or animal starch. -Between meals,
when the blood glucose level drops below nor-mal, the
pancreas secretes glucagon, which breaks down the
glycogen into glucose and returns it to the blood cir-
culatory system for distribution to body cells. Thus, the
glucose level in the blood plasma remains at a nearly
constant level so that it does not remain elevated after a
m eal, nor does it drop too low between meals.
Body temperature regulation is another important
example of homeostasis. When we go out on a hot sum-mer
day and our body temperature rises above 98.6°F, the
hypothalamus of the brain detects this change and sends
signals to various organs so that we sweat (sweating is a
cooling process). As water is excreted by the sweat glands
onto the skin, it evaporates in the air (evaporation is a
cooling mechanism). In addition, our blood vessels dilate to
bring the blood near the skin’s surface to dissipate body
heat. When our body tempera-ture falls below 98.6°F, such
as when we go out on a cold winter day, the hypothalamus
sends signals to muscles, causing us to shiver to raise our
body temperature; it also causes our blood vessels to
constrict to conserve body heat.

Our body must constantly monitor itself to correct any
major deviations in homeostasis. It does this by us-ing what
is referred to as a negative feedback loop. Feed-back
responses that revise disturbances to our body’s

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condition are examples of negative feedback. A good

example of a negative feedback loop is the relationship

between your home thermostat and your furnace. You set

the thermostat at a temperature of 72°F. When the

temperature in your home drops below 72°F, the furnace

turns on to raise the house temperature. When the tem-

perature goes above 72°F, the thermostat causes the fur-

nace to turn off. See Figure 1-10 for an example of how

negative feedback controls body temperature. Positive

feedback is an increase in function in response to a stim-

ulus. For -example, after the first contraction during la-bor,

the uterus continues to contract with more strength and

frequency.

Our organ systems help control the internal envi-

ronment of the body and cells so that it remains fairly

constant. Our digestive, urinary, circulatory, and respi-

ratory systems work together so that every cell receives the

right amount of oxygen and nutrients, and so waste

Body temperature is lowered by

change in external environment^

Relieves^

Condition

Sensory nerve cells in skin

detect change in temperature

(SENSOR)

Sends information to the

The Types of Muscle.

(CONTROL CENTER)

Sends signals to blood vessels

in skin to dilate skeletal muscles

to shiver; liver to release

glucose to produce metabolic

heat (EFFECTORS)^

(^)
Heat production causes (^)
increase in body temperature
®^
(^)
Learning^
Negative Feedback
Cengage^
(^)
2016
©^
(^)
Figure 1- 10 An example of how negative
feedback controls body temperature.