Download Maintaining a Dynamic Equilibrium The Need for Homeostasis

THEME III
Maintaining a Dynamic
Equilibrium
10
The Need for Homeostasis
HOMEOSTASIS
Hair
Surface of skin
Organisms live in a world of changing conditions. But, to remain alive, every organism
needs to keep the conditions inside of itself
fairly constant. An organism must have ways
to keep its internal conditions from changing
as its external environment changes. This
ability of all living things to detect deviations and to maintain a constant internal environment is known as homeostasis.
An obvious change that has occurred in the
course of evolution is the development of
larger multicellular organisms from microscopic, single-celled ones. Is there an advantage to being multicellular? Being microscopic
and single-celled makes it difficult for an organism to maintain homeostasis. Having a
multicellular body makes possible many types
of protection against changes in the environment. In other words, an organism with many
cells is able to have structures and systems
that protect its individual cells from external
changes, thus helping it to stay alive. (See Figure 10-1.)
To maintain homeostasis, organisms actually must make constant changes. That is why
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Blood vessels
Figure 10-1 Multicellular organisms have systems and
structures that help them maintain homeostasis. For
example, our skin has features that detect and respond
to changes in external temperature.
homeostasis is often referred to as maintaining a dynamic equilibrium. Dynamic means
“active,” and equilibrium means “balanced.”
Homeostasis requires active balancing.
THE CELL AND ITS
ENVIRONMENT
One of the most fascinating facts about our
bodies is that each of our many, many cells is
Chapter 10: The Need for Homeostasis
Capillary Lymph
Red blood cells
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Skull
Cerebrum
Lymph vessel ICF Tissue cells
Figure 10-2 All the cells in our body are surrounded by
intercellular fluid (ICF). Materials are exchanged between
the cells and the fluid, which helps to maintain stable
conditions inside each of the cells.
Medulla
Cerebellum
Spinal cord
surrounded by liquid. The smallest blood vessels in our bodies, the capillaries, are close to
every cell. There is a small amount of space
between the capillaries and the body cells.
This space is filled with fluid. The fluid that
surrounds cells is made up mostly of water,
with many substances dissolved in it. This intercellular fluid is important in helping to
maintain stable conditions inside each of our
cells. Many materials are exchanged between
the cells and the fluid. In turn, materials may
be exchanged between the fluid and the blood
in the capillaries. All of this is done to make
sure that each and every body cell is able to
maintain homeostasis and remain healthy.
(See Figure 10-2.)
Vertebrae
Figure 10-3 A structure in the brain (the medulla) monitors the amount of CO2 in the body, adjusting the breathing rate to maintain proper levels.
sends signals to the chest to increase the rate
of breathing and the amount of air taken in on
each breath. These changes in breathing increase the exchange of gases in the lungs,
lowering the CO2 levels in the body. These
lower levels are then detected in the brain,
which in turn sends a signal to reduce the
breathing rate. This process is an example of
a feedback mechanism. Feedback mechanisms are important in maintaining homeostasis. (See Figure 10-3.)
MAINTAINING HOMEOSTASIS
WHEN WE EXERCISE
FEEDBACK MECHANISMS
Exercise involves increased muscle activity.
This activity creates changes within the body.
To maintain homeostasis, the body needs to be
able to respond to these changes.
An example of a change that occurs when
we exercise is the increase in the body of carbon dioxide (CO2), produced by muscle cells as
a result of cellular respiration. The level of
CO2 increases in both the intercellular fluid
and the blood. To maintain homeostasis, the
body first must be able to detect this change
and then respond to the change.
A structure in the brain detects the increased CO2 level in the blood passing
through the brain and in the fluid around the
brain cells. As a result, this part of the brain
Carbon dioxide levels in your body are regulated somewhat as a thermostat regulates the
temperature of your house. A thermostat
measures the temperature of the air in a
room. When the air temperature in the house
falls below a preset figure, the thermostat
turns a furnace on. The furnace produces
heat, and the temperature of the air in the
house increases. When the temperature of the
air rises above the preset temperature, the
thermostat tells the furnace to shut down.
The temperature in the house stops rising, the
air begins to cool, and the thermostat continues the cycle of telling the furnace to produce
heat or to shut down. (See Figure 10-4.)
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Reviewing the Living Environment: Biology
Thermostat
Sensor
Stomate
Temperature
sensors in brain
Body temperature
control
Skeletal
muscles
Hypothalamus
Temperature
sensors
in skin
Sweat
glands
Chloroplasts
Guard cells
Nucleus
Leaf cells
Stomate
closed
Nucleus
Stomate
open
Figure 10-5 Special openings in the surface of a leaf
function to maintain water balance in plants.
Superficial
arteries
Home
heating
system
Figure 10-4 Both CO2 levels and body temperature are
regulated by feedback mechanisms, much as a thermostat controls the temperature in a room.
In this type of feedback mechanism, a
change occurs that produces another change,
which in turn reverses the first change. This is
an important process in maintaining homeostasis.
The following are parts of a feedback mechanism used in maintaining homeostasis:
♦ Sensor. Something must be able to detect
a change. A thermometer attached to a
thermostat is a sensor. In the body, structures in the brain detect changes in CO2
levels.
♦ Control unit. Something must know what
the correct level should be. A thermostat
in a house is set to a particular comfort
level. Information in the brain is preset at
the correct CO2 level.
♦ Effector. Something must take instructions from the control unit and make the
necessary changes. In a house, the effector
would be a furnace or an air conditioner.
In the body, the effector for CO2 levels
would be the muscles in the chest that are
used for breathing.
MAINTAINING HOMEOSTASIS:
WATER BALANCE IN PLANTS
Maintaining water balance is a major concern
for all living things. Plants as well as animals
must maintain water balance. Openings in
the surface of a leaf are adapted to control the
loss of water. Each opening is surrounded by
two guard cells. These guard cells, like any
cells, allow water to diffuse through their cell
membrane. When water is abundant, it moves
into the guard cells. The increased quantity of
water increases the pressure within the cells.
Guard cells are somewhat curved in shape;
when they are filled with water, they become
even more curved. The space between them
expands, the opening widens, and excess water is allowed to evaporate out of the air
spaces inside the leaf to the air that surrounds the plant. (See Figure 10-5.)
When water becomes scarce, the guard cells
become less curved in shape and the opening
closes. Water loss is reduced and the plant is
able to maintain its water balance.
SYSTEMS FOR MAINTAINING
HOMEOSTASIS
Multicellular animals have evolved highly organized, complex organ systems especially
suited to maintaining a relatively constant internal environment. These organ systems include the excretory system, which regulates
the chemistry of the body’s fluids while removing harmful wastes; the nervous system,
which uses electrochemical impulses to regulate body functions; the endocrine system,
which produces hormones—chemical messengers essential in regulating the functions and
behavior of the body; and, finally, the immune
system, which uses a set of defenses to protect
the body from dangerous substances and microorganisms that could upset the internal
balance on which life itself depends.
Chapter 10: The Need for Homeostasis
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Chapter 10 Review
Part A—Multiple Choice
1. Organisms undergo constant chemical changes
as they maintain an internal balance known as
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2
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4
interdependence
synthesis
homeostasis
recombination
2. What characteristic has evolved that helps to
maintain homeostasis?
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2
3
4
taller bodies with larger cells
shorter bodies with fewer cells
multicellular bodies with many cells
multicellular bodies with fewer cells
3. A system in dynamic equilibrium
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2
3
4
makes constant changes
changes in intervals or steps
changes very infrequently
never changes at all
4. Intercellular fluid is made up mostly of
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2
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4
water
blood
mineral salts
cytoplasm
5. Intercellular fluid is important for the exchange
of materials between
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2
3
4
body cells and arteries
body cells and veins
veins and capillaries
body cells and capillaries
6. As a result of exercise, CO2 levels increase in
the
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2
3
4
blood only
intercellular fluid only
blood and intercellular fluid
muscles only
7. The brain sends a signal to increase the breathing rate when the CO2 level has
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2
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4
not changed for a while
decreased
increased
increased, then decreased
8. The increased breathing rate signaled by the
brain serves
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2
3
4
to increase the CO2 level in the body
to decrease the CO2 level in the body
to decrease the O2 level in the body
no function in changing O2 and CO2 levels
9. In adjusting the CO2 level, the part of the body
that acts like a thermostat in the home is the
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2
3
4
brain
chest
lungs
muscle tissue
10. If an organism fails to maintain homeostasis, the
result may be
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disease only
death only
disease or death
none of the above
11. A change in the body results in another change.
This second change reverses the first change in
order to maintain homeostasis. This describes a
type of
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2
3
4
control mechanism
feedback controller
feedback mechanism
effector mechanism
12. The effector for adjusting the CO2 level in the
body would be the
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4
blood tissue
brain
lungs
chest muscles
13. Why might a blood clot be important to maintaining homeostasis?
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2
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4
It slows the flow of blood through the body.
It prevents the loss of blood from the body.
It increases the amount of water in the blood.
It adds more cells to the blood tissue.
14. The changing shape of a plant’s guard cells
helps to
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2
3
4
allow the plant to grow stronger
prevent the plant from losing food
regulate the temperature of the plant
maintain the plant’s water balance