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Section 1: Homeostasis
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 Bellringer
 Key Ideas
 A Changing Environment
 Responses to Change
 Humans and Homeostasis
 Summary
Bellringer
Imagine you are playing softball on a warm summer day.
Write down two ways that your body would adjust to keep
you cool and to keep your muscles working.
Key Ideas
 How does a change in the external environment affect
an organism?
 How does an organism respond to changes, and what
happens if the organism fails to respond to changes?
 What organ systems do humans need in order to
survive?
A Changing Environment
 The external environment around an organism is
constantly changing.
 These changes threaten the stability of an organism’s
internal environment.
 The maintenance of a relatively stable internal
environment is called homeostasis.
A Changing Environment,
continued
 Change in seasons, light level, water availability, or other
changes to the external environment can change an
organism’s internal environment.
 Different organisms deal with these changes in different
ways.
 In a colder environment, for example, organisms may eat
more food, puff out fur or feathers, fly to warmer areas, or
hibernate.
Responses to Change
 Organisms must carry out many chemical reactions to
grow, obtain energy, and reproduce.
 Many organisms also move, breathe, produce heat,
and do other tasks.
 All of these activities require the organism to maintain a
relatively stable internal environment.
Responses to Change,
continued
 Organisms detect and respond to both internal and
external environments in a variety of ways.
 Detection and response can take place at both the
cellular and organismal level.
 Failure to respond to change can result in an
organism’s death.
Responses to Change,
continued
From Cells to Organ Systems
 All organisms respond to changes in the environment.
 The organelles in some single-celled organisms work in a
way that is similar to the way tissues and systems in
multicellular systems work.
 This allows single-celled organisms to perform all the life
processes needed for homeostasis.
Responses to Change,
continued
 A Euglena is a single-celled photosynthetic protist.
 If you place the Euglena in a darkened tank of water
and turn a light on at one end of the tank, the
Euglena’s eyespot will sense the light.
 In response, the protist’s flagellum will propel the
Euglena toward the light.
Responses to Change,
continued
 The chloroplast in the Euglena will begin to perform
photosynthesis.
 By moving toward the light, the Euglena is able to produce
enough energy for its cell to maintain homeostasis and stay
alive.
 Multicellular organisms can respond to change at the
cellular, tissue, organ system, or organismal levels.
Responses to Change,
continued
 The pores on a plant’s leaf help the plant to regulate
water loss and gas exchange.
 During the day, tissues perform photosynthesis.
 Guard cells near these active tissues swell with water.
This opens pores, letting in the carbon dioxide that is
needed for photosynthesis
Responses to Change,
continued
 Open pores also allow water to exit the leaf.
 At night, when photosynthesis slows, water exits the
guard cells.
 The cells shrink and close the pores, preventing excess
water loss at night.
Control of Stomatal Opening
Responses to Change,
continued
 If an organism is not able to respond to changes in its
environment, disease or death can result.
 Consider what would happen if a plant could not slow
water loss.
 When the amount of water in a plant’s tissues got too
low, the plant would wilt, collapse, and die.
Responses to Change,
continued
 Humans must also control internal water levels.
 About 2/3 of the human body is made of water.
 If a person loses too much water, by sweating or urination,
dehydration will occur.
 Dehydration causes blood pressure to drop significantly until
death occurs.
Responses to Change,
continued
 Some deep-water fish must be adapted to deal with
high water pressure.
 The coelacanth is a fish that lives in the deep waters off
the coast of Africa.
 At the depth where the fish lives, water pressure is
several hundred times greater than the water pressure
at the ocean’s surface.
Responses to Change,
continued
 If a person tried to swim at this pressure, the person’s body
would be crushed.
 The coelacanth will actually die in the low-pressure
environment at the water’s surface.
 The coelacanth’s body is adapted to require high pressure,
in order to keep gases from bubbling out of the fish’s
bloodstream and blocking flow to the fish’s heart.
Humans and Homeostasis
 Humans are constantly subjected to environmental
stresses that threaten to upset the delicate balance that
exists within our cells and tissues.
 Humans require multiple systems for digestion,
respiration, reproduction, circulation, movement,
coordination, and immunity.
 These systems interact to maintain homeostasis.
Humans and Homeostasis,
continued
 The organs and systems of the human body help to provide
all of the cells with their basic needs.
 Each cell needs a constant supply of nutrients and oxygen,
removal of waste materials, and protection from disease.
 With the exception of the reproductive system, each of the
human body systems performs a task that helps stabilize the
body’s internal environment.
Visual Concept: Overview of
Organ Systems
Click the button below to watch the Visual Concept.
Humans and Homeostasis,
continued
 In the human body, the main components of
homeostasis are the following:




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the concentration of salts
The pH of the internal environment
The concentration of nutrients and waste products
The concentration of oxygen and carbon dioxide
The volume and pressure of extracellular fluid
Humans and Homeostasis,
continued
 When these five components are adequately
controlled, homeostasis is maintained and the body is
most likely healthy.
 No organ system can maintain homeostasis by itself.
 Organ systems must work together in a synchronized
manner.
Humans and Homeostasis,
continued
 For example, the concentration of oxygen and carbon
dioxide is controlled by four different body systems.
 First, the respiratory system brings oxygen into and
carbon dioxide out of the body.
 The circulatory system distributes the oxygen to the
body’s tissues and picks up carbon dioxide.
Humans and Homeostasis,
continued
 If the level of carbon dioxide gets too high, the nervous
system instructs the muscular system to make the
muscles around the rib cage work harder.
 This action allows the lungs of the respiratory system to
breathe faster, to get rid of excess carbon dioxide.
Summary
 The external environment around an organism is
constantly changing. These changes threaten the
stability of an organism’s internal environment.
 Organisms detect and respond to change in a variety of
ways: both at the cellular level and at the organismal
level. Failure to respond to change can result in an
organism’s death.
Summary, continued
 Humans require multiple systems for digestion,
respiration, reproduction, circulation, movement,
coordination, and immunity. The systems interact to
maintain homeostasis.