Chapter 9:
Homeostasis & Circulation
Section 9.1:
Homeostasis: Life in the Balance
pg. 300-303
In a room, I have a thermostat
which has a thermometer that
records the room’s
temperature. The thermostat is
connected to my furnace and is
set at 20° Celsius.
Explain what happens on a cold
day.
Like your thermostat,
multicellular organisms
must maintain near-constant
conditions within their internal
environment so that all of their
individual cells can continue
working together.
(i.e. blood glucose = 100 mg/mL, blood pressure
= 120/80, body temperature = 37 degrees C)
Homeostasis- “steady
state”
The body’s maintenance of a
relatively stable internal
environment despite a changing
external environment.
e.g. blood glucose concentration, blood
pH, blood pressure, body temperature
Dynamic EquilibriumState of balance
achieved within an
environment as the
result of internal
control mechanisms
that continuously
oppose outside forces
that tend to change that
environment.
Homeotherms:(homeo = same, therm = temperature)
These organisms keep their body temperature
relatively constant.
E.g. birds & mammals
Poikilotherms – animals whose body temperature
fluctuates depending on the temperature of their
environment.
(i.e. cold-blooded animals like turtles, lizards, snakes)
What are some behavioural ways that
humans control their body temperature?
Physiological Means of
Maintaining Body Temperature
• The normal human body temperature is
37.5 degrees C. Your body generates heat
as a by-product of metabolic processes.
• As warm blood from the
interior of the body passes
by the skin, heat is lost.
• The constriction of blood
vessels is called
vasoconstriction.
• The dilation of blood
vessels is called
vasodilation.
• The body maintains homeostasis through
the use of feedback loops
– Negative Feedback loops
– Positive Feedback loops
Negative Feedback Loop
• The body’s reaction to
increased or
decreased
temperature.
• A process that detects
and reverses
deviations from
normal body
temperature or other
constants.
Negative Feedback Loop
• Involves three parts:
– A sensor
– An integrator
– An effector
Negative Feedback Loop
• Sensory receptors
– found in every organ and
body tissue
– Its function is to send
nerve impulses to the
brain in response to
environmental info
– Monitor pH, blood sugar,
blood pressure and many
other internal conditions
on a continual basis
– i.e thermoreceptors in the
skin
Negative Feedback Loop
• Integrator
– The brain
(hypothalmus)
– Takes info from sensor
– Compares this info to
desired set point
– Sends a message to
the effector
Negative Feedback Loop
• Effector
– Any structure that
takes action to return
the value to the
desired set point
– Causes the change in
internal condtions
– i.e. blood vessels
constrict or dilate
Negative Feedback Loop
• Moves a system
toward balance and
stability
• Essential to maintain
homeostasis.
Positive Feedback Loop
• Rare in healthy bodies
• Acts to move a system away
from balance and stability
• Usually associated with
disease or change
– i.e. Addictions, child birth
Body systems play an
important role in
maintaining homeostasis.
Equilibrium is maintained
as long as the system
is active.
Many Body Systems Interact to Maintain
Homeostasis (Fig. 9.2, pg. 301)
1. Why do we need a
circulatory system?
2. What does this system
transport?
3. In humans, is this system
open or closed? (Does
blood ever leave the
system)
Section 9.1
Review Questions
• Do question #’s 1-6 on page 303
Section 9.1
Answers
1. Explain how the brain helps to regulate body
temperature.
The hypothalamus of the brain is the body’s
thermostat. It regulates temperature by
controlling blood flow to skin, sweating and
shivering (muscle contraction).
2. Provide one example of dynamic equilibrium
in the body.
Temperature regulation, control of blood
sugar levels, etc.
3. Describe the role of the skin in maintaining
homeostasis in the body.
The skin provides physical protection from
environmental hazards (e.g. excess sunlight),
acts as a barrier to infectious agents, and
regulates body temperature.
4. Explain the difference between hypothermia
and hyperthermia.
Hypothermia = core body temperature falls
below the normal temperature range (37.5
degrees Celsius).
Hyperthermia = an elevation of core body
temperature above the normal range.
5. Explain the graph showing the relationship
between body temperature and ambient
(environmental) temperature in a cat and a
lizard. How is the cat able to maintain a more
constant body temperature than the lizard?
In mammals, metabolic reactions in the body
generate heat internally and various
physiological mechanisms work to maintain
this body temperature.
Question 5 (continued)
In reptiles, body heat is primarily absorbed
from external sources, so body temperature is
affected by changes in environmental
temperature.
6. Explain the difference between a negative
feedback loop and a positive feedback loop.
Negative feedback loops are systems that
reverse the original stimulus.
Positive feedback loops are systems that
reinforce the original stimulus.