Unit 3
HOMEOSTASIS
WHAT IS HOMEOSTASIS?
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Homeostasis: process by which a constant internal
environment is maintained despite the changes in the
external environment. “homoios” = similar, “stasis” =
standing still.
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Severe strenuous excersize: body temperature can increase
to more than 39⁰C  temperature associated to fever.
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
Heat is dissipated through __________.
How does your body respond to an increase in body
temperature?
Sweat  loss of water  drop in blood pressure  pressure on
kidneys (to conserve water)
 Sweat  salts (nerve function, muscle contraction)  pressure on
kidney’s to maintain electrolyte balance.
 Energy needs  blood glucose needs to be kept constant to
maintain ATP supplies  etc. etc. etc.
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
What dangers exist if your body is unable to
regulate the fluid balance of your tissues?
7.1 – HOMEOSTASIS AND CONTROL SYSTEMS
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Ideal conditions of a human body
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External environments in Canada
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37⁰ C.
0.1% blood sugar level
pH = 7.35
-40 ⁰ C and 40 ⁰ C.
Foods rarely contain 0.1% blood sugar level and pH of 7.35.
Pressures you put on your body
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Excercise
Digesting large meals
Drinking to much water/not enough water
Eating too much salt
Etc.
Homeostasis requires the interaction of several
regulatory systems.
Information about blood sugar, fluid balance,
body temperature, oxygen levels, and blood
pressure are relayed to a nerve coordinating
centre once they move outside the normal
limits.
From the coordinating centre, regulators bring
about the needed adjustments.
BASIC COMPONENTS OF A HOMEOSTATIC
CONTROL SYSTEM
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All homeostatic control systems have three
functional components:
1)
A monitor
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2)
A coordinating centre
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3)
Located in organs
Sends signal to coordinating centre when organ
operates outside of its normal limits.
relays information to appropriate regulator
A regulator

Helps to restore the normal balance.
EXAMPLE: BODY DEALING WITH INCREASED CO2
Exercise  CO2 increased  receptors in brain
stem stimulated  nerve cells carry impulses
to muscles which control breathing  muscles
increase depth and rate of breathing  helps
flush excess CO2 from the body.
 Exercise  O2 decreased in blood  receptor
in neck artery detects low O2  nerve impulse
sent to the brain  impulses sent to muscles
that control breathing
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DYNAMIC EQUILIBRIUM & HOMEOSTASIS
Dynamic:
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dy·nam·ic (d-nmk)adj. also dy·nam·i·cal (--kl)
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1. a. Of or relating to energy or to objects in motion.
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2. Characterized by continuous change, activity, or progress
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3. Marked by intensity and vigor;
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4. Of or relating to variation of intensity, as in musical sound.
Equilibrium:
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e·qui·lib·ri·um (kw-lbr-m, kw-)n. pl. e·qui·lib·ri·ums or e·qui·lib·ri·a (-r-)
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1. A condition in which all acting influences are cancelled by others, resulting in a stable,
balanced, or unchanging system.
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3. Physics The state of a body or physical system at rest or in unaccelerated motion in which the
resultant of all forces acting on it is zero and the sum of all torques about any axis is zero.
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4. Chemistry The state of a chemical reaction in which its forward and reverse reactions occur at
equal rates so that the concentration of the reactants and products does not change with time.
DYNAMIC EQUILIBRIUM: CONDITION THAT REMAINS STABLE WITHIN FLUCTUATING LIMITS.
Homeostasis is often referred to as a dynamic equilibrium.
LEVELS REMAIN CONSIDERABLY STABLE WITH
CHANGES IN ENVIRONMENT
HOMEOSTASIS AND FEEDBACK
Negative feedback: process by which a
mechanism is activated to restore conditions to
their original state.
 Positive feedback: process in which a small
effect is amplified (less common).
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NEGATIVE FEEDBACK
Makes adjustments to bring body back within
acceptable range.
 Change in the variable being monitored triggers
the control mechanism to COUNTERACT any
further change in the same direction.
 Resists change.
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Prevents small changes from becoming too large.
 Analogy: room temperature regulation
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 Monitor
(thermometer): monitors the temperature. If
temperature falls below a set point, sends info to thermostat.
 Coordinating centre (thermostat): sends info to the regulator.
 Regulator (furnace): is switched on when it receives this
information. Normal conditions are re-established.
 Opposite would occur with temperature above the set point.
POSITIVE FEEDBACK
Reinforce change
 Move controlled variable further away from
steady state.
 Allows small physiological event to be
accomplished rapidly.

 Example:
BIRTH.
 Decrease
in progesterone  contractions  release of
oxytocin  STRONGER contractions  baby to cervix 
stronger contraction  baby OUT
SEATWORK/HOMEWORK
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Page 337, 1-5
7.2 - THERMOREGULATION
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Thermoregulation: maintenance of body
temperature within a range that enables cells
to function efficiently.
 How
can extreme temp. affect cell processes?
ECTOTHERMS VS. ENDOTHERMS
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Ectotherm: invertebrates, most fish, amphibians,
reptiles.
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Depend on air temp. to regulate metabolic rates
Activity partially regulated by environment
Behavioural adaptations: ‘basking,’ retreating to shaded
areas.
Endotherm: mammals and birds.
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Can maintain constant body temp regardless of
surroundings.
Cold: increase cellular resp. (generate heat)
Hypothalamus: region of the vertebrate’s brain responsible
for coordinating many nerve and hormone functions

“thermostat”
HEAT STRESS
Rise in body temperature  hypothalamus 
sweat glands (sweat).
 Rise in body temperature  hypothalamus 
blood vessels dilate  increase in ‘cooled’
blood  blood cools internal organs.
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COLD STRESS
Temperature drops  thermoreceptors (skin) 
hypothalamus  blood flow limited  limits heat
loss from skin  retains heat in internal organs.
 Temperature drops  thermoreceptros (skin) 
hypothalamus  smooth muscles (in skin:
‘goosebump’)  hair traps air next to skin 
reduced heat loss.
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What type of animal would this be most effective?
Temperature drops  thermoreceptros (skin) 
hypothalamus  skeletal muscles  shivering.
PROLONGED COLD STRESS
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Hormonal response
 Elevates
metabolism
 ‘brown fat’: adipose tissue capable of converting
chemical energy into heat.
 Newborns:
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lack ability to shiver.
Hypothermia
 Body
core falls below normal temp. Range.
 Mammalian diving reflex:
 When
in cold water  heart rate slows  blood diverted
to VITAL organs.
FREEZING CELLS
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Possible?
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Ice crystals form
 Act
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as little knives: membranes are torn, tissues destroyed.
Melting
 Cells
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fill with water  lyse
Frogs
Frozen in winter.
 Can ‘defrost’ and live afterwards.
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How?!
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Antifreeze: protein that prevents ice crystals from
forming.
SEATWORK/HOMEWORK
Page 341
#1-4, 6-8, 10.
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