Homeostasis
What is homeostasis?
Homeostasis is the maintenance of an organism’s
internal environment within set limits.
Body temperature and blood concentration
are both controlled by homeostasis. The limits
on these conditions may be quite broad, but if
factors vary outside them it is damaging.
Internal conditions are regulated by
the endocrine and nervous systems, as
well as by behavioural patterns.
Organisms increase their survival chances by modifying their
internal environment in response to external changes.
Why is homeostasis important?
Enzyme activity – Enzymes operate best over a specific range of
conditions. By maintaining pH and temperature in the body all
enzyme-linked reactions proceed efficiently.
Cell size – Changes in the water potential of the blood will affect
the amount of water in the tissue fluid and cells. This could cause
animal cells to desiccate, or swell and burst.
Independence from external
conditions – Animals with a constant
internal environment can maintain a
constant level of activity regardless of
their environment.
Negative feedback
To maintain their internal environment organisms need a self
regulating mechanism. In most animals this is achieved by negative
feedback.
Negative feedback works by initiating corrective mechanisms
whenever the internal environment deviates from its normal or
acceptable level.
Thermoregulation: an example of a negative feedback loop
body temperature
increases
corrective
mechanisms
normal body
temperature
normal body
temperature
body temperature
decreases
corrective
mechanisms
Negative feedback
Positive feedback
In positive feedback, a deviation from
normal conditions is amplified, leading to a
further deviation.
Positive feedback is rare in biology because
it tends to produce harmful, unstable
conditions.
In early childbirth the release of the
hormone oxytocin stimulates uterine
contractions. The contractions trigger a
positive feedback reaction. More oxytocin
is released, initiating more contractions.
further increase
positive feedback
levels increase
set point
levels decrease
positive feedback
further decrease
Beware the cold!
Nervous or endocrine control?
The nervous system and endocrine system are both involved in
controlling the internal conditions of a mammal.
signal type
nervous control
endocrine control
electrical
chemical
(action potential)
(hormones)
speed of signal
reach the target
relatively slow,
cells in milliseconds travelling in the
bloodstream
duration of signal
short lasting
long lasting
When do you think endocrine control could be advantageous over
nervous control in some situations?
Homeostasis in mammals
Visualizing homeostasis
Thermoregulation
Animals regulate their body temperature using a process called
thermoregulation. Thermoregulation is crucial as it helps to
maintain the body at the optimal temperature for its enzymes.
Some animals live in thermostable
environments, such as the open
ocean, and have to do very little to
maintain their body temperature.
Instead their enzymes are adapted to
work at their environmental
temperature.
Environments with fluctuating temperatures provide the greatest
challenge for thermoregulation.
Heat transfer
The balancing act
Balance/equilibrium
Gain:
Loss:
Metabolism
Radiation
Radiation
Conduction
Conduction
Convection
Evaporation
How is heat gained and lost?
Ectotherms and endotherms
There are two major strategies for thermoregulation in the
animal kingdom: endothermy and ectothermy.
Ectotherms do not exert physiological control
over their internal body temperature. They
instead rely on external sources of heat to
warm their bodies. Despite this, many
ectotherms maintain a near-constant body
temperature during their hours of activity.
Reptiles and amphibians are ectotherms.
Endotherms generate their heat metabolically. Most also maintain a
constant internal body temperature, higher than the external
environment. Mammals and birds are endotherms.
Thermoregulation in ectotherms
Thermoregulation in endotherms
Endotherms generate most of their heat metabolically.
As they are warmer than their environment they tend to lose
body heat to their surroundings.
Adaptations for heat conservation include:

Insulation – insulating layers of fat,
fur or feathers to prevent heat loss.

Metabolism – metabolic rate can be
varied to generate more or less heat.
Endotherms also use their skin to vary
their rate of heat loss and maintain a
constant core body temperature.
TASK:
Q Evaluate the advantages and disadvantages of
endothermic and ectothermic temperature control
• energy use efficiency – link to feeding and growth
• activity: temporally and geographically
• Predator/ prey considerations
The importance of the skin
Thermoregulation in humans
Q. Suggest how the following may help temperature
regulation in non-human animals:
dog panting with tongue lolling out of mouth
• blood vessels close to surface of tongue
• saliva evaporates
• cooling air currents
hippopotami semi-submerged in water during the day
• water has high specific heat capacity and is cooler than
the air
fennec fox (desert living) staying in burrow during the day
• burrow is shaded and cooler
Hyperthermia and hypothermia
Prolonged exposure to high temperatures can lead to
hyperthermia. The body’s core temperature rises above 41°C
causing its thermoregulatory mechanisms to break down.
When this happens, positive feedback occurs and the person’s
body temperature can spiral out of control. A core body
temperature of 43°C and higher usually causes death.
If the body’s core temperature falls
below 35°C a person may suffer from
hypothermia. Again the body’s
thermoregulatory mechanisms fail and
positive feedback occurs leading to a
further decrease in temperature.
Physiological thermoregulation
Thermoregulatory control system
Q. Suggest how the following might help temperature regulation in
ectotherms:
butterfly resting with its wings outstretched and orientated towards the
sun
• large surface area is exposed to sun’s rays, absorbing the heat energy
crocodiles bask on the bank during the early part of the day but move to
the water later in the day
• air and bank heat up more quickly than the water (early in day)
• water is cool, being in the water stops overheating (later in day)
Q. Suggest why ectotherms put on more growth than endotherms when
fed the same amount of food
• less food is used to generate heat to maintain body temperature, so
more is available for growth
Endotherm (exemplified by mammal)
Meaning of the term:
High temperature
Physiological responses:
•
Behavioural responses:
•
•
•
•
•
•
Low temperature
Physiological responses:
•
Behavioural responses:
•
•
•
•
•
•
Endotherm (exemplified by mammal)
Meaning of the term: generate heat internally to maintain body temperature
High temperature Physiological responses:
 Vasodilation
 blood close to surface, so
heat easily lost by radiation
 Sweating
 evaporative cooling
 Hairs lie flat
 insulating layer of air not
trapped
 Panting
 evaporative cooling from the
respiratory system
Low temperature Physiological responses:
 Vasoconstriction
 blood not close to surface so
less heat lost
 Increased metabolism (liver)
 more heat produced
 Shivering
 more heat produced
 Pili-erection
 traps insulating layer of air
Behavioural responses:
 Seek shade
 it is cooler
 Move less
 generate less internal heat
 Other
 cool drinks, ice cream ...
Behavioural responses:
 Move more
 generates more internal
heat
 Huddle
 get heat from neighbour
 Other
 warm drinks and food,
shelter ...
Ectotherm
Meaning of the term:
High temperature
Low temperature
Physiological responses:
•
Behavioural responses:
•
•
•
Physiological responses:
•
Behavioural responses:
•
•
Ectotherm
Meaning of the term: body temperature varies with environmental temperature
High temperature Physiological responses:
Behavioural responses:
 Increase breathing movements  Hide in burrow
(insects)
 cool and shaded
 increase evaporative cooling
Low temperature
 Increase mucus secretion from
skin
 increases evaporative
cooling
 Dive into warm water
(amphibian)
Physiological responses:
 Contract flight muscles before
takeoff (bees and large moths)
 heat generation warms the
muscles up
Behavioural responses:
 Expose body to the sun or warm
surface
 sun’s rays or surface heats
the animal up
 Direct blood away from the skin
(diving reptiles)
 prevents unnecessary
cooling