L2 What is homeostasis
Homeostasis is the maintenance of a
steady state in the body despite changes
in the external environment
¨  The steady state is the optimum level for
the body functions
¨  Eg.
¨  Body temperature
¨  pH of blood
¨  Sugar levels in blood
¨  Oxygen levels in tissues
¨ 
L2 A system in homeostasis needs
¨ 
¨ 
¨ 
¨ 
¨ 
Sensors to detect changes in the internal environment
A control centre which monitors the change and “knows” the
set point of the system (e.g. body temperature).
A communication system to link the different parts
together
Effectors which bring the system back to the set point
Feedback control. Negative feedback stops the system over
compensating (going too far)
A system in homeostasis needs
L2 Communication systems
These should
consist of the
following
components
Stimulus
Sensor
Linkage
system
Effector
Reponse
L2 Examples of Set Points
¨ 
Body temperature: Human average 36.8
n You
are feverish at 38
n  Blood
vessels dilate
n  Sweating occurs
n At
¨ 
35 you have hypothermia
Blood pH: between 7.35 and 7.45
n Variations
in this value can cause headaches,
confusion, tremors, coma, muscle weakness
L2 Maintaining the body temperature
Keeping warm
Staying cool
Increased insulation,
subcutaneous fat reduces
the conduction of heat from
the body
Increase blood flow to skin,
increases conduction and
radiation of heat from the
body
Reduced sweating
decreases evaporation
Increased sweat secretion,
increases evaporation
Increased shivering,
increases heat produced by
muscle tissue 2 to 5 times
Reduced activity
Core body
temperature
>37°C
Thermoreceptors
Hypothalamus
nerves
Muscles
reduce
activity
Sweat
glands
increase
secretion
Muscles of
skin arteriole
walls relax
Skin arteries dilate
More blood to the
skin.
More radiation &
conduction of heat
More water covers the
skin.
More evaporation
Less heat generated
NEGATIVE
FEEDBACK
Thermoreceptors
Core body
temperature
>37°C
Thermoreceptors
Blood
temperature
Muscles of
skin
arteriole
walls relax
nerves
Hypothalamus
Sweat
glands
increase
secretion
Muscles
reduce
activity
Body
loses
heat
Return
to 37°C
Core body
temperature
<37°C
Thermoreceptors
Hypothalamus
nerves
Muscles
shivering
nerves
Sweat
glands
decrease
secretion
More heat
generated
Muscles of
skin arteriole
walls
constrict
Less water covers the
skin.
Less evaporation
Skin arteries
constrict
Less blood to the
skin.
Less radiation &
conduction of heat
NEGATIVE
FEEDBACK
Thermoreceptors
nerves
Core body
temperature
<37°C
Thermoreceptors
Blood
temperature
Muscles of
skin
arteriole
walls
constrict
Sweat
glands
decrease
secretion
Hypothalamus
nerves
Muscles
shivering
Body
loses
less
heat
Body
gains
heat
Return
to 37°C