HOMEOSTASIS
Balancing the body
WHAT IS HOMEOSTASIS
 Maintaining
a relatively stable state
internal environment
 Keeping factors within narrow
optimal limits
 Reducing the impact of fluctuating
external factors
WHAT IS CONTROLLED?
Temperature
 pH (hydrogen ion concentration)
 Water
 Blood pressure/volume
 Oxygen/Carbon Dioxide concentration
 Ion concentration

THE ROLE OF SYSTEMS
System
Factors controlled
Nervous
Transmits electrical impulses
Endocrine
Produces hormones
Respiratory
Regulates gases and pH levels
Circulatory
Transports gases, hormones and nutrients
Digestive
Obtains nutrients, water and salts.
Excretory
Removes excess water, salts and wastes. Regulates pH.
Integumentary Creates a barrier. Regulates temperature. Assists
immunity.
HOW ARE THE FACTORS
CONTROLLED

1.
2.
3.
4.
By a stimulus-response mechanism
RECEPTORS pick up the change in the stimulus
A MESSAGE is relayed to the control centre
The control centre sends a message to the
EFFECTORS
The EFFECTORS create a change in the internal
environment to respond to the original stimulus
NEGATIVE FEEDBACK
TWO TYPES OF RESPONSES
Negative Feedback
Positive Feedback

Response acts to
counteract the effect
of the stimulus
For example:

If photoreceptors sense a
decrease in light, a
message will be sent to the
pupil to dilate, increasing
the available light
 More common
During birth, a hormone
oxytocin is released to help
the delivery. The presence
of oxytocin sends a
message for more oxytocin
to be released.
 Less common
Response acts to
enhance the effect of
the stimulus
For example:
NEGATIVE FEEDBACK
Negative feedback is more common because it
keeps factors at their optimum level
 For Example: If the external temperature drops,
the body must counteract this to stay at 37℃. If
the body drops below this temperature
(hypothermia), proteins (like enzymes) will no
longer work at their optimum rate.

EXAMPLE: TEMPERATURE
EXAMPLE: BLOOD GLUCOSE
EXAMPLE: WATER BALANCE
THE TWO SYSTEMS
The Nervous System
Via neurons
 Messengers –
impulses and
neurotransmitters
 Fast acting
 Short-lived

The Endocrine System
Via blood
 Messenger - hormones
 Slower action
 Long lasting

THE NERVOUS SYSTEM
Option 1
THE NERVOUS SYSTEM
RED: The central
nervous system acts
as the “control centre”
BLUE: The peripheral
nervous system
receives messages and
produces the response
NEURONS
The basic units of the nervous system are neuron
cells
 They are specialised cells with a large surface
area, particularly at the dendrites
 They can carry electrical impulses to and from
the CNS
 The CNS is also made of neurons
 There are multiple types of neurons




Motor neurons connect to effector cells
Sensory neurons monitor the changes in stimuli
Interneurons connect the neurons in a path
THE TYPES OF NEURONS
HOW THE NERVOUS SYSTEM
WORKS
Information is received by the sensory neurons in
the Peripheral Nervous System
 Electrical impulses are sent to the Central
Nervous System via connecting neurons
 The information is processed
 A message is sent to the effectors, via connecting
neurons, to alter the internal environment
 The dendrites increase the surface area, which
enables greater communication between neurons
 The myelin sheath protects the axon and controls
the direction of the impulse

BASIC NEURON STRUCTURE
IMPULSES
o To carry an impulse,
ion channels are inserted
into the axon membrane
o Positively charged ions
flow into the axon
o Because it takes time
for the ions to move back
across the membrane
(the refractory period)
impulses can only travel
in one direction
ACTION POTENTIALS
TO MOVE BETWEEN NEURONS
To move between
neurons the impulse
must travel across a
gap (the synapse)
 Packages of
neurotransmitter are
received by the
receiving neuron
 An enzyme breaks
down the
neurotransmitter
after the impulse is
sent.

HOW TO STOP THE MESSAGE
REFLEX ARCS
REFLEX ARCS
Do not travel to the brain
 The response is controlled by the spinal cord
 It produces an automatic and immediate
response
 The brain still recognises the stimulus, the reflex
arc is responsible for the initial reaction

EXAMPLE:
 Touching a hot stove, your reflex arc will pull the
hand up and away from the source of pain. The
brain will perceive the pain itself.