Homeostasis refers to the maintenance of a steady state in the body despite changes in the external environment. A system in homeostasis requires sensors to detect changes, a comparator to set a reference point, and effectors to bring the system back to the reference point using negative feedback. There are two main communication systems - the endocrine system uses hormones transported via blood, and the nervous system uses nerve impulses transmitted via neurons. Both systems help regulate homeostasis.

1. HOMEOSTASIS
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
© 2016 Paul Billiet ODWS

2. A system in homeostasis
needs
 Sensors to detect changes in the internal
environment
 A comparator to fix the set point of the system
 Effectors which bring the system back to the
set point
 Feedback control. Negative feedback stops
the system over compensating (going too far)
 A communication system to link the different
parts together. © 2016 Paul Billiet ODWS

3. A system in homeostasis
needs
Sensor
Perturbation in
the internal
environment
Return to
normal
internal
environment
Effector
Comparator
Sensor
Negative
feedback
© 2016 Paul Billiet ODWS

4. Communication systems
These should
consist of the
following
components Linkage
system
Effector
Response
Sensor
Stimulus
© 2016 Paul Billiet ODWS

5. In animals there are two
communication systems
 The endocrine system based upon
hormones
 The nervous system based upon
nerve impulses.
© 2016 Paul Billiet ODWS

6. Hormones
 Organic substances
 Produced in small quantities
 Produced in one part of an organism (an
endocrine gland)
 Transported by the blood system
 All over the body
 Target organs or tissues respond to the
hormone.
© 2016 Paul Billiet ODWS

7. The endocrine system
 Chemical signals
 Specific messages
 Effects may be very slow (e.g. growth
hormone over years)
 Some are very fast (e.g. adrenaline which
acts in seconds).
© 2016 Paul Billiet ODWS

8. Nerve impulses
 All nerve impulses look the same
 Signals travel along nerves…
 …to specific parts of the body
 The nerve impulses travel very quickly
Affect their target tissues in milliseconds.
© 2016 Paul Billiet ODWS

9. The nervous system
 The nervous system is composed of
excitable cells called neurons
 Neurons have long thin extensions which
carry electrical nerve impulses
 The electrical signal of the nerve impulse
needs to be converted into a chemical signal
(a neurotransmitter) so that it can pass from
nerve cell to nerve cell
 The neurotransmitter is destroyed after the
signal has passed on. © 2016 Paul Billiet ODWS

10. The nervous system
organisation
 A Central Nervous System (CNS) made of the
brain and spinal cord…
 and peripheral nerves connecting it to
sensors and effectors
Stimulus
Effector
eg muscle or gland
Central Nervous
System
Receptor or Sensor
eg photoreceptor
Sensory nerve
Response
Motor nerve
© 2016 Paul Billiet ODWS

11. Hormone pathway
Hormone secreted
into the blood
stream
Endocrine cell
Hormone stored
in vesicles
Hormone
precursors
Hormone
synthesis
Stimulus acts on
receptor site or
directly inside cell
Chemicals stimulating the
release of the hormone © 2016 Paul Billiet ODWS

12. Hormone pathway
Hormone
receptor site
Target cell
Metabolic effect
Hormone
destroyed
© 2016 Paul Billiet ODWS