1. 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
© 2008 Paul Billiet ODWS

2. A system in homeostasis needs
Sensors to detect changes in the internal environment
A comparator which fixes the set point of the system (e.g. body temperature).
The set point will be the optimum condition under which the system operates
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
© 2008 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
© 2008 Paul Billiet ODWS

4. Communication systems
These should consist of the following components
Linkage system
Effector
Reponse
Sensor
Stimulus
© 2008 Paul Billiet ODWS

5. In animals there are two communication systems
The endocrine system based upon hormones
The nervous system based upon nerve impulses
© 2008 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
To a target organ or tissue where it has a profound effect
© 2008 Paul Billiet ODWS

7. The endocrine system The endocrine system produces chemical signals
Each hormone is different and they travel relatively quickly through the blood stream all over the body
Their effects may be very slow (e.g. growth hormone over years)
Some are very fast (e.g. adrenaline which acts in seconds)
© 2008 Paul Billiet ODWS

8. Nerve impulses All nerve impulses look the same
So the nervous system sends signals along nerves to specific parts of the body
The nerve impulses travel very quickly and affect their target tissues in milliseconds
© 2008 Paul Billiet ODWS

9. The nervous system
The nervous system is composed of excitable cells called neurones (also neurons)
Neurones, characteristically, have long thin extensions which carry electrical nerve impulses
This 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
© 2008 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
© 2008 Paul Billiet ODWS

11. Hormone pathway Endocrine cell Hormone synthesis Hormone precursors
Hormone stored in vesicles
Stimulus acts on receptor site or directly inside cell
Hormone secreted into the blood stream
Chemicals stimulating the release of the hormone
© 2008 Paul Billiet ODWS

12. Hormone pathway Hormone receptor site Metabolic effect Target cell
© 2008 Paul Billiet ODWS