1. HOMEOSTASIS
The maintenance of a relatively stable internal environment in the face of changes in either the external or internal environment.

2. Cellular Environments
External environment – the medium immediately surrounding an organism.
Internal environment – also called extracellular fluid, is the fluid that surrounds cells in multicellular organisms.
Intracellular fluid – the fluid content of cells.
Note: the average human contains approx. 40L of water. 15L is extra cellular fluid, 25l is intracellular fluid.
Blood makes up 5L and contains both intracellular and extracellular fluid.

3. Environmental Requirements
Unicellular organisms - obtain all of their requirements and perform all of their activities for themselves to survive. The cell membrane regulates the composition of the cytoplasm.
Multicellular animals have an internal environment (body fluid) that is distinct from their external environment that remains relatively constant. Changes in the internal environment arise from within the organism. Nutritional and gas requirements change as a result of varying activity levels, stage of growth or reproduction.

4. REGULATION
Organisms that can regulate their internal environment are able to live in a wide range of conditions.
Organisms that have little ability to regulate their internal environment have low tolerance limits in environmental conditions.
Example: Decorator Crab - Osmoconformor & the Common Shore Crab – Osmoregulator, (page 99).

5. HOMEOSTATIC MECHANISMS IN VERTEBRATES
Homeostasis involves Integration & Regulation by the hormonal and nervous systems, largely through NEGATIVE FEEDBACK mechanisms.
The circulatory system plays a central role, with most complex homeostatic mechanisms regulating the internal environment of mammals and birds, including a relatively constant body temp. This is very important for biochemical reactions to occur efficiently & it requires energy expenditure.

6. STIMULUS RESPONSE MODEL - a
CONTROL CENTRE
RECEPTOR
nerves
EFFECTOR
RESPONSE
hormones

7. STIMULUS RESPONSE MODEL - b
CONTROL CENTRE
RECEPTORS
Misalignment Disturbance
detectors detectors
nerves
RESPONSE
EFFECTOR
hormones

8. NEGATIVE FEEDBACK SYSTEMS
Stimulus-response mechanism that reduces the effect of the original stimulus, (ie. the response has a negative effect on the stimulus).
Eg. When you exercise your body produces heat & temp. increases. Your body responds by initiating cooling processes to cool your body temp. The stimulus is an increase in body temp. and the response reduces the stimulus, cooling the body. This is a negative feedback mechanism.

9. Responding to changing conditions.
Stimuli: These are changes in either the internal or external environment of an organism. Many internal & external environmental factors act as stimuli and are monitored for the maintenance of homeostasis.
Internal Conditions that are monitored include:
Chemical Factors – such as levels of Oxygen, Carbon Dioxide, Glucose, Ions, Wastes & Water.
Physical Factors – such as Temperature, Blood Pressure and Balance.
2. External Conditions that are monitored include:
Physical Factors – such as Light, Temperature, Gravity, Sound & Day Length.
Chemical Factors – such as Food, Oxygen, Carbon Dioxide, Water & Specific Chemicals.

10. RECEPTORS Types of Receptors
Receptors: Specialised structures capable of responding to specific stimuli by initiating signals in the nervous system or triggering the release of a hormone.
Types of Receptors
Chemoreceptors – These are stimulated by specific chemicals in the external and internal environment.
Mechanoreceptors – These are stimulated by anything that changes the shape of the receptor.
Photoreceptors – These detect light. In some animals they also detect colour and form images.
Thermoreceptors – These detect external heat and cold through receptors near the surface and internal body temp. deeper in the body by receptors in the major arteries and hypothalamus.

11. Detecting Stimuli.
The intensity of a stimulus must be sufficient to reach the threshold of the receptor. This is the weakest stimulus to which the receptor can respond. Receptors then stimulate effectors to produce a response.