1. Detecting & Responding
Homeostasis
Detecting & Responding

2. Internal Vs. External Environment
External environment can vary greatly
Internal environment of the body is kept constant
Within a very narrow tolerance range over the lifespan
Vital for survival

3. The Internal Environment
This includes:
The cells (Intracellular fluid)
Extracellular Fluid
Tissue Fluid (fluid surrounding cells)
Plasma (fluid component of blood in capillaries)
Wastes and nutrients are readily exchanged between all of these fluids

4. Internal Vs. External Environment
Range between 0-29C
Range between 36.5 and 38C

5. Environmental conditions
Some examples of conditions that need to be kept constant are:
Temperature
Oxygen concentration
Blood pressure/volume
pH levels
Salt concentration
Water balance
Glucose concentration

6. Homeostasis
The term homeostasis refers to the maintenance of a stable internal environment within narrow limits despite changes in the external environment
Range between 36.5 and 38C

7. Homeostasis
All body systems contribute to the internal environment in some way
Monitoring of the internal and external environment is carried out by two different systems:
Nervous – nerves/electrical
Endocrine – hormones/chemical
Both interact to maintain homeostasis
Usually the nervous system triggers the endocrine

8. Homeostasis
Control of internal environmental conditions is usually through a negative feedback system
When a change is detected in an environmental condition, an action occurs to oppose the change and bring the condition back to normal
Any change in environmental conditions that stimulates a homeostatic response such as this is called a stimulus

9. Negative Feedback
In a negative feedback system, there are 2 main stages involved in reacting to a stimulus:
Detecting the stimulus: The change is detected by a receptor or sensor which then transmits the message to the control centre which coordinates a response
Counteracting the stimulus: The control centre transmits a message to tissues or organs which are able to create a response. These are called effectors.

10. Definitions
Stimulus: A change in the internal or external environment that is able to bring about a response (Stimuli for pl.)
Receptor: Specialised cell which is able to detect changes in a certain internal or external environmental condition and transmits the information to the Control Centre
Control Centre: Usually a component of the Central Nervous System (CNS). It coordinates the action to be taken and transmits the message to the tissue/organ which will respond
Effector: The tissue or organ which responds

11. Receptors Some examples of receptors include:
Chemoreceptors: Detect changes in chemicals (eg. taste, glucose, salt, pH)
Mechanoreceptors: Detect changes in pressure and movement (eg vibrations, sound, muscle stretch, blood pressure)
Photoreceptors: Detect light (eg vision)
Thermoreceptors: Detect changes in temperature

12. Effectors
The types of tissues/organs that normally act as effectors include:
Muscles
Endocrine glands

13. Negative feedback Receptor Control Centre Effector STIMULUS
(Change in environmental condition)
Return of environmental conditions to normal range
Negative Feedback
Receptor
RESPONSE
Transmission of message
Control Centre
Transmission of message
Effector

14. (Decrease in body temperature) Return to normal body temperature
An Example
STIMULUS
(Decrease in body temperature)
Return to normal body temperature
Negative Feedback
Thermoreceptor in skin
RESPONSE:
Shivering, increased metabolism
Transmission of message
Control Centre (CNS)
Transmission of message
Effector:muscles, thyroid

15. In Summary Stimulus Receptor Control Centre Effector Response
Transmission
Negative
Feedback

16. Excess Corrective mechanism
Negative feedback
* Negative feedback is when the response counteracts or cancels out the original stimulus/disturbance
Normal (set point)
Normal (set point)
Negative feedback
Corrective mechanism
Deficiency

17. Nervous & Endocrine
The nervous and endocrine systems are able to maintain homeostasis by:
Detecting changes; and
Initiating a response
This is made possible because cells of the nervous and endocrine systems are able to communicate with other cells through the use of:
Chemical messages – signalling molecules – used by both nervous and endocrine systems
Electrical messages – nerve cells only

18. Signalling molecules Signalling molecules Production in Transport
Targets
Hormones
Endocrine glands
In blood circulation and extracellular fluid
Receptors on target cells
Neurotransmitters
Nerve cells – stored in secretory vesicles in axon terminals
Across synaptic gap
Dendrites of other neurons
Pheromones
Exocrine glands
Released into external environment
Other members of same species – usually a scent