1. Stimulus/Response

2. Internal Stimulus A stimulus that comes from inside your body.
EX. hunger pains, emotions, and a full bladder.

3. External Stimulus Any stimulus from outside the body.
Ex. flashing lights, scary or sad events.

4. Response A reaction to a stimulus.
When you are hungry you eat, when you are sad you cry, when you are scared you run…

5. Internal Stimuli & Responses
Think of all the changing conditions that an organism might need to respond to. Here’s
just a sample:
excited
thirsty
And here are some specific
responses to these internal
stimuli.
Forget shivering…
Where’s my hoodie?!?
roots absorb H2O
Rex wags tail
too hot
sleepy
Joe sweats
yawn
pain
tired
scream
lion rests
hungry
owl grabs mouse
too cold
afraid
Joe shivers
kitten hides

6. External Stimuli & Responses
External stimuli can be a little more complicated. Let’s see how.
EXTERNAL STIMULUS RESPONSE
moonlight = external stimulus
howling = response
Bright sunlight
Squinting (ok, this one is pretty simple)
Shorter days, colder
temperatures
Trees drop their leaves in autumn
Dogs shed their thick coats in late spring
Increasingly hot weather
Sunlight, motion of the Sun.
Plants grow toward the Sun’s position.
Hibernation (chipmunks sleep through cold months)
Migration (geese fly to warmer weather & easy food)
Dormancy (grass turns brown in winter instead of
trying to grow & stay green)
Difficult survival conditions
coming up
There really are countless more examples, from as simple to an itch on your arm to the
complicated conditions that cause plants to realize winter is over and fire back into life.

7. #22 Write. Stimulus (changing condition) and Response
Internal Stimulus – detected inside an organism
ex: hunger, emotions, temperature
External Stimulus – detected outside
ex: light, dark, climate change
Organisms respond to these stimuli to help increase their chances at
survival. When you learn more about the human body, you might be
astonished to discover the numerous stimuli, both internal and external,
that your nerves and brain can respond to.

8. Looks like Joe is prepared to handle some stimuli…
Yep, sure am.
And don’t think I forgot to pack one other thing…

9. And That’s…
THE END!

10. A state of balance in the body
Notes
Homeostasis
A state of balance in the body
Whoa

11. Definition of Homeostasis
homeo = same; stasis = standing
Homeostasis is the term we use to describe the constant state of the internal environment.
Homeostasis is a state of balance in the body.
The processes and activities that help to maintain homeostasis are referred to as homeostatic mechanisms.

12. Introduction
You are exposed to ever changing environmental conditions. For example, you may walk out of an air conditioned room into the hot summer sun.
However the cells in your body work best when their surroundings are kept constant. Your body has many mechanisms that keep the cells surroundings constant even though your external environment is changing. This is homeostasis.
Homeostasis is very important because when it fails you become ill and may die.  

13. Maintaining Homeostasis
Chemistry is the reason why we must maintain homeostasis.
Biochemical processes (the chemical reactions) that occur within us) are vital to life and occur efficiently only within a limited temperature range and at a specific pH.

14. Internal Communication
The body must have good internal communication, using the endocrine and nervous systems, to maintain homeostasis.
Feedback inhibition limits the operation of a system or causes it to shut down when it senses too much of a certain product (such as water, glucose, salt, heat, CO2 etc.)
It will cause the system to “turn back on” when there is too little of this product.

15. A Temperature Control System
To help us understand homeostasis in living organisms, let us first look at a non-living system. We will use a temperature control system for a room which has many similar features to homeostatic mechanisms…. Click on the thermostat.

16. Human Body Temperature Control
You have just studied how the temperature of a room can be controlled. Now you will examine how similar mechanisms operate to control body temperature in humans. Go to this web address…. Click on the hot man.

17. A Review Example: thermostatic heating system in a home
Components of an automatic control system
Variable is the characteristic of the internal environment that is controlled by this mechanism (internal temp in this example).
Sensor (receptor) detects changes in variable and feeds that information back to the integrator (control center) (thermometer in this example).

18. Summary
Changes in temp are detected by thermometer, which feeds info about the actual temp back to thermostat.
Thermostat has been previously set to ideal (set point) value; thermostat compares actual value to set point value and sends signal to furnace.
Furnace fires up and changes the internal temp back toward set point.
(Furnace will shut down when thermostat determines actual temp is now higher than set point temp).

19. Human Example of Negative Feedback
Human example: shivering in response to cooling of body during cold weather or sweating when their core temperature gets too hot.
Homeostasis allows an organism to remain in balance with its environment.
If homeostasis is not maintained, it can harm or kill the organism.