1. KEY CONCEPT Homeostasis is the regulation and maintenance of the internal environment.

2. A Major Biological Theme: Organisms must keep their cells in specific conditions in order to survive in diverse environments.
Homeostasis is the maintenance of constant internal conditions – it makes sure our cells are kept healthy.
Our bodies (and other creatures) must be able to modify their activities to stay healthy.
Examples: Avoiding temperature changes, maintaining blood sugar levels, staying hydrated, heart rate, calcium balance, and many more.

3. Homeostasis
Homeostasis – ability to maintain a relatively stable internal environment in an ever-changing outside world
Your internal environment includes the amount of important molecules in your system (such as oxygen, glucose, calcium, water, vitamin D, etc.) along with the amount of physical factors within your body (like your blood pressure, body temperature, and so on)
The internal environment of the body is in a state of dynamic equilibrium (“changing balance”) – the exact amounts of water, nutrients, and temperature levels vary from moment to moment (they’re dynamic), but the overall levels remain constant (they’re in equilibrium)
Chemical, thermal, and neural factors interact to maintain homeostasis

4. WARNING:
Many textbooks use body temperature (thermoregulation) as their primary example of homeostasis – it is a great example, but the books are not always clear that it is only one kind of homeostasis, and that all kinds of homeostasis are equally important for health!

5. Note: Thermoregulation is just ONE example of homeostasis!

6. Note: Thermoregulation is just ONE example of homeostasis!

7. Question: What other actions does the body take to stay warm?
Note: Thermoregulation is just ONE example of homeostasis!

8. Negative feedback loops are necessary for homeostasis.
Homeostasis is usually maintained through negative feedback.
Negative feedback means that any change is canceled out – it’s a good thing for our survival!
Negative feedback systems return a condition to its normal value (its “set point”).

9. Negative Feedback
In negative feedback systems, the output shuts off the original stimulus
Negative feedback does the opposite of whatever the change was
Homeostasis usually operates by negative feedback to bring the body back into balance
Examples: Regulation of room temperature, body temperature, blood sugar levels, calcium balance, oxygen and CO2 levels….

10. Negative feedback loops are necessary for homeostasis.
Feedback compares current conditions to set ranges.
Negative feedback counteracts change.
Negative Feedback Loop
O2 / CO2 level returns to normal
Holding breath, CO2 levels rise,
Control system forces exhale, inhale

11. Positive feedback increases change.
Positive feedback happens when a change builds up on itself, increasing the change instead of blocking it.
Positive feedback increases change.
platelets
fibrin
white blood cell
red blood cell
blood vessel
clot
Torn blood vessel stimulates release of clotting factors
growth hormones stimulate cell division

12. Not used for homeostasis Example: Regulation of blood clotting
Positive Feedback
In positive feedback systems, the output enhances or exaggerates the original stimulus – the event builds on itself more and more
Not used for homeostasis
Example: Regulation of blood clotting
Figure 1.6

13. Behaviors and adaptations can help maintain homeostasis.
Looking for a warm place to sleep is a behavior to retain warmth, while having hollow hairs that trap heat is an inherited adaptation to stay warm……

14. Control Center Communication System Sensor Target
Parts of the brain that compare current conditions to ideal conditions (aka, the set point).
If conditions are not ideal, they send instructions to get the body in balance again.
Control Center
Brainstem
Communication System
Target
Sensor
Nerves that send messages from one part of the body to another.
Specialized cells that recognize bodily conditions.
Muscle or glands that follow instructions from the control center.

15. Homeostatic Control Mechanisms
Sensor
Control systems help maintain homeostasis.
sensors gather data
control center receives data, sends messages
communication system delivers messages to target organs, tissues
targets respond to change
Control Center
Target

16. Homeostatic Control Mechanisms
Variables produce a change in the body
The three interdependent components of homeostatic mechanisms:
Receptor (or affector or sensor) – monitors the environment and responds to changes (stimuli) by sending messages to the control center
Control center (often in brainstem)– compares current conditions to the set point (ideal conditions); sends messages to effectors if needed
Effector (or target) – follows control center’s instructions to act on body and restore balance – muscles and glands - can be automatic or voluntary actions

17. Variable Out of Balance Variable In Balance Variable In Balance
Change
detected
by receptor
Stimulus:
Produces
change
in variable
Input:
Information
sent along
afferent
pathway to
Receptor (sensor)
Effector
Control
center
Variable (in homeostasis)
Response of
effector feeds
back to
influence
magnitude of
stimulus and
returns variable
to homeostasis
Output:
Information sent
along efferent
Imbalance 2 3 4 5 1
Variable Out of Balance
Variable In Balance
Variable In Balance
Figure 1.4

18. Signal wire turns
heater off
Set
point
Control center
(thermostat)
Receptor-sensor
(thermometer in
Thermostat)
Heater
off
Effector
(heater)
Stimulus:
rising room
temperature
Response;
temperature
drops
Imbalance
Balance
Response;
temperature
rises
Stimulus:
dropping room
temperature
Imbalance
Heater on
Effector
(heater)
Set
point
Receptor-sensor
(thermometer in
Thermostat)
Signal
wire turns
heater on
Control center
(thermostat)
Figure 1.5

19. Conditions within the body must remain within a narrow range.
Homeostasis involves keeping the internal environment within set ranges.

20. A disruption of homeostasis can be harmful.
Homeostasis can be disrupted for several reasons.
sensors fail
targets do not receive messages
injury
illness

21. Homeostatic Imbalance
Disturbance of homeostasis or the body’s normal equilibrium makes us feel sick
Overwhelming the usual negative feedback mechanisms allows destructive positive feedback mechanisms to take over
This results in diseases or disorders

22. Short-term disruption usually causes little or no harm.
Long-term disruptions can cause more damage.
Diabetes is a serious long-term disruption of homeostasis. 1 2 3 4
Pancreas cells attacked;insulin declines
Blood glucose levels rise,
Body burns fat; blood more acidic,
Cells impaired; all organs damaged.
pancreas
fats
kidneys