1. Introduction to
Homeostasis

2. What is Homeostasis?
Ability of an organism to maintain a stable internal environment, despite changes to its internal or external environment
All organ systems work together to achieve homeostasis
all organisms do this!

3. Organization of a Human
cells  tissues  organs  organ systems

6. What things in your body need to be kept within a normal range?
Body Temperature
Blood pressure
Blood pH
O2 and CO2 concentration
Osmoregulation-Water balance
Blood glucose
Calcium levels

7. How does homeostasis work?
Feedback pathways
A cellular relay race! (dynamic equilibrium)
organs and structures communicate with each other in response to changes in the body
Keeps levels of certain processes within a normal range
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8. Cellular Relay Race Stimulus Receptor Integrating center Effector
Response
Reverses the stimulus
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9. Homeostatic control systems
3 components:
Monitor (receptors)
Monitors send signals through sensory pathways to the coordinating centre
Coordinating centre (brain)
The coordinating centre sends signals through the motor pathway to the regulator to restore normal balance
Regulator (muscles, glands etc.)
Monitor
Coordinating centre
Regulator
Normal balance
Change in balance

10. Negative feedback
Process by which a mechanism is activated to restore conditions to their original state
It ensures that small changes don’t become too large.
Why is a thermostat a negative feedback system?
Conserves resources
Cellular Materials
Energy (ATP)

11. Homeostasis Example: household thermostat
Room T at 22 oC
T below: thermostat turns on the furnace
T above: thermostat turns off the furnace

12. Example: household thermostat
Monitor- Thermometer
Detects decrease in temperature
Coordinating centre- Thermostat switches on furnace
Regulator- Furnace
Thermostat detects temperature increases over “normal”
Furnace turns off

13. Dueling Mechanisms What goes up, must come down!
1. Thermoregulation
Sweating (cooling) vs. shivering (warming)
2. Sugar levels
insulin vs. glucagon
3. Osmoregulation (later!)
Hypotonic vs. hypertonic
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14. Thermoregulation Core Temperature
Humans have a normal temperature of around 36.2 to 37.2 degrees Celsius
Above (hyperthermia): infection
Below (hypothermia): cell death

16. a) Heat stress
monitor
Thermoreceptors of the peripheral nervous system detect an increase in body temperature
send message via sensory neurons to brain (central nervous system, CNS)

17. ii. coordinate
Hypothalamus (CNS) signals via motor nerves to sweat glands

18. sweat glands initiate sweating
regulate
sweat glands initiate sweating
evaporation of sweat off the skin causes cooling
blood vessels in the skin dilate
dilation allows for more blood flow to the skin
heat from blood is lost to the skin so blood can return to core of your body & cool the internal organs.

19. Result: body temperature returns to normal; hypothalamus turns off cooling system

20. A Cooling Glove  The heat extractor increases heat loss and allows the body to perform at a higher level in severe conditions.

21. b) Cold stress
monitor
- thermoreceptors message the hypothalamus using sensory neurons
ii. coordinate
hypothalamus sends a message via the motor neurons

22. iii. regulate
Arterioles of the skin constrict, therefore limiting blood flow and reducing heat loss from the skin

24. Smooth muscles that surrounds the hair follicles in your skin contract, causing the hair to “stand on end” trapping warm air.
Skeletal muscles contract causing shivering and increasing your metabolism to make heat.

25. Result: body temperature increases and hypothalamus turns off the heating system

26. 2. blood sugar levels

27. Dueling Hormones What goes up, must come down!
Insulin
Produced by -cells of the pancreas
Released into circulatory system when blood glucose is high
Facilitates the transport of glucose into target cells
Glucagon
Produced by -cells of the pancreas
Released into the circulatory system when blood glucose is low
Signals the liver to break down glycogen into simple glucose
Life Sciences-HHMI Outreach. Copyright 2009 President and Fellows of Harvard College.
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28. Positive Feedback

29. Food for thought…
On average, how many organ systems are involved in each of the processes we’ve explored?
Are there any organ systems that you see in all of these processes?
What might happen to these pathways if just one system was not functioning properly?
Life Sciences-HHMI Outreach. Copyright 2009 President and Fellows of Harvard College.
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