1. Homeostasis
Biology 12

2. Joke of the day:

3. Tim and Moby video:

4. What is homeostasis?
Process by which a constant internal environment is maintained despite changes in the external environment
Maintain – keep up
Constant – the same
Internal – inside the body
Environment – surroundings of the body

6. What things in your body need to be kept within a range?
Body Temperature
Blood pressure
Blood pH
O2 and CO2 concentration
Osmoregulation (water balance)
Blood glucose
all images generated by Microsoft Clip Art Gallery

7. How does it work?
All organ systems work together to achieve homeostasis
Nervous and endocrine systems most important in homeostasis
The body has mechanisms to keep the cells in a constant environment.
Negative feedback
Positive Feedback

8. Negative Feedback
Mechanisms that make adjustments to bring the body back within an acceptable range

9. Positive Feedback Less common
Positive feedback occurs when the rate of a process increases as the concentration of the product increases
Ex. Hormones in child birth, blood clotting
Stress
Sensor
Control Center
Intensifies
Effector

10. Example: Clotting
Positive feedback “mini-loops” are built into pathway to speed up production of chemicals needed to form the clot.
But the entire sequence of clotting is a negative feedback pathway

11. Maintaining Homeostasis
Example 1
Maintaining Homeostasis

12. Thermoregulation:
The maintenance of body temperature within a range that enables cells to function efficiently
Ectotherm: an organism whose body temperature changes with the temperature of its surrounding environment (reptiles)
Endotherm: an organism whose body temperature does not change with its surrounding environment (mammals)

13. Humans: All mammals maintain a constant body temperature.
Human beings have a body temperature of about 37ºC.
E.g. If your body is in a hot environment your body temperature is 37ºC
If your body is in a cold environment your body temperature is still 37ºC

14. Response to heat stress:
Thermoreceptors detect an increase in body temperature
Hypothalamus signals to the sweat glands to initiate sweating.
Evaporation of the sweat off the skin causes cooling.

15. Sweating
When your body is hot, sweat glands are stimulated to release sweat.
The liquid sweat turns into a gas (it evaporates)
To do this, it needs heat.
It gets that heat from your skin.
As your skin loses heat, it cools down.

16. Vasodilation: Blood carries most of the heat energy around your body.
The hypothalamus also sends message to blood vessels in the skin causing them to dilate.
Dilation allows for more blood flow to the skin.
Heat from the blood is lost to the skin so blood can return to core of your body & cool the internal organs.

17. Summary Heat Stress Thermoreceptors monitor body temperature
Hypothalamus turns on cooling system
Skin blood vessels dilate and sweat glands initiate sweating
Result: body temperature decreases; hypothalamus turns off cooling system

18. Response to cold stress:
Thermoreceptors detect a decrease in body temperature
The hypothalamus sends message to blood vessels in the skin causing them to constrict.
Messages are also sent to smooth and skeletal muscles

19. Vasoconstriction: This is the opposite of vasodilation
The capillaries underneath your skin get constricted (smaller diameter).
This takes the blood away from the surface of the skin so less heat can be lost.

20. Shivering and “goose bumps”:
Smooth muscle contracts the muscle that surrounds the hair follicles in your skin causing the hair to “stand on end” trapping warm air.
Skeletal muscle to contract causing shivering and increasing your metabolism to make heat.

21. Summary Cold Stress Thermoreceptors monitor body temperature
Hypothalamus turns on heating system
Skin blood vessels constrict and skeletal muscles contract
Result: body temperature increases; hypothalamus turns off heating system

22. To Do :
READ p P
Page 341
Questions: 1,2,3,6,7,8