1. HOMEOSTASIS

2. 2nd Law of Thermodynamics
One of the fundamental laws of nature.
It states that the universe and everything in it is always moving in the direction of increasing disorder

3. 2nd Law of Thermodynamics
Yet, life appears to be the exact opposite of this, for life is very organized and living organisms maintain that organization until they die.
This is this property that separates life from non-life.

4. HOMEO – THE SAME, STASIS – TO STAY
The ability of life to maintain a constant, stable, and organized internal environment is called HOMEOSTASIS.
Along with evolution, this is one of the guiding principles of modern biology.

5. Even the simplest life form, an amoeba, maintains a distinctly ordered internal environment separate from its external environment. Why?
The more complex the life form, the more elaborate and efficient the homeostatic mechanisms they have evolved are.

6. In humans, we have (as much as possible) physically separated all of our cells from the external environment; thus, improving the quality of our ‘internal seawater’.
We have evolved ‘physiology’ which allows us to monitor and adjust our internal environment within very narrow parameters.

7. Homeostatic Mechanisms
Sensitive receptors monitor each condition under homeostatic control
For example:
37oC
Blood 120/80
Sugar 0.07%
Osmolarity (salt 0.1M 7

8. 2. These monitors are linked in some way (usually by nerves or hormones) to mechanisms capable of correcting changes in the particular state (ie: warm the body temperature).
3. A linked seriesof receptors and corrective mechanisms is called a FEEDBACK LOOP.

9. Feedback Loops There are two types of feedback loops:
Negative Feedback Loops
Positive Feedback Loops

10. Negative Feedback Loops
This type of a feedback loop occurs where the output from some body system inhibits itself (shuts itself off) at a certain set point.
Example: with both a thermostat in the furnace at home or in the brain (hypothalamus)
When the heat increases, the furnace is shut off
When the heat decreases, the furnace is turned on
In humans temperature homeostasis is controlled by the thermoregulatory centre in the hypothalamus. It receives input from two sets of thermoreceptors: receptors in the hypothalamus itself monitor the temperature of the blood as it passes through the brain (the core temperature), and receptors in the skin monitor the external temperature. Both pieces of information are needed so that the body can make appropriate adjustments. The thermoregulatory centre sends impulses to several different effectors to adjust body temperature:

12. The hypothalamus maintains a set point of 37. 5 ± 0
The hypothalamus maintains a set point of 37.5 ± 0.5 °C in most mammals.
The temperature is regulated within a range determined by a ‘high’ and ‘low’ set point (± 0.5 °C )
Whenever a change occurs in a system, the change automatically causes a corrective mechanism to start, which reverses the original change and brings the system back to normal.

13. How It Works
STIMULUS

14. How It Works
STIMULUS

15. How It Works
STIMULUS
RECEPTOR

16. How It Works
STIMULUS
RECEPTOR

17. How It Works
STIMULUS
RECEPTOR
REGULATORY CENTER

18. How It Works
STIMULUS
RECEPTOR
REGULATORY CENTER

19. How It Works
STIMULUS
RECEPTOR
REGULATORY CENTER
EFFECTOR

20. How It Works
STIMULUS
RECEPTOR
REGULATORY CENTER
EFFECTOR

21. How It Works
STIMULUS
RECEPTOR
REGULATORY CENTER
RESPONSE
EFFECTOR

22. How It Works
STIMULUS
RECEPTOR
REGULATORY CENTER
RESPONSE
EFFECTOR

23. How It Works
STIMULUS
RESPONSE
EFFECTOR
REGULATORY CENTER
RECEPTOR

24. STIMULUS: Body Temperature rises above 37oC
How It Works: an example
STIMULUS: Body Temperature rises above 37oC

25. STIMULUS: Body Temperature rises above 37oC
How It Works: an example
STIMULUS: Body Temperature rises above 37oC

26. STIMULUS: Body Temperature rises above 37oC RECEPTOR: thermoreceptor
How It Works: an example
STIMULUS: Body Temperature rises above 37oC
RECEPTOR: thermoreceptor

27. STIMULUS: Body Temperature rises above 37oC RECEPTOR: thermoreceptor
How It Works: an example
STIMULUS: Body Temperature rises above 37oC
RECEPTOR: thermoreceptor

28. How It Works: an example
STIMULUS: Body Temperature rises above 37oC
RECEPTOR: thermoreceptor
REGULATORY CENTER: hypothalamus

29. How It Works: an example
STIMULUS: Body Temperature rises above 37oC
RECEPTOR: thermoreceptor
REGULATORY CENTER: hypothalamus

30. How It Works: an example
STIMULUS: Body Temperature rises above 37oC
RECEPTOR: thermoreceptor
REGULATORY CENTER: hypothalamus
EFFECTOR: smooth muscle in arterioles, sweat glands, muscles in skin, skeletal muscles, and thyroid gland

31. How It Works: an example
STIMULUS: Body Temperature rises above 37oC
RECEPTOR: thermoreceptor
REGULATORY CENTER: hypothalamus
EFFECTOR: smooth muscle in arterioles, sweat glands, muscles in skin, skeletal muscles, and thyroid gland

32. How It Works: an example
STIMULUS: Body Temperature rises above 37oC
RECEPTOR: thermoreceptor
REGULATORY CENTER: hypothalamus
RESPONSE: arterioles dilate (blood moves to extremities), sweat, thyroid gland decreases metabolism
EFFECTOR: smooth muscle in arterioles, sweat glands, muscles in skin, skeletal muscles, and thyroid gland

33. How It Works: an example
STIMULUS: Body Temperature rises above 37oC
RECEPTOR: thermoreceptor
REGULATORY CENTER: hypothalamus
RESPONSE: arterioles dilate (blood moves to extremities), sweat, thyroid gland decreases metabolism
EFFECTOR: smooth muscle in arterioles, sweat glands, muscles in skin, skeletal muscles, and thyroid gland

34. How It Works: an example
STIMULUS: Body Temperature rises above 37oC
RECEPTOR: thermoreceptor
REGULATORY CENTER: hypothalamus
RESPONSE: arterioles dilate (blood moves to extremities), sweat, thyroid gland decreases metabolism
EFFECTOR: smooth muscle in arterioles, sweat glands, muscles in skin, skeletal muscles, and thyroid gland

35. Another Example:
Blood Sugar Control

36. Positive Feedback Loops
This type of a feedback loop occurs when the product of one pathway stimulates or causes another pathway to increase activity.
Example: Labour (the pressure of the babies head on the uterus) stimulates the pituitary gland in the brain to release the hormone oxytocin.
The oxytocin causes the uterus to contract, which leads to increased pressure.
Contractions cause the pituitary to produce more oxytocin and the cycle continues until you give birth.

37. Positive Feedback Loops
Positive feedback is a mechanism that brings about an ever greater change in the SAME DIRECTION.
This type of loop helps the body to complete a process that has a definite cut off point, such as:
a. Blood clotting
b. Digesting proteins in the stomach
c. Birth
d. Body temperature during a fever
The feedback loop keeps going until the stimulus that initiated the loop stops.

38. Positive Feedback Loops
Ex: during a fever, a presence of a pathogen is the stimulus that starts the positive feedback loop.
Chemicals called pyrogens are released by the body
These cause the body temperature to increase
White blood cells work better under warmer temperatures, so they can do a better job of killing off the pathogen
Pyrogens will keep being released (and body temperature keeps rising) until the stimulus (pathogen) is gone.
Note: your body will also keep doing negative feedback at the same time, which is why you sweat so much when you have a fever: negative feedback is trying to cool you down!!