1. Introduction to
Homeostasis

2. What is Homeostasis?
= The body’s attempt to maintain “normal” levels within your body
Homeostasis is often referred to as a dynamic equilibrium- which is a mechanism to ensure that all body systems function within an acceptable range to sustain life.

3. What is Homeostasis?
Homeostasis depends on the action and interaction of a number of body systems to maintain a range of conditions within which the body can best operate.
Because the external environment is constantly changing and homeostatic reactions respond to the change and bring the body back to a given set point, it is often referred to as a dynamic equilibrium.
Dynamic equilibrium  a condition that remains stable within fluctuating limits.

4. What is Homeostasis?
In order to function properly, homeostatic mechanisms are required to:
1. Regulate respiratory gases
2. Maintain water and salt balance
3. Regulate energy / nutrient supply
4. Maintain constant body temperature
5. Protect against pathogens
6. Make repairs when injured
KNOW THESE!

5. Negative feedback
= A feedback in which the system responds in an opposite direction to the disturbance.
Your body responds in such a way to reverse the change that is happening.
This way, small changes don’t become too large
Negative feedback is the primary homeostatic mechanism that our bodies use.

6. Homeostatic control systems
3 components:
Monitor (sensor)
sends a signal to the coordinating centre that the normal levels have changed.
Coordinating centre
Usually the brain
Regulator (effector)
The coordinating centre then messages the regulator which helps restore normal balance
Monitor
Coordinating centre
Regulator
Normal balance
Change in balance

7. Core Temperature
SARS Virus – Causes high fever
Humans have a normal temperature of around 36.2 to 37.2 degrees Celsius
Body temperature goes above normal temperature then likely suffering from an infection and your body had to raise its temperature to fight off the infection.
If your body goes below this range it indicates hypothermia. Therefore if left untreated it could lead to cell damage and possibly death.

8. Homeostasis Example: household thermostat
Room temperature is set to 22 degrees Celsius. When the temperature falls below the “normal” temperature of 22 degrees, the thermostat recognizes change in “normal” temperature and switches on the furnace. When the thermometer detects a temperature above the “normal”, the thermostat switches off the furnace.

10. Homeostasis 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
Whole control system is called a negative feedback system

11. Practice
In the above example, what would be the bodily equivalent to the monitor, coordinating system, and the effector? 1. 2. 3.
*JUST SEE IF YOU CAN ANSWER THESE IN YOUR HEAD FOR NOW

12. Negative Feedback Example: Thermoregulation
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.

13. Negative Feedback Example: Thermoregulation
Heat Stress
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.

14. Negative Feedback Example: Thermoregulation
Cold stress
Thermoreceptors message the hypothalamus
Hypothalamus sends a message via the nerves to:
Arterioles of the skin cause smooth muscles to contract, constricting arterioles & limiting blood flow =reduced heat loss from the skin and retains heat in the body.
Smooth muscle contract 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.

16. Practice
In the above example, what would be the bodily equivalent to the monitor, coordinating system, and the effector?
1. nerve receptors in skin / core
2. temperature centres (hypothalamus) in brain
3. Too hot?
skin blood vessels dilate and sweat glands initiate sweating
Too cold?
muscles shiver, hair follicles close, blood vessels constrict , mitochondria ↑ metabolism

17. Positive Feedback Systems
= A feedback in which the system responds in the same direction as the disturbance.
When a change to the ‘normal’ happens, your body reacts by amplifying the change that is happening
A positive feedback mechanism can sometimes be harmful
Ex: when a fever causes metabolic changes that push the fever still higher. Death occurs at a body temperature of about 45oC
Still, positive feedback loops such as those involved in blood clotting, the stomach’s digestion of protein and childbirth assist the body in completing a process that has a definite cutoff point.

18. The head of the baby begins to push on the cervix, stimulating the sensors there.
These sensors send nerve impulses to the brain (coordinating center).
The brain causes the pituitary gland (effector) to secrete the hormone oxytocin.
Oxytocin travels in the blood and causes the uterus to contract. As labor continues, the cervix is ever more stimulated, and uterine contractions become stronger until birth occurs.

19. More Examples of Positive Feedback Systems
One example is the onset of contractions in childbirth, known as the Ferguson reflex. When a contraction occurs, the hormone oxytocin causes a nerve stimulus, which stimulates the hypothalamus to produce more oxytocin, which increases uterine contractions. This results in contractions increasing in amplitude and frequency.
Another example is the process of blood clotting. The loop is initiated when injured tissue releases signal chemicals that activate platelets in the blood. An activated platelet releases chemicals to activate more platelets, causing a rapid cascade and the formation of a blood clot.
Lactation also involves positive feedback in that as the baby suckles on the nipple there is a nerve response into the spinal cord and up into the hypothalamus of the brain, which then stimulates the pituitary gland to produce more prolactin to produce more milk.
A spike in estrogen during the follicular phase of the menstrual cycle causes ovulation.
The generation of nerve signals is another example, in which the membrane of a nerve fibre causes slight leakage of sodium ions through sodium channels, resulting in a change in the membrane potential, which in turn causes more opening of channels, and so on. So a slight initial leakage results in an explosion of sodium leakage which creates the nerve action potential.
In excitation–contraction coupling of the heart, an increase in intracellular calcium ions to the cardiac myocyte is detected by ryanodine receptors in the membrane of the sarcoplasmic reticulum which transport calcium out into the cytosol in a positive feedback physiological response.

20. Just to clarify…
Negative feedback systems and positive feedback systems are BOTH ESSENTIAL for organisms to cope with enviornments.
Yes, negative feedback systems are more common, but negative feedback is NOT the “good one” and positive feedback is NOT the “bad one”. They are just different.

21. Did you know?
Many diseases are a result of disturbance of homeostasis, a condition known as homeostatic imbalance.
As it ages, every organism will lose efficiency in its control systems. The inefficiencies gradually result in an unstable internal environment that increases the risk for illness.
In addition, homeostatic imbalance is also responsible for the physical changes associated with aging.

22. Test notice
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23. Homeostasis video 18 mins

24. If you’re done:
Put your worksheets in the INBOX in the back of the room
Think of a question that can be answered by SCIENCE!!!