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Homeostasis and Feedback


Organisms need to control things like body temperature, water level, salt level, and sugar level, to keep them from changing too much.

The process of keeping levels within a safe zone is called homeostasis. It is effected through a process called feedback.

Feedback is when the effect of an activity is connected to its cause

1. Negative feedback pushes a system back to its initial condition

2. Positive feedback pushes a system away from its initial condition

Homeostasis is how the body keeps conditions inside it the same. It is the maintenance of a constant internal environment.

Where is negative feedback in your life?

You are outside on a hot summer day. Your internal body temperature is 37o C (98.6 o F). But as you exercise outdoors, your body warms up above this temperature. That’s bad.

Fortunately your body senses this, and takes corrective action – your body begins to push its’ temperature back to it’s initial condition.

You sweat: you bring water from your blood vessels, out to the surface of your skin.

As the water evaporates, it carries heat off of your body (and up into the air)

Sweating reduces your heat, returning your temperature back to normal.

{ US forest service, http://www.fs.fed.us/t-d/pubs/htmlpubs/htm10512316/ }


{ http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=jpg::640::480::/sites/dl/free/0072919345/63806/0751l.jpg::Homeostasis%20and%20temperature%20control }

Controlling blood pH (acid & base levels) with negative feedback

Sometimes blood pH levels drops (becomes too acidic )

So the body increases the pH, back up to normal.

Sometimes blood pH levels rises (becomes too basic )

So the body drops the pH, down up to normal.

{ http://www.ctec-chemicals.com/en/concept-ph }

Where is positive feedback happening in your life?

Normally, blood clotting is bad. You don’t want your blood to thicken and clot up. Blood flow would drop, and your body cells will lose oxygen, and eventually die.  But sometimes blood clotting is necessary.

What is our blood made of?

* red blood cells – carry O2 to cells, and CO2 away from cells

* white blood cells – fight pathogens. Part of our immune system

* platelet cells – These bind together when they recognize damaged blood vessels.

If you get cut then you could bleed out, which is very dangerous. You need a way to stop this.

Once a blood vessel is damaged, platelets will cling to the injured site.

Platelets then release chemical signals that attract more platelets

These added platelets release even more chemical signals

These attract even more platelets. And so on, and so on… this is a positive feedback loop!

Very quickly, the platelets clot up and seal the wound.


Positive feedback: Contractions in childbirth: the Ferguson reflex

When contraction occurs, oxytocin (hormone) is released into the body

Oxytocin stimulates stronger contractions.

Results in more oxytocin being released

Which stimulates stronger contractions

(the cycle repeats, until it is broken by birthing)

Ferguson reflex positive feedback oxytocin

Where do we find positive feedback at a musical event?

You sing into a microphone. That sounds goes to an amplifier.

The sound that comes out of the amplifier is louder, and is supposed to go out to the crowd.

But what happens if some of that sound goes back into the microphone?

Then a positive feedback loop is created.

Within a fraction of a second, a very loud noise is created.

{ http://electronics.howstuffworks.com/gadgets/audio-music/question263.htm }

Where do we find negative feedback in our homes? Toilets
Consider a toilet water tank. It has an outlet, and a float-operated inlet valve.

When you flush the toilet, water flows out of the reservoir (and into the bowl)

But now the reservoir water level is too low. We need to bring it back up!

When water flows out of the reservoir then the float will sink.

This opens the inlet valve – which causes more water to flow in

This causes the float to rise up, thus closing the inlet valve.



Regulating sugar in your blood:

1. If blood sugar levels gets too high then the PANCREAS releases insulin,

Causes muscle and fat cells to take in more sugar -> DECREASES blood sugar levels

Promotes glycolysis (using sugar to make ATP molecules, for energy)

Promotes protein synthesis (making new protein)

2. If blood sugar levels get too low then the PANCREAS releases glucagon,

In the liver, and muscle cells. it causes cells to break down glycogen in to glucose -> INCREASES blood sugar levels

STOPS muscle and fat cells taking in more sugar
STOPS glycolysis, in those cells
STOPS protein synthesis in cells

(image from http://www.imm.dtu.dk/~jbjo/diabetescontrol.html )

Glucose Pancreas Homeostasis with graph

Why do we need to regulate blood sugar levels?

If blood sugar levels are too high: damage to blood vessels, tissue dies.

-> Leads to damage of hands, feet, and open sores, even blindness

If blood sugar levels are too low then: hypoglycemia (low blood sugar levels)

-> feeling bad, faint, to seizures,  falling unconscious brain damage

Sample questions

When astronauts are in low-gravity environments, their bodies begin to release stored calcium. As a result, which of the following most likely occurs when an astronaut returns to Earth?
A. The risk of inflamed tendons increases.
B. The chance of breaking a bone increases.
C. The stomach’s level of functioning decreases.
D. The blood’s ability to carry oxygen decreases.


*. At high altitudes, air is less dense than at sea level because of decreased air pressure. This means that a person who ascends to high altitudes takes in fewer oxygen molecules per breath.

a. Describe and explain an immediate response that occurs in the respiratory system when a person first reaches high altitudes.

b. Describe and explain an immediate response that occurs in the circulatory system when a person first reaches high altitudes.

If a person lives at high altitudes for an extended period of time, other body responses occur. One response is an increase in the number of red blood cells.

c. Explain how this response is helpful to the body.

At high altitudes, air temperature is also generally colder than at sea level.

d. Describe and explain one internal body response that occurs when a person has been
outside long enough for his core temperature to drop.


Learning Standards



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