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Homeostasis and Feedback

Why study this?

Cells only work correctly in a narrow range of conditions.  Too much or too little of anything, and they begin to malfunction or die. Homeostasis means keeping all these things in dynamic balance: it keep’s the cell’s environment in a safe zone.

What does life need to control in cells?

Amount of water

Amount of salts

Amount of enzymes

Amount of waste products


Level of acidity (pH level)

Negative feedback

This is the most common way that we control things in our body.

If the level of [anything] gets too high, then reduce the level of [anything]

If the level of [anything] gets too low, then increase the level of [anything]

Positive feedback

Sometimes we do want something to get bigger, then bigger again, and then yet even bigger – up to some huge point. A system that makes this happen is called positive feedback.

Example: Negative feedback in our homes

Let’s examine a practical example of negative feedback in your home: how the toilet works!

A toilet water tank has an outlet, and a float-operated inlet valve.

Toilet flushing GIF feedback

When you flush, water flows out of reservoir (and into 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.

Toilet flushing GIF feedback water rising

This opens inlet valve – lets water flow in

Causes the float to rise, thus closing the inlet valve.

Neg feedback: body temp

You are outside on a hot day. Your body temp is 37 C (98.6 F).  As you exercise your body warms up – which if it continues, would be dangerous.

So your body senses the temp increase and takes corrective action:

Your brain sends nerve signals to your skin: causes some water to be pulled out of your blood, and out to surface of your skin.

(Some salt and urea also comes out with this water.)

As heat evaporates the water, that escaping water vapor carries heat away.

This lowers your temperature back to normal.

Now let’s follow both possible pathways:

Neg feedback: blood pH

You first may want to review: What are acids and bases? What is pH?

Neutral liquids are around pH 7

Acid liquids are below pH 7.

Basic (alkaline) liquids are above pH 7

pH scale examples

Every cell is water-based, with some amount of acid or base.

The amount of acid or base is called pH

Sometimes some body tissue pH levels slightly drop (becomes too acidic )

Correction? the tissue will increases the pH, back up to normal.

Sometimes some body tissue pH levels slightly rise (becomes too basic )

Correction? the tissue drops the pH, back down to normal.


Neg feedback: Regulating blood sugar

If blood sugar too high then 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)

If blood sugar levels too low then Pancrease releases glucagon,

-> causes liver & muscle cells to break down glycogen into glucose

> INCREASES blood sugar levels

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

Why do we need to regulate blood sugar levels?

If [blood sugar] too high

-> damage to blood vessels of hands, feet, eyes. Leads to and open sores, or blindness.

If [blood sugar] too low

-> hypoglycemia (low blood sugar levels) feeling bad, faint, seizures, brain damage

Liver pancreas homeostasis GIF

from dynamicscience.com.au, feedbackloops

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


Positive feedback at a concert

Sing into a microphone. The sound goes to an amplifier.

Most of that amplified sound goes to the crowd – but some goes back into the microphone!

Then the mic amplifies that louder sound, making an even louder sound.

As the diagram below shows, we get a loop which keeps increasing.

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

{ from electronics.howstuffworks.com }

Positive feedback: healing a cut

Blood clots are necessary in some circumstances, and life-threatening in others. We never want random blood clotting: Blood flow would drop, body cells will lose oxygen, and eventually die.

But sometimes blood clotting is necessary: We need it to heal cuts.

What is our blood made of?

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

* white blood cells – fight pathogens.

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

Once a blood vessel is damaged, platelets 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.

platelets stopping bleeding

Source (to be found)

Positive feedback: Contractions in childbirth

the Ferguson reflex

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

oxytocin stimulates stronger contractions.

So more oxytocin released

Which stimulates stronger contractions

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

Ferguson reflex positive feedback oxytocin

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

Learning Standards

LSH-PE.5.5.1 Construct a model that represents the molecular communication that takes place between cells that are in direct cell-to-cell contact, and a model that represents the molecular communication among cells in which molecules are released from one cell and target other cells in the vicinity of the emitting cell (e.g., neurotransmitters, local hormones, growth factors).

LSH-PE.5.5.2 Construct a model to describe the communication between distant cells (e.g., hormones, pheromones, chemotaxis) and the process by which molecular communication between distant cells leads to particular outcomes.

LSH-PE.5.5.4 Construct a simple representation of a feedback mechanism that maintains the internal conditions of a living system within certain limits as the external conditions change. Describe, using the representation, the response of the system to some particular system imbalance (e.g., lack of water causing stomata to contract).

LSH-PE.5.5.5 Construct a representation of the interaction of the endocrine and nervous systems (e.g., hormones and electrochemical impulses) as they interact with other body systems to respond to a change in the environment (e.g., touching a hot stove). Explain how the representation is like and unlike the phenomenon it is representing.

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