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Lion Track icon Lion Den » AP » AP 1 » Mini Lessons »Homeostasis

Learning Outline

Mini Lesson: Homeostasis

The body's balance


Introduction to homeostasis

homeo = same; stasis = standing

Homeostasis = relative constancy of the internal fluid environment slide slide

The concept was first articulated by [Frenchman] Claude Bernard in 1860s

"I think I was the first to urge the belief that animals have really two environments: a milieu extérieur in which the organism is situated, and a milieu intérieur in which the tissue elements live. The living organism does not really exist in the milieu extérieur (the atmosphere it breathes, salt or fresh water if that is the element) but in the liquid milieu intérieur formed by the circulating organic liquid which surrounds and bathes all the tissue elements; this is the lymph or plasma, the liquid part of the blood which, in the higher animals, is diffused through the tissues and forms the ensemble of the intercellular liquids and is the basis of all local nutrition and the common factor of all elementary exchanges. A complex organism should be looked upon as an assemblage of simple organisms which are the anatomical elements that live in the liquid milieu intérieur." — Claude Bernard

Claude Bernard

Claude Bernard (1813-1877)

The term "homeostasis" was first used by [American] Walter Bradford Cannon in 1920s

"The coordinated physiological processes which maintain most of the steady states in the organism are so complex and so peculiar to living beings--involving, as they may, the brain and nerves, the heart, lungs, kidneys and spleen, all working cooperatively—that I have suggested a special designation for these states, homeostasis. The word does not imply something set and immobile, a stagnation. It means a condition—a condition which may vary, but which is relatively constant." —Walter B. Cannon

Cannon was the one who really established homeostasis as a unifying concept of human physiology

Walter Bradford Cannon

Walter B. Cannon (1871-1945)

Fishbowl model

Fish are like the cells of the [multicellular] body slide slide

Internal environment of the fishbowl is a fluid that must be maintained in relatively constant conditions for the fish to survive in health

In the human body, physiological mechanisms keep oxygen up and carbon dioxide down (respiratory system), nutrients up (digestive system), wastes low (kidneys), temperature constant (muscles, sweat, etc)


Fishbowl Model

Fishbowl Human Body



Body fluid 

Internal environment

Glass bowl


Barrier (internal vs. external)



Stay alive

Air pump


Keep O2 level constant (high)



Keep nitrogen wastes constant (low)



Keep temperature constant (high)


Digestive system

Keep nutrient levels contant (high)

Everything working together

Relatively constant conditions

To visit Kevin's fish Clyde click here
For a REAL fish story click here for a SCARY fish story click here


Engineered control system (thermostat) model

Example: thermostatic heating system in a home

Components of an automatic control system

Changes in temp are detected by thermometer, which feeds info about the actual temp back to thermostat, which has been previously set to ideal (setpoint) value; thermostat compares actual value to setpoint value and sends signal to furnace, which fires up and changes the internal temp back toward setpoint (furnace will shut down when thermostat determines actual temp is now higher than setpoint temp)

Negative feedback

Positive feedback

feedback thermostat

Click to enlarge

feedback animation

feedback body

Click to enlarge

Engineered Room Control

Feedback Loop

Human Body Thermoregulation

Room Temperature

The characteristic that is controlled

Body temperature


Detects the value of the variable

Nerve receptors


Compares the actual value of the variable to a pre-determined setpoint value

Brain (hypothalamus)


Instrument that has an effect on (changes) the variable

Muscles (shivering)

Wallenda model

The Flying Wallendas are a family of circus performers famous for their high-wire acts (this model is that of a high-wire artist)

Karl Wallenda (former patriarch of the family acts) made "sky walks" famous

If the video player above is blank (or not visible) click here

Elements of the
Wallenda Model

Variable: position of body

Setpoint: directly over the wire

Sensors: nerve receptors (eyes, inner ears, muscle stretch receptors, etc.)

Integrator: brain

Effectors: skeletal muscles

tightrope man

Seven-person pyramid - famous Wallenda trick involved multiple balances to maintain pyramid of 7 artists

If the video player above is blank (or not visible) click here

Click here to see a video taken by one of the Wallendas as he crosses the wire in the middle row of the seven-person pyramid

For more on the Wallenda model of homeostasis see p. 95 of
Survival Guide for Anatomy & Physiology

Kevin and Wallendas
Kevin (left) with The Flying Wallendas (Alida, Tino, Olinka) when they all performed on the same program in Circus Flora at Faust Park in 1987.
Click image to enlarge it

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This page updated on 4-jan-15