homeo = same; stasis = standing

Relative constancy of the internal fluid environment   pp   pp

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."

Fish are like the cells of the [multicellular] body

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

	Optimum temperature, high nutrient level, high oxygen level, low carbon dioxide level, low nitrogen-waste level, optimum pressure, etc.
	Devices such as heaters/chillers, auto feeders, aerators, filters keep conditions relatively constant

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)

Example: thermostatic heating system in a home

Components of an automatic control system

	Variable is the characteristic of the internal environment that is controlled by this mechanism (internal temp in this example)
	Sensor (receptor) detects changes in variable and feeds that information back to the integrator (control center) (thermometer in this example)
	Integrator (control center) integrates (puts together) data from sensor and stored "setpoint" data (thermostat in this example)
	Setpoint is the "ideal" or "normal" value of the variable that is previously "set" or "stored" in memory
	Effector is the mechanism (furnace in this example) that has an "effect" on the variable (internal temperature in this example)

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

	Occurs when feedback (from sensor to integrator) results in a reversal of the direction of change
	In example, thermostat's response causes temperature decrease to reverse and become a temperature increase
	Negative feedback tends to stabilize a system, correcting deviations from the setpoint
	Human example: shivering in response to cooling of body during cold weather

Positive feedback

	Occurs when feedback (from sensor to integrator) results in an amplification of the change (same direction as deviation from setpoint)
	In example, positive feedback would occur if the thermostat's response to a dropping temperature was to switch off the furnace or to switch on the air conditioner (chiller)
	Another example: audio "feedback" occurs when amplified sound is picked up by microphone and then amplified again then picked up and amplified again, and again, and again --each time getting louder Can be stopped only if "feedback loop" is broken
	Human example: increased labor contractions stimulated by oxytocin (OT) hormone Fetus's head moves into birth canal (vagina) at start of labor, which causes the birth canal to stretch beyond its setpoint amount of stretch, which is detected by sensors (stretch receptors) in the vaginal wall and fed back to hypothalamus of brain, which releases OT, which stimulates stronger and more frequent uterine (womb) contractions, which pushes the fetus, which causes more vaginal stretch, which produces more OT, and so on --greatly amplifying and speeding up labor contractions Broken when baby is born (no more stretch, thank goodness)

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 Single artist walking across single wire (very high, very long --as in Busch Stadium walk) Click here to see current family patriarch, Tino, doing these daring skywalks Uses negative feedback to maintain relatively constant position on wire (setpoint is having wire walker's center of gravity directly over wire) Karl died in a skywalk in Puerto Rico when, sadly,  he couldn't maintain a constant position over the wire That's how we all die: we each eventually lose homeostatic constancy (but hopefully ours won't be shown on the news)

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

	Fell in 1962 when front artist lost grip on balance pole, all 7 lost their balance
	Two died in accident, one permanently paralyzed from waist down, all were injured
	Recreated by Wallendas in late 1990s (premiering at Detroit, then Forest Park in St. Louis's Circus Flora) . . . and then in 2001, they achieved a 10-person pyramid Click here to see photos of how the pyramid is arranged and a video clip of the "7" in action

Wallenda model illustrates these concepts of homeostasis:

	Negative feedback maintenance of variable
	Homeostasis is a dynamic, energy-consuming process
	Homeostatic balance is vital to healthy survival (you die when you lose it)
	Balance of one homeostatic variable is often interdependent with the balance of other variables