I remember my high school bio teacher telling us that our body maintains homeostasis through a feedback loop like a house furnace maintains temperature. The furnace kicks on whenever it is needed to heat the house back up to the desired temperature and then shuts off again when that temperature is reached. But wouldn’t it be more efficient to just stay on at a low level always maintaining that temperature? Sort of how a car would maintain its speed. Cruise control works by maintaining the desired speed. Not speeding back up to that speed once it slows to a different speed. Right?
In: Biology
Most people have a thermostat that maintains a constant temperature and runs a furnace. That’s probably why your teacher chose this example.
Things that have engines also have an engine controller. Let’s think about an engine controller that maintains a constant spinning speed (RPM) [1].
The difference is this:
– The furnace either runs full blast or not at all. The only control is “on” or “off.”
– The engine runs all the time, but it has a throttle that opens wider to allow more fuel input, or closes to reduce fuel input. The throttle control is any number from 10% (minimum throttle) to 100% (maximum throttle).
You’re hung up on this difference between the engine and the furnace. You’re missing the common feature.
What is the common feature? Both of them need to have the following parts:
– A control that can alter its running (furnace on/off, throttle 10%-100%)
– A sensor that checks something (temperature or RPM)
– A circuit that changes the control based on the sensor (temperature / RPM too low -> give me more!, temperature / RPM too high -> back off!)
These three features are what is meant by the term “feedback loop.” A feedback loop is about checking part of the world, then using that information to take action to alter the world, in a way that affects the part of the world you’re checking.
The reason these parts are needed is that the system has an unknown load from the environment. You can’t just say “25% load gives me 3600 RPM, so set it to 25% and call it a day,” because the engine might have different mechanical loads. For example if you engage a gear that runs some heavy machinery [2], your engine will slow the heck down unless its fuel is ramped up to compensate.
In biology, the molecular mechanisms for maintaining temperature have no idea what the weather is going to be, day / night, sun / shade, use of clothing, indoors / outdoors, swimming, etc. All this can change from moment to moment. So the biological temperature regulation system’s able to maintain a constant temperature in a wide range of conditions.
[1] For example to run an electrical generator.
[2] Or, if your engine’s an electrical generator, if the user turns on a light switch to connect some electrical load. (*Electrical load* on a generator’s electrical output actually applies a *mechanical load* to its spinning. This is called “back EMF.”)
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