Eli5 feedback loop


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

> But wouldn’t it be more efficient to just stay on at a low level always maintaining that temperature?

Yes, it would be.

Unfortunately, the way that air conditioners work means they are “On” or “Off”. It is difficult to make a well-functioning air conditioner that can turn itself up or down in the way you are describing.

“Cruise control works by maintaining the desired speed”

There are a few assumptions baked into that statement, right? I mean, cruise control feels super-smooth when the road is level, straight, and even the wind and temperature are constant. But try using it in the mountains, and you’ll have a different experience.

There is no set point in thermoregulating organisms which wouldn’t require the same things the cruise control system on your car require: sensors to monitor the current state, a signaling system to communicate its findings, a computer to determine the difference between current and desired, and a means to effect necessary changes.

My point is that to my knowledge there is no better design and our understanding of how it really works is pitifully small.

Cruise control works the same way only within tighter tolerances. It does go up and down but within smaller limits. If you hit 70mph it may be allowed to fluctuate up to 70.2 mph and then drop down to 69.8 mph and target in between those 2 speeds. The body isn’t able to respond as fast . How fast it approaches the limits as well as how far it is allowed to overshoot is all part of the design behind the software and hardware capabilities.

Your blood pressure is pretty tightly controlled, and something like hunger is less tightly controlled with larger bounds of what is considered normal.

The system is working to control itself with influence from outside forces acting on it. It can’t ever be static as that would not maintain its desired set point.

It is more difficult for things that are “on” or “off” with no throttles like cruise control on a car has. You could introduce a “throttle” into a heater by say having 2, 4, or all 6 burners going, but that introduces complexity. How to determine how many burners are needed, separate solenoids and gas lines for them, etc. People expect reliability from their HVAC because it is important, the more moving parts you have the more likely it is to fail.

Homeostasis is the process of keeping a parameter at an optimum in the face of external agents and stochastic fluctuations that deviate it. There are countless variables governing even the simplest biological parameters, those are internal like changes in metabolism in different organs or stochastic secretions of hormones and so on and external like fluctuations in ambient temperature or stressors that activate the sympathetic nervous system. You can’t avoid these deviations, but you can rectify them. Negative feedback is a very stable approach to do this, and there are many types of it, not to mention other kinds of feedback. And regarding your example of cruise control, you have to think about it a little deeper, there is still negative feedback there, but many mathematical parameters are optimized to minimize delay and also minimize sudden changes by having small step changes, so you think you’re set on one speed, but you’re actually continuously hovering, jumping over and going under by a small amount, with feedback governing this rectification.

Source; I’m a physiologist

Edit: English