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Principles of homeostasis and negative feedback

The internal environment of our bodies is constantly kept within strict limits. Take for example temperature. It must be a challenge keeping our fleshy selves at 37°C while the outside fluctuates wildly! Or how about blood glucose concentration? We’ve all heard of diabetes – if it goes too high our organs sustain damage, if it goes too low a coma may be induced or even death.

Welcome to homeostasis – the maintenance of physiological parameters within optimal range.

Core Body temperature and Blood pH

Why, though, is it so important to keep a strict temperature and pH? From a physiological viewpoint, think of enzyme activity. Its sensitivity to both temperature and pH means that the only conditions of optimal function will be strictly defined. Lots of enzymes denature past 40°C and are inactivated below 35°C.

Similarly, extreme pH is detrimental to optimal enzyme function, so blood pH must be tightly regulated.
Blood Glucose Concentration

The effects of suboptimal blood glucose concentration are twofold:

1. If it is too high, the very low water potential will result in water being drawn from cells. This is clearly cause for concern, since water is at the heart of so many reactions that take place within cells, and is in fact the solvent for all the contents of the cells themselves! A long term effect of hyperglycaemia is heart disease.

2. If it is too low, vital organs like the brain are starved of essential energy. This is very dangerous and can result in coma.
Therefore it is crucial to maintain an internal environment where energy is supplied where needed, and not wasted where not needed.

Negative and positive feedback loops are very straightforward. This is a simplified description of their actions:

Something happens –> feedback “NO” –> something stops happening

Something happens –> feedback “YES” –> something keeps happening

Negative Feedback

The operation of an oven is an easy example of negative feedback acting both ways, which is how it usually acts. That means that there are deviations in two opposing directions. If an oven is set at 220°C, both a decrease and an increase in temperature is a deviation. So if the temperature drops or rises, a sensor picks that up and commands the heater to turn on or off.

This is negative feedback because it returns the system to its original state.

Sometimes completely separate sensors control a rise or fall in temperature. This separation gives a high level of control.

Positive Feedback

In contrast with negative feedback, positive feedback drives a system further away from its original state, often starting out with small disturbances which cause a factor which further stimulates the disturbance. For example, a panicking sheep will cause more sheep around to get panicked, potentially causing a stampede.

Unlike the sharp control available in negative control loops, positive feedback may end up spiralling itself out of control and self-destructing.





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