Allostasis vs. Homeostasis: Differences & Relationship

Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will clearly define homeostasis and allostasis and then distinguish exactly how they are truly different concepts or if they're actually exactly the same ones.

Allostasis vs. Homeostasis

If you've ever taken even a basic biological science course, you were probably taught about a concept known as homeostasis. Maybe you know what this is, maybe you forgot, or maybe you actually never came across this concept at all. In any case, fewer people are aware of a related concept, known as allostasis. This lesson is going to describe each one and then discuss how the two are related.

What is Homeostasis?

Let's start off by defining the more well-known term of homeostasis. This term can actually be defined in more than one way, but it really boils down to a simple concept. Homeostasis refers to a biological system's ability to maintain a dynamic internal equilibrium in the face of external or internal changes. More properly, homeostasis actually refers to the state of being in such an equilibrium. More on that important distinction later.

So let's just break the definition down, shall we? A biological system can refer to something like a cell or a collection of cells, like our body. By internal, we mean inside of this system. A dynamic equilibrium refers to the fact that this equilibrium is not static, like a rock. It constantly adapts to various factors in order to maintain a relatively steady state.

Still confused as to what a dynamic equilibrium is? Then straighten your arm out in front of you. Try to keep it completely steady, like a rock. If you look closely, you can't. Your body is always adjusting the arm's position in very tiny increments in order to keep it relatively steady (so it appears steady like a rock) but without actually being able to keep it in the same exact position the entire time. This neuroanatomical metaphor demonstrates what a dynamic equilibrium is with respect to the biochemical and physiological changes seen within a biological system as it maintains homeostasis.

And what external or internal changes play a part in forcing the body to maintain homeostasis? They could be external ones, like changing temperatures or internal changes, like changes in blood sugar levels after you've eaten. Your body must react to these changes in order to maintain homeostasis.

What is Allostasis?

OK, so what is allostasis then? In simple and literal terms, allostasis means stability through change. Doesn't sound much different than homeostasis, does it? More on that in a bit.

What you should be aware of is that there is more than one version of allostasis; more than one explanation of what this term actually is, all depending on which proponent of this concept you read. Originally, allostasis was suggested as a concept separate from homeostasis back in 1988 because it was believed, by the original authors, that homeostasis was a concept that denoted a state where an organism maintained equilibrium by holding ALL of its internal parameters constant.

The original way allostasis was described and differentiated from homeostasis can be explained with the following example. Imagine the concept of blood pressure. From a very simple understanding of homeostasis, we would say that the body tries to keep its blood pressure within a relatively small and normal range (relatively constant). If something were to cause the blood pressure to drop too much, the body would release various hormones in order to raise it back to the normal range and thus maintain equilibrium (homeostasis).

But is the relatively normal homeostatic range of blood pressure during the time you're sleeping the same as that during the time you're awake? It may not be. Thus the body may maintain one 'homeostatic range' for blood pressure during sleep but an entirely different one when the person is awake.

Thus, when all physiological parameters (a 'normal blood pressure range' in this case) are set to new levels in accordance with circumstances (sleep vs. awake in this case), this is termed allostasis. In other words, allostasis refers to the idea that ALL of an organism's internal parameters are varied in order to appropriately match them to external stressors, and thus homeostasis is wrong because by their definition, homeostasis required ALL internal parameters to be constant.

The Misunderstanding

Original

But is this what homeostasis actually is? If you're confused, don't worry, so are many scientists.

If you were to read the original concept of homeostasis, as provided by Walter Cannon in 1929, it would make clear the following facts:

The primary parameters that are maintained by the body at a relatively stable level are those that are directly most critical to a cell's survival and include the temperature, pH, and oxygen/nutrient supply of a given tissue.
Other parameters, like those that maintain the environment around the cell, must also be considered within the sphere of homeostasis. These parameters include the likes of blood pressure and heart rate.

Cannon specifically mentioned that these other parameters might be found within 'normal ranges' when the body is at rest but will most certainly be actively changed when necessary, such as during exercise.

Put another way, Cannon never said that all parameters (primary and other) must always be at relatively stable levels at all times. No! He explicitly said that homeostasis involves primary parameters that are held at a dynamic equilibrium but also other parameters can be actively changed to 'new normal' levels when necessary. In other words, Cannon already covered the concept of allostasis as it was described before. The term allostasis, in this case, is redundant.

New

Now, there is one other definition of allostasis; a newer one. Allostasis is sometimes defined as the process for actively maintaining homeostasis. If you recall the definition for homeostasis, when properly phrased, it refers to a biological system's state of internal dynamic equilibrium with respect to external or internal changes placed upon that biological system. Homeostasis, originally, never referred to the process for actively maintaining homeostasis. Cannon didn't call this homeostasis; he called these processes homeostatic mechanisms. The endpoint (dynamic stability) is homeostasis. The processes involved in achieving homeostasis are called homeostatic mechanisms.

Today, however, homeostasis is misrepresented by many as being both the endpoint and/or the process to achieve the endpoint. Ergo, allostasis was termed to represent the process. But there is no need for the new term, allostasis, when Cannon termed 'homeostatic mechanisms' for the process a very long time ago.

Lesson Summary

Homeostasis, when properly defined, refers to a biological system's state of internal dynamic equilibrium with respect to external or internal changes placed upon that biological system.

Allostasis has been variously defined as:

Stability through change.
The variation of an organism's internal parameters in order to appropriately match them to external stressors.
The process for actively maintaining homeostasis.

All such definitions are based on a misunderstanding or misrepresentation of the original concepts of homeostasis and homeostatic mechanisms and are thus redundant.

Medical Disclaimer: The information on this site is for your information only and is not a substitute for professional medical advice.

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Allostasis vs. Homeostasis: Differences & Relationship