Allostasis is defined as “the ability to achieve stability through change”. Now many of you will be thinking, “this is identical to what homeostasis means”, and that is correct. Allostasis does have the same definition as homeostasis, yet
1) the range of things it applies to is wider (homeostasis traditionally only applies to maintaining functions which are vital for life).
2) the concept is paired with allostatic-load which defines long term health implications of homeostatic/allostatic functions.
In many cases allostasis and homeostasis are used interchangeably and there is nothing wrong with this at all. As long as the core concept relates how life is supported by dynamically maintaining balance through change to use personal resources to meet environmental demands and then enact other behaviours to recoup spent resources, the two words are both semantically okay.

The core concept of allostasis can be applied equally regarding all reactions to environmental events, yet its roots are in biology. The principle revolves around how afferent nerves send signals from different parts of the body (such as the organs) to the central nervous system and how a measured response is then enacted so as to deal with the perceived event. Notice how afferent nerves send signals from both internal and external sources. The brain receives updates about the internal environment from, say, the liver, lungs and bones whilst simultaneously receiving stimuli from the external environment via, say, the eyes, ears and skin. Our subconscious assesses how many resources we have internally, to support continuing to live, whilst keeping track of available resources externally, so if an internal resource drops below a homeostatic baseline, we are then motivated to gather the resource from the external environment.
For an external example, if the body needs vitamin B, we feel motivated to get some meat. In the process of getting the meat we perspire excessively which reduces our sodium levels, so then we are motivated to eat some salty crisps. And on, and on, and on. This is the principal concept of homeostasis.
For an internal example, if signals are received that there is too much activity in a neuron circuit, perhaps the neurotransmitter gama-aminobutyric acid (GABA) will be released to inhibit the circuit.
The above examples are relatively simple ones which would be considered necessary to support life and so within the traditional definition of homeostasis.
Nevertheless the principles of this type of behaviour extend into all behavioural life, and so when the mind logically starts to consider a future threat that’s about to materialise, the vigilant disposition which is enacted to prepare for it (including priming cognitive biasses which will detect threatening features faster than normal) has the same homeostatic-like principles, yet as the threat is social and perhaps not life threatening, it’s beyond what is normally encompassed by the term homeostasis, thus it’s more appropriate to refer to these adaptations as allostatic, for allostasis has no such traditional limits.

Procreation drives evolution and behaviour, of that there can be no doubt, yet people must survive to procreate, and that means honouring homeostatic needs of hunger, hydration, warmth, cleanliness and add every other biological process which is needed to survive.
The various emotions are what motivate us to fulfil these unavoidable goals. They are at the very heart of evolution, always have been, and always will be.
To think of allostasis as a simple procedure executing rudimentary functions is a colossal misjudgement. A mindboggling number of intricately measured sensory inputs are orchestrated into a symphony of synergistic enactments which are all designed to keep the organism in which they reside performing optimally. The process is interwoven throughout bodily systems. Those systems serve allostatic needs. A continual flow of inputs and reactions transverse all levels.

Neocortex <-> Hypothalamus <-> Brainstem <-> Viscera

How many hundreds of thousands, millions or billions of pre-programmed instinctual solutions for environmental challenges have accumulated in our gene pool throughout evolution is anyone’s guess.
What is sure, however, is that emotions are what motivate us to perform, or not to perform, actions which serve these goals.
Emotions are at the heart of evolution and allostasis too.
One thing that sets allostasis apart is its emphasis on the biological functions which are required to enact changes. As the body constricts the cardiovascular system to raise blood pressure, for say an angry response, a number of neurobiological actions are required that exact a cost on the metabolism. Only that’s not the only cost, for while that angry state is being enacted, other states (e.g. a calm reflective state) are blocked from experience. Thus if an angry state is enacted vastly too much over many years, the cardiovascular system’s blood pressure will be kept at elevated levels and other states which serve homeostatic needs will be blocked from being enacted. Therefore the cost of inappropriately enacted emotional states and mood dispositions have a very real cost that can accumulate and cause wear and tear through a lifetime – allostatic-load.
A high allostatic-load baseline score gives an increase risk to cardiovascular disease, decline in physical and cognitive functioning to name a few. The allostatic processes that seek to server short term homeostatic needs and maintain good health can in the long term cause damage and disease – especially if inappropriate emotional states are often enacted. The implications are clearly more immediate for people with mental health conditions which revolve around stress, anxiety and conduct disorders for example.

Three stages of allostatic-load are defined and studied.
Primary Mediator Outcomes: Cortisol, noradrenalin, epinephrine and DHEA are four chemical mediators which have widespread influence in allostatic processes and predict a variety of secondary and tertiary outcomes.
Secondary Outcomes: These are outcomes of the primary effects to tissues and organs from the primary mediators. More focused on specific organs and diseases, secondary outcomes measure such things as abnormal metabolism and risk of cardiovascular disease via waist hip ratio, blood pressure, cholesterol/HDL ratio, HDL cholesterol, elevated glucose and insulin resistance.
Tertiary Outcomes: These are the actual diseases and conditions caused by allostatic-load. Alzheimer’s disease, vascular dementia, cancer, specific immune system dysfunctions, metabolic syndrome and cardiovascular disease, decreased physical capacity and severe cognitive decline would be some examples of tertiary outcomes.

Homeostasis