Allostasis is a theory used to explain how an organism regulates its internal systems. It was first developed in the 1980s as an alternative to homeostasis. The main difference between these two theories is that the principle of homeostasis assumes that an organism attempts to keep internal stability, while the theory of allostasis states that the organism attempts to regulate its internal systems in the way that is most adaptive to the current situation, based on the organism's prior experiences. The principles of allostasis claim that stability is less important than adaptability and that internal systems are not designed to be perfectly stable.
There are a number of principles that define allostasis. The first is that organisms have evolved so that their internal systems are efficient. The needs for each system are based on the average amount that an organisms can metabolize and the average amount that each system needs. An organism is also designed to be able to put more energy into a given system, such as the digestive system after eating or the respiratory system while running, as needed.
Since the average input and output is not always attainable, an organism is able to make trades between different systems. These trades are regulated by the organism's brain, which assesses situations in order to determine which systems need extra energy and which can do with less for a time. The adrenal response is one example of this. When confronted with a dangerous situation, the adrenal gland releases a hormone that causes respiration, heart rate, and blood pressure to increase, the digestive system to shut down, and certain aspects of vision, such as the ability to see color, to disappear.
One of principles of allostasis, which differs considerably from homeostasis, is that an organism uses prediction in order to adjust the amount of resources that go into each system. This predictive ability means that an organism will leave its average state willingly when it anticipates that a change will be necessary. An example of this is the increase in the amount insulin in the bloodstream in anticipation of eating, such as when smelling food. The insulin level is adjusted before glucose enters the bloodstream, forcing the organism to undergo a chemical change. These adjustments assume that the organism has learned, either through evolution or adaptation, that adjustments will be needed in the future in order to remain in a state of allostasis.