Homeostasis is a fundamental concept in physiology and refers to the ability of an organism to maintain a stable internal environment despite changes in the external environment. For example, your body temperature is tightly regulated around a set point of approximately 98.6 degrees Fahrenheit, or 37 degrees Celcius. When your body temperature starts to rise due to external factors like exercise or exposure to heat, your body responds by sweating, increasing blood flow to the skin, and other mechanisms that help to dissipate heat and bring your temperature back down to the set point.

Homeostasis is accomplished through a series of feedback mechanisms that regulate various physiological processes. For example, the hypothalamus in the brain plays a key role in regulating body temperature, as well as other processes like hunger, thirst, and sleep. The hypothalamus receives input from sensors in the body that detect changes in temperature, blood sugar levels, and other parameters, and then sends out signals to other parts of the body to restore balance.

Allostasis, on the other hand, is a more recently developed concept that refers to the process by which the body adapts to stressors in order to maintain stability. Unlike homeostasis, which seeks to maintain a constant set point, allostasis involves a dynamic process of responding to and adapting to changing conditions.

One of the key differences between homeostasis and allostasis is the timescale over which they operate. Homeostasis is designed to work over short timescales and is particularly effective at maintaining stability in response to acute changes in the environment. For example, your body can quickly adapt to a sudden change in temperature or hydration status in order to maintain balance.

Allostasis, on the other hand, is designed to work over longer timescales and is particularly effective at maintaining stability in response to chronic stressors. Chronic stressors can include things like sleep deprivation, poor nutrition, chronic pain, exposure to toxins, and bad working conditions. In response to these stressors, the body activates various physiological systems that can help to buffer the effects of stress and maintain stability.

Two of the key physiological systems involved in homeostasis and allostasis are the sympathetic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis. These systems are involved in the body's response to stress and involve the release of various hormones, including adrenaline (epinephrine) and cortisol, that help to prepare the body for fight or flight. When the sympathetic nervous system and the HPA axis are activated in response to a stressor, it can lead to changes in heart rate, blood pressure, and other physiological processes that allow the body to respond to the stressor.

However, if the HPA axis is activated over a prolonged period of time, it can lead to wear and tear on the body and increase the risk of disease. For example, chronic stress has been linked to a variety of health problems, including cardiovascular disease, diabetes, and mental health disorders.

Another key difference between homeostasis and allostasis is the degree of plasticity involved. Homeostatic mechanisms are designed to maintain a relatively stable set point, and deviations from this set point can be seen as a failure of the system. Allostasis, on the other hand, involves a degree of plasticity that allows the body to adapt to changing conditions.

For example, if you are exposed to a new stressor, your body may initially respond with a strong stress response. However, over time, your body may adapt to the stressor and become less responsive. This adaptation can be seen as a positive outcome of the allostasis process, as it allows the body to maintain stability in the face of ongoing stressors.

In summary, while homeostasis and allostasis are both important concepts in physiology, and especially in stress physiology, they operate on different timescales and have different mechanisms. Homeostasis is designed to maintain stability over short timescales and operates through feedback mechanisms that regulate various physiological processes around a set point. Allostasis, on the other hand, is designed to maintain stability over longer timescales and involves a dynamic process of responding to and adapting to changing conditions.

While allostasis can be adaptive in the short term, prolonged activation of these physiological systems can contribute to wear and tear on the body and increase the risk of disease. For example, chronic activation of the HPA axis has been linked to a variety of health problems, including cardiovascular disease, diabetes, and mental health disorders.

One of the key challenges in understanding allostasis is determining what constitutes a "healthy" level of adaptation. Some level of adaptation is necessary for maintaining stability in response to changing conditions, but too much adaptation can be harmful. For example, chronic exposure to stressors like poverty, discrimination, and trauma can lead to dysregulation of the stress response system and contribute to a variety of health problems.

Researchers have identified a variety of factors that can influence the degree of adaptation to stressors, including genetic factors, early life experiences, and social support. For example, individuals with certain genetic variations may be more or less susceptible to the effects of stressors, while individuals who experience chronic stress early in life may be more likely to develop health problems later in life. Similarly, individuals who have strong social support networks may be better able to adapt to stressors than those who lack social support.

Homeostasis through allostasis

Overall, allostasis represents an important concept in understanding how the body adapts to stressors and maintains stability over time. While homeostasis remains an important concept in physiology, allostasis provides a more nuanced understanding of how the body responds to changing conditions and how this response can impact health outcomes.

The relationship between homeostasis and allostasis is perhaps best illustrated by the phrase "homeostasis through allostasis". The body can only be in equilibrium, or homeostasis, thanks to complex mechanisms in the body that can maintain this equilibrium. Or set a new one that is perhaps better if circumstances have changed. These complex mechanisms work together: allostasis.

If the body has to work hard to maintain equilibrium, it is said that there is a high allosteric load. This illustrates that allostatistic mechanisms can be intense or can last a long time.

By better understanding the mechanisms of allostasis and the factors that influence adaptation, researchers may be able to develop new interventions that can help individuals better cope with stressors and maintain long-term health.

If you want to read more about homeostasis, allostasis and stress, please consult our free article on the definition of stress according to science.