The body can maintain biological equilibrium even when outside forces are changing. Walter Bradford Cannon was a physiologist who developed this concept and called the phenomenon homeostasis. Homeostasis is the ability of a system to remain relatively stable by regulating variables that can affect that system. Cannon identified four main features of homeostasis: 1) mechanisms that adjust to maintain stability, 2) factors that resist change when it occurs, 3) mechanisms that act together, and 4) organized mechanisms, rather than chance occurrences.

Mechanisms That Maintain Stability

When outside factors change, the internal system has mechanisms that adjust to meet that change. For example, if water levels in a mammal's body fall, they are restored first by water flowing out of the cells and into the blood, then by the cells of the throat and mouth becoming dry, activating thirst centers in the brain and stimulating the animal to drink. This is a mechanism that acts via negative feedback to maintain stable water levels in the body.

Change Is Met with Resistance

Resistance to change involves adaptation. Adaptation can refer to changes in behavior, developmental changes or evolutionary changes in traits. One example would be moving physically from an unsuitable environment to one that is more suitable. Adaptation also can involve compensation for change over a longer period, such as changes in an organism's sensitivity to certain smells. If those smells are irrelevant to an organism's survival fitness, the brain may become desensitized to those smells because they are irrelevant. Harmful changes are met with resistance and compensation.

Regulation Requires Mechanisms That Work Together

In order for a system like the body to maintain homeostasis, many mechanisms work together either simultaneously or in a succession of actions. If oxygen content falls in the blood, the brain has sensors that detect that change and then send signals to the heart to pump faster in order to increase blood flow and deliver more oxygen to the body. In turn, the respiratory system breathes deeper and faster in order to gather more oxygen and release carbon dioxide. This is regulation that happens through several mechanisms working together.

Stability by Organization, Not Chance

Cannon's fourth feature of homeostasis is organization. This can be seen in the organization of the body into cells, tissues, organs, and organ systems. This organization and synchronization of parts is required for a system to manage changes in the environment. Cannon believed that responses to change are not the result of chance reactions that happen to result in a restoration of balance, but that homeostasis was the result of organized systemic responses.