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Homeostasis is commonly used as a word to describe any system that is in a stable state, self-correcting and sustaining. It can be applied to a closed system, such as a nuclear-powered robotic rover for space exploration. More often, it is applied to open systems which have input and output channels for interacting with its environment or with other systems. Very complex systems, however, are seldom perfect, so the term describes an ideal, hypothetical state. Among the most complex processes is life, and biological homeostasis has been the most rigorously studied.
The term was first coined in the 1920s as a concept of human functioning. Given that people survive in a wide range of environments, under varying conditions, and with diverse diets, the assumption is that the human body possesses inherent adaptive mechanisms. Despite the many differences in external input or stimulus, and the body’s corresponding different reactions, they belie a systematic internal state which is essentially the same in all humans. Biological homeostasis can be applied to an entire organism, as well as to its interdependent sub-systems.
One of the most commonly used examples to explain biological homeostasis is internal temperature regulation. For humans, the ideal temperature is precisely 98.6° Fahrenheit (37° Celsius). Whether the fever is from the summer sun or from illness, if the body’s temperature rises above normal, it begins to sweat. Evaporation of the water in sweat cools the body. If internal temperature falls below this fine line, the body begins to shiver because one of the by-products of muscle contraction is heat.
Other organisms may regulate their temperature differently. Cold-blooded reptiles, for example, might need to absorb radiant heat from the sun or a warm rock to raise their body temperature to the level necessary for physical activity. Kangaroos of the arid Australian desert cool their bodies by licking their paws. In all cases, the objective is the same — to maintain a critical internal equilibrium.
Another example of biological homeostasis is the need to maintain proper pH, or level of acidity. The stomach, for example, is highly acidic. The pH of human blood, on the other hand, has a narrow range of tolerance that is slightly more alkaline than the neutral measure of pure water. Each is critical for healthy function.
The mechanisms by which the body achieves correct balance is, in principle, typical of homeostatic systems. First, a receptor of some kind must sense the system’s current condition and relay this information to a control center of some type. In humans, this could be nerves carrying electrical signals to the brain. With set knowledge of the system’s optimal state, the control center subsequently sends a command to an effector whose activation results in an adjustment to the system’s condition. The human brain might send signals to a particular organ that releases hormones which chemically restore the balance.
Biological homeostasis is the regulation of an organism’s internal environment, as external forces or environment constantly changes. The fundamental, typical process is a correspondingly constant feedback loop. Whether the feedback is positive or negative, the link between receptor, control center and effector is cyclical. With perpetual plus adjustments combined with negative adjustments, the result is a zero state equivalent to healthy function. A broad theory of disease defines them as an imbalance or malfunction of this regulatory feedback loop.