Life within a cell, tissue or body depends on a supply of essential nutrients, mechanisms to process them, and arrangements for disposal of waste. Just as importantly it requires a network of control systems which adjust the inputs and outputs to ensure that chemical transformations occur in an environment of relative constancy. The vitality of all animals, from singlecelled protozoans to multi-celled whales, depends ultimately on the efficient operation of the regulatory systems that control the conditions within their bodies. Homeostasis is an approximately constant state which varies only within tolerable limits; the regulatory systems which maintain the internal homeostasis of the body are said to be homeostatic.
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