Osmotic and ionic regulation in reptiles

Claude Bernard was the first to advance the concept of a protected milieu intérieur composed of the physical elements within the tissues which are maintained at levels and concentrations different from those found in the external environment by the operation of homeostatic mechanisms. Bernard did not use the term homeostasis; Walter Cannon coined it in 1929. The notion of homeostatic mechanisms regulating the functional behavior of machines is now commonplace and has been a powerful organizing paradigm in biology. It is important to note, however, one important difference between machines and living organisms. Homeostasis in engineered artifacts results from inbuilt design constraints, whereas that in animals is more often the result of purposive behavior, which may be modulated by specific chemicals secreted internally in the form of hormones. Evidence of homeostasis in animals may thus be sought at two levels: in the temporal stability of the chemical structure of the tissues themselves and by the identification of hierarchical control systems impacting on the composition of the tissues through the maintenance of set points.