A century ago, Claude Bernard 1 pointed out that the cells of most higher organisms do not really come into direct contact with the external environment but exist in their own internal sea—the extracellular fluid. Subsequently2 he deduced from a relatively few facts that it was the stability of this milieu intérieur which constituted the "primary condition for freedom and independence of existence."
In recent years much attention has been given to the role of the kidney in regulating the volume and osmolality of the body fluids. Nevertheless, it must not be overlooked that the kidney can subserve this office only by excreting excesses—it cannot furnish water or solutes to replace deficits. Hence, an adequate intake of water is of primary importance for body fluid homeostasis in man, since inevitable losses occur continually, both through renal and extrarenal channels.
Under usual circumstances water deficiency manifests itself promptly by thirst,
STRAUSS MB. Thirst—The Acquisition of Water. AMA Arch Intern Med. 1958;101(2):216-227. doi:10.1001/archinte.1958.00260140048009