Dehydration refers to the process of reducing body water either through illness, physical
exertion, thermal stress, or water deprivation. Generally, during dehydration, body water is
conserved by defense mechanisms, which act to reduce water output by the kidney. This
process is mediated by the release of an antidiuretic hormone and its action on the
functional part of the kidney (the nephron) to conserve water. Antidiuretic hormone is
released during dehydration by signals from blood that reflect the tonicity (salt
concentration) and volume of blood held within the blood vessels. The secretion of
antidiuretic hormone during dehydration is not decreased with aging, yet the ability of the
kidney to conserve water is reduced. There are several possible explanations for this
problem. First, the number of nephrons per kidney begins to decrease by about 10% per
decade after the age of 40. Second, nephrons from older kidneys appear less responsive to
a given level of antidiuretic hormone than younger nephrons. Thus, both a reduction in
number of nephrons and their sensitivity to antidiuretic hormone limit the ability of older
individuals to conserve water and predisposes the older individual to dehydration.