To clinically estimate the sodium content of a senior or geriatric cat, examine the cat for signs of volume contraction or expansion. Clinical signs consistent with impaired tissue perfusion such as dehydration indicate reduced sodium content, while signs of volume expansion such as edema indicate an increase in sodium content. Sodium concentration is the ratio of sodium in milliequivalents to water in liters. Because water equilibrates across semipermeable cell membranes, the sodium concentration reflects the tonicity of all body fluids. Hyponatremia is evidence supporting intracellular fluid volume expansion.
Hypernatremia is evidence supporting intracellular volume contraction.
The serum sodium concentration does not provide information on the body's sodium content. Tonicity receptors are located within the CNS and are located in close proximity to the thirst center and the ADH release center. Tonicity receptors apparently detect changes in tonicity as a function of changes in their cell volume. Changes in tonicity are virtually synonymous with changes in serum sodium concentration. While tonicity is the natural stimulus influencing thirst and ADH secretion, a sufficient reduction in extracellular fluid volume will override osmoreceptor control.