Salt-Appetite Mechanism for Controlling
Extracellular Fluid Sodium Concentration and Volume
Maintenance of normal extracellular fluid volume and sodium
concentration requires a balance between sodium excretion and sodium intake. In
modern civi-lizations, sodium intake is almost always greater than necessary
for homeostasis. In fact, the average sodium intake for individuals in
industrialized cultures eating processed foods usually ranges between 100 and
200 mEq/day, even though humans can survive and function normally on 10 to 20
mEq/day. Thus, most people eat far more sodium than is necessary for
home-ostasis, and there is evidence that our usual high sodium intake may
contribute to cardiovascular disorders such as hypertension.
Salt appetite is due in part to the fact that animals and humans
like salt and eat it regardless of whether they are salt-deficient. There is
also a regulatory component to salt appetite in which there is a behavioral
drive to obtain salt when there is sodium deficiency in the body. This is
particularly important in herbivores, which nat-urally eat a low-sodium diet,
but salt craving may also be important in humans who have extreme deficiency of
sodium, such as occurs in Addisonâ€™s disease. In this instance, there is
deficiency of aldosterone secretion, which causes excessive loss of sodium in
the urine and leads to decreased extracellular fluid volume and decreased
sodium concentration; both of these changes elicit the desire for salt.
In general, the two primary stimuli that are believed to increase salt appetite
are (1) decreased extracellular fluid sodium concentration and (2) decreased
blood volume or blood pressure, associated with circulatory insuffi-ciency.
These are the same major stimuli that elicit thirst.
The neuronal mechanism
for salt appetite is analo-gous to that of the thirst mechanism. Some of the
same neuronal centers in the AV3V region of the brain seem to be involved,
because lesions in this region frequently affect both thirst and salt appetite
simultaneously in animals. Also, circulatory reflexes elicited by low blood
pressure or decreased blood volume affect both thirst and salt appetite at the