Salt

Common salt is a mineral substance composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of ionic salts; salt in its natural form as a crystalline mineral is known as rock salt or halite. Salt is present in vast quantities in the sea where it is the main mineral constituent, with the open ocean having about 35 grams of solids per litre, a salinity of 3.5%. Salt is essential for animal life, and saltiness is one of the basic human tastes. The tissues of animals contain larger quantities of salt than do plant tissues; therefore the typical diets of nomads who subsist on their flocks and herds require little or no added salt, whereas cereal-based diets require supplementation. Salt is one of the oldest and most ubiquitous of food seasonings, and salting is an important method of food preservation.

Sodium Metabolism

Sodium is so intimately related to other intracellular ions, extracellular ions, and water that it is difficult to consider the factors that regulate its metabolism independently. Nevertheless, the following text is limited to the regulation of sodium retention and excretion.

 

Sodium, chloride, potassium, and magnesium ion concentrations (mmol/L) in intracellular fluid and in four extracellular fluids
Source	Sodium	Chloride	Potassium	Magnesium
Intracellular fluid	8	150	31	10
Extracellular fluids
Sweat	15–53	4–8	2–5	15–70
Urine	32–224	43–60	8–10	39–218
Blood plasma	96–110	3–6	1–2	135–145
Saliva	11–45	11–23	0.1–0.4	10–75

 

At rest, the kidneys filter circulating blood at the rate of 1.0 to 1.5 L/min, causing the kidneys to generate approximately 180 L of fluid during a 24-hour period. Because the average urine volume of normal adults totals 1.3 L/day, almost all of the renal filtrate is reabsorbed and returned to the bloodstream. The amount of sodium excreted into the urine depends upon the body’s need for sodium. If excess sodium is consumed without water, the kidney excretes urine with a high concentration of sodium. If dietary sodium is restricted, the kidneys are capable of producing a dilute urine that maintains the concentration of sodium in body fluids at a normal level.