In addition to the control of fluid balance, it is also necessary to have control of the salt balance in the body.
As with water, there are two main factors which control salt balance:
- ingestion of salts
- loss of salts
Absorption of salt in the body occurs primarily through the intake of food and drink, with a small amount of salts being released in the body during metabolism. Low salt ingestion is rarely a problem for us. Physiologically speaking, there are far greater challenges associated with the over consumption of salt in our bodies. In other words, it is the regulation of salt secretion which is a challenge for the organism. Salt loss is experiences continuously through the production of faeces, urine and sweat.
By salts, we mean the body’s total salt content, including sodium, calium, calcium, magnesium, zinc etc. It is in any case, primarily sodium, which we in physiology are concerned most about. This is because sodium is the dominant salt in in extracellular fluid, and constitutes 90-95% of the salts there, both as sodium bicarbonate (NaHCO3) and sodium carbonate (NaCl). This means that sodium is deciding for most of the osmotic pressure we have and is crucial for the control of the extra cellular fluid in the interstitial space.
A physiologically interesting phenomenon is that sodium loss does not trigger signs of faulty sodium concentration. This is owed to the fact that water almost always follows sodium. By this, we mean that when we lose sodium, we will also lose water, and therefore total sodium concentration will be unchanged. Curiously, there are no sodium receptors in peripheral tissues, thus salt balance is regulated in a bit of a special way in your body.