It is very important to regulate the bodies sodium level. If it falls too low then water and ECF volumes also fall and sodium dependant transporters are also disturbed. If it rises too far the transport systems are still disturbed but body water and ECF volume increases. Sodium is therefore maintained within near constant parameters. With the amount being excreted/lost corresponding to that ingested. Salt intake is not really regulated and thus it falls to the kidneys to maintain salt levels via excretion.
The total body content of sodium is regulated rather than the actual plasma concentration. It would be impossible to regulate actual plasma concentration for 3 reasons:
- When sodium is reabsorbed water follows it so the volume of the plasma is altered rather than the concentration of sodium changing
- Angiotensin 2 and aldosterone affect sodium but they also affect ECF volume so only quantity affected not concentration
- ADH and thirst response also work together to dilute the ECF if concentrations of sodium are high so although there is more NaCl the actual concentration is not really changed.
The kidneys are the most important regulatory organs of sodium in the body. They adjust their excretion to match the amount taken in through the digestive tract taking into account the amount lost through sweat. The handling of sodium by the kidneys is also essential to allow the reabsorption of many other important nutrients which would otherwise be lost in the urine. These nutrients include glucose, amino acids, chloride, bicarbonate and phosphate. It is also exchanged for the likes of potassium and hydrogen ions to aid in their secretion. As a result sodium transport accounts for over 80% of the energy metabolism in the kidneys