A number of hormones are used to control salt levels in the body. Aldosterone and ANP - atrial natriuretic peptide are the key players. They influence the kidneys to control sodium losses. ADH and Angiotensin II stimulate thirst to control how much water we consume to dilute salty blood. Cortisol and some other hormones can also influence levels(1).
Changes in salt levels leads to changed blood volume as water follows salt. Our cells shrink or swell as water moves out or into them to dilute salty blood or increase the strength of dilute blood. This triggers a number of feedback responses.
The RAA system retains salt
The Renin-Angiotensin II-Aldosterone is triggered when the kidneys detect reduced blood pressure. In the short term a low salt diet or too much water will reduce the osmolarity of interstital fluid pushing water into cells and out of the bloodstream, lowering blood pressure. This triggers the release of renin. Renin then causes angiotensin II to constrict blood vessels (raises blood pressure). Angiotensin II then stimulates the adrenals to release aldosterone which makes the kidneys retain salt. As the salt is reabsorbed, water follows via osmosis, raising blood volume and therefore blood pressure.
There is good evidence that a diet too low in salt can lead to a chronically raised level of renin(2). This, along with raised triglycerides could result in negative health outcomes for those consuming low levels of salt.
The ADH system retains water
The ADH system is triggered when the brain detects high osmotic pressure. This is normally after events that can lower blood pressure such as heavy sweating, haemorrhage and diarrhoea. A salty meal would also trigger the ADH system. ADH or vasopressin constricts blood vessels to raise blood pressure again, and gets the kidneys to hold onto more water (not salt), by inserting water holes in the kidney reabsorption tublules. It also reduces sweat losses of water.
The ANP system reduces salt
When blood pressure is high, perhaps due to drinking a lot of fluid, the increased blood volume is detected by the atria in the heart which release ANP - atrial natriuretic peptide. This increases the kidneys excretion of salt. Water then follows, reducing blood pressure.
Problems occur when kidneys are damaged
If the kidneys are damaged then the above systems may not work properly. However in most cases they do and a large range of intakes of salt can be coped with and blood pressure is maintained within range.