It's been known for several thousand years that an excess intake of common salt can have adverse effects on the cardiovascular system. (ref.)

Recent research has shown unequivocally that long-term high intakes of salt can cause a range of problems in otherwise healthy individuals. Dietary specialists worldwide have now come to an agreement that salt intake for adults should be no more than 5g. per day.

A large number of epidemiologic, evolutionary, and clinical studies have confirmed that salt intake is an important factor in elevating the BP ^{[ Blood Pressure ]} in humans. The first double-blind controlled study of moderate salt restriction was performed in the early 1980’s by MacGregor et al."

Source : Electrolyte Blood Press 12:7-18, 2014

It's now thought that the ways in which salt affects blood pressure is considerably more complicated than was first suggested, and the exact mechanisms which lead to salt-induced persistent high blood pressure and cardiovascular damage are still not fully understood.

“Salt” and “sodium” are used synonymously. In fact, salt is only 40% sodium; 1 g of salt has 400 mg sodium. The remaining 60% of salt— chloride—is an often forgotten but likely important part of the link between salt and blood pressure. Replacing sodium chloride with sodium citrate abolished the increase in plasma volume and blood pressure induced by sodium chloride. Similar effects have been observed when sodium chloride was replaced by sodium phosphate or sodium bicarbonate."

Source : The American Journal of Medicine 125, 433-439

Notes :

1) The biological processes which regulate blood pressure in general are also poorly understood - see : Hypertensionplugin-autotooltip__plain plugin-autotooltip_bigHypertension

Only 5 -10% of cases of hypertension (high blood pressure) have causes which can be clearly identified - e.g. due to narrowing of the arteries. Put another way, of those patients who have been identified as having abnormally high blood pressure, physicians will not be able to pinpoint a specific cause in more than 90% of cases.

2) It's not clear why humans (and some other mammals) are so keen on consuming high levels of salt, see : Salt preferencesplugin-autotooltip__plain plugin-autotooltip_bigSalt preferences

Sodium deficiency is rare in humans - the physiological need for salt has been calculated at around 1.3 grams per day for an adult - but, given the opportunity, most people consume far more than this. Leading to the idea that there must be an inbuilt preference to consume more salt than is actually required.

3) The 5g. per day recommendation is for the total salt intake - and generally there is poor public awareness that many highly-processed foods are manufactured and marketed with extremely high intrinsic salt levels.

4) Some individuals have very acute reactions to higher salt levels - being so-called 'salt intolerant' - again implying that the underlying bio-mechanisms are far more complex than textbook explanations sometimes suggest.