Salt will cause high blood pressure, but only under certain circumstance.
physiologic hormonal compensations for increased blood volume:
- Atrial natriuretic peptide -- increases sodium excretion, hence this is INCREASED with high blood volume
- Antidiuretic horomone -- ADH increases water reabsorption in the kidney, hence this will be DECREASED with high blood volume
- Renin/Aldosterone -- increases sodium reabsorption from ultrafiltrate back into blood, hence it will be DECREASED
these explainations are a bit reverse (saying that these horomones have these effects and that is why they are released) for the sake of simplicity, and I'm not discussing the mechanism of release of these horomones (aka under what circumstances they are released in the place where they are released from)...
while these horomones help with the kidney regulation of sodium balance and blood volume, this still depends on the kidneys being given the circumstance where they can be effective.
Pathologic circumstances, when elimination of excess sodium fails:
Some people simply have a higher sodium sensitivity and there are a variety of factors that have been statistically correlated to sodium sensitivity, but often the mechanism is unknown. Here are various factors statistically correlated to affect salt sensitivity:
the above article shows that no organ systems need be compromised for excess sodium to raise blood pressure, but just for the sake of discussing more macroscopic things I want to discuss these organ systems based reasons for the kidney being unable to compensate for excess sodium in the diet=
- The kidney:
A patient with renal disease may not be able to regulate the excretion of sodium. Even in normal humans, the kidney's capacity to excrete sodium declines with age, and smaller increases in salt intake induce a rise in arterial pressure, as GFR (the renal filtration of the blood sent to the kidney) falls by about 40%, though individual variations are wide and some people lose less.
Other than the kidney:
Other diseases can also interfere with renal excretion of sodium. A problem with maintaining blood volume will cause constriction of the renal artery. This would occur before a drop in blood pressure would occur, as this is a way of preventing a drop in blood pressure. This occurs in congestive heart failure and liver cirrhosis.
- A patient with congestive heart failure may not have adequate renal
perfusion. Hence, to maintain adequate blood pressure the renal artery is constricted.
- Also, liver cirrhosis may decrease renal perfusion
because of the lack of blood proteins (proteins found inside the
blood vessels that keep water in the blood vessels via osmosis)
holding fluids in the intravascular space and having them escape into
the extra-vascular (outside the blood vessels but still outside the
cells of the body) space. Again, to maintain adequate blood pressure the renal artery is constricted.
- burns, also because of loss of blood proteins--> leakage of intravascular fluid to extravascular space--> decreased blood volume and increased sympathetic nervous system output--> reduction of renal blood flow... same as for cirrhosis
In addition to reduction of salt in the diet, diuretics (ex. loop and thiazide) can help the body excrete even more sodium into the urine than what your body can do on it's own.
this article describes the renal handling of excess salt in the diet: