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Due to high blood pressure, my doctor has recommended I go on a low-sodium diet. So, that got me wondering what it is about sodium that drives this recommendation?

What does sodium do to a body that, by its presence or absence, affects blood pressure in humans?

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Don't have the time to write a proper answer, but maybe you want to check out this review (and if someone wants to write an answer starting from it he/she is very welcome to do it!). Dietary Sodium and Blood Pressure –  nico Nov 1 '12 at 21:55

4 Answers 4

Here is a simple answer, if you want it.

Eating salt raises the amount of sodium in your bloodstream and wrecks the delicate balance, reducing the ability of your kidneys to remove the water. The result is a higher blood pressure due to the extra fluid and extra strain on the delicate blood vessels leading to the kidneys.

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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: http://physrev.physiology.org/content/85/2/679

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: http://booksite.elsevier.com/brochures/thekidney/PDFs/TheKidneyCh36_P088488.pdf

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As salt accumulates in thebody, it also indirectly increases the extracellular fluid volume for two basic reasons: 1. When there is excess salt in the extracellular fluid,the osmolality of the fluid increases, and this in turn stimulates the thirst center in the brain, making the person drink extra amounts of water to return the extracellular salt concentration to normal. This increases the extracellular fluid volume. 2. The increase in osmolality caused by the excess salt in the extracellular fluid also stimulates the hypothalamic-posterior pituitary gland secretory mechanism to secrete increased quantities of antidiuretic hormone. The antidiuretic hormone then causes the kidneys to reabsorb greatly increased quantities of water from the renal tubular fluid, thereby diminishing the excreted volume of urine but increasing the extracellular fluid volume. Thus, for these important reasons, the amount of salt that accumulates in the body is the main determinant of the extracellular fluid volume. Because only small increases in extracellular fluid and blood volume can often increase the arterial pressure greatly, accumulation of even a small amount of extra salt in the body can lead to considerable elevation of arterial pressure.

enter image description here

Progressive changes in important circulatory system variables during the first few weeks of volume-loading hypertension.

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in addition to the increased thirst, and increased ADH mediated water reabsortion in the collecting duct, salt will also cause a shift of fluid from the intercellular space to extracellular space. –  Jasand Pruski Jan 12 at 20:33
    
oops, thats supposed to say intracellular space, not intercellular space... –  Jasand Pruski Jan 12 at 20:42

The basic reason is osmosis, the tendency of solutes to move from an area of high concentration to one of low concentration across a permeable barrier. So, ingesting large amounts of salt results in a high sodium concentration in the blood stream. This in turn causes water to enter the blood vessels by osmosis. More water in the blood means a greater volume of liquid and, therefore, higher blood pressure.

This process occurs primarily in the kidneys, specifically in the part of the nephron called the loop of henle (the image below was taken from wikipedia and is originally from Grey's Anatomy):

enter image description here

With low salt concentrations, water exits the loop and is retained by the body. When sodium concentration is high, the opposite occurs, water enters the blood stream by osmosis and increases blood pressure.

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Great answer! I read that there may be variations in the blood pressure over the course of the day, and that the renal function itself has a circadian rhythm. Is there anything circadian in this loop that may alter how Sodium affects the blood pressure based on the time of day? –  Alex Stone Nov 2 '12 at 20:40
    
Blood pressure is far more attuned to maintain mean arterial pressure, and as a result, the level of activity and intensity someone is engaging. So during the day, when one is more likely to be active, blood pressure is overall greater, and then at night, when activity tends to decrease, so does blood pressure. This may be conflated with circadian rhythms (which likely do exist, but probably don't affect blood pressure all that much). –  leonardo Nov 2 '12 at 21:33
    
This is a nice answer. As blood sodium concentration increases, osmosis causes an increase in blood volume, which cause a greater degree of atrial and vessel stretching, leading to a secretion of atrial natriuretic peptide (ANP) which tries to regulate blood pressure. Read more about ANP at its wikipedia page (en.wikipedia.org/wiki/Atrial_natriuretic_peptide) –  leonardo Nov 2 '12 at 21:39

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