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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
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2 Answers

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):

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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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I found this Sodium fact sheet that mentions effects on blood pressure, but does not directly indicate why it is rising.

Off the top of my head, I know that Sodium, Potassium and Calcium are 3 substances that affect sodium-potassium channels, which control the flow of nutrients in and out of the cell. Maybe this is a part of the answer to your question?

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It has rather to do with how kidneys handle dietary sodium. –  nico Nov 1 '12 at 21:52
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