Why is it always Sodium that we talk of in blood pressure?

Question

Why is that we emphasize so much on sodium in blood pressure? I understand that it is a major extracellular ion, but, it also is tightly regulated. So when something is that tightly regulated, won't small concentrations of the potassium matter? And what about the chloride and bicarb? They are all there in the blood in comparable quantities but we never talk of their osmotic importance. Sodium retention and blood pressure - why not chloride retention?

Is it not required (like calculating sodium includes them too)? or is it that they actually don't matter? Also aldosterone, we conserve the sodium but aren't we pushing out potassium? Potassium also is an ion with oncotic activity right? Why is Natriuresis so important why not say, Chloriuresis?

So all in all, why does sodium steal the limelight?

Answer 1

The key is in fluid compartments and osmotic activity. For the purposes of this discussion, let us say that there are two fluid compartments the ExtraCellular Fluid (ECF) and IntraCellular Fluid (ICF). Only the ECF contributes to blood volume and blood pressure.

Na is not only the major ECF ion, but it is also confined there. The ICF concentration of Na is much lower than in the ECF, and it is continuously, invariably and actively kept out of the ICF. In other words, the vast majority of ingested Na ends up in the ECF, and drag the associated amount of water (ie volume, thereby increasing blood pressure). It is the opposite for K, which is mainly in the ICF. As for Cl, it is more or less equal in the ICF and ECF, and is allowed to move freely between them, meaning that you need to ingest twice as much as a given amount of Na for a similar effect on volume distribution. Bicarb is a minor ion, and moves freely between compartments. These are the reasons why, clinically, if someone is hypovolemic (ie dehydrated, losing blood) they are given a sodium based fluid or a protein based fluid (colloid), because both will drag volume and stay in the ECF.

As for the other part of your question, isn't it tightly regulated? Yes, but extra sodium takes some time to get excreted (much less than 1% of sodium that goes through the kidneys is excreted). In other words, any extra sodium (i.e. extra volume as we have established) will stay there for some time, and if you are continually having high sodium meals, every day, you will walk around with extra sodium most of the time. Therefore, your blood pressure will be higher than it would be, if your diet was lower in sodium.

So, no, sodium is not stealing the spotlight from anyone when it comes to ECF. But the other ions have their glory under other spotlights.