how does sodium increase blood pressure?

Retention of water .

Explanation :

Sodium like other substances dissolved in the blood is osmotically active . That means that , a high concentration of sodium on any side of a filtration membrane (like in the kidney) will lead to water moving towards that side .

For example in Diabetes , because of the high blood sugar in the blood , a lot of sugar escapes to the urine , that sugar takes a lot of water with it , thats why people with diabetes often pee all the time . (polyurea) .

When too much sodium is taken in the diet , the high blood sodium will likewise pull a lot of water to the blood , the blood VOLUME then increases , and consequently the BLOOD PRESSURE will increase .

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The kidneys actively re-absorb sodium back into the body. When the sodium atom gets re-absorbed, it drags 5 molecules of water with it. So, water also gets re-absorbed. The effect of the water re-absorption is water retention. The retained water causes the blood volume to increase within a relatively finite vascular space. As a result, the blood pressure increases.

I have heard it is the chloride portion of sodium chloride that raises blood pressure. Increased fluid retention, I would surmise. Pressure is inversely proportional to volume. Just a conjecture, perhaps a bio-medical type will comment.

Hypertension (high blood pressure) is something that is not well understood at all. While there are two types of hypertension, primary and secondary, the overwhelming majority of cases are considered primary, or IDIOPATHIC. Idiopathic is never a good thing, it means basically we have no idea what causes it. So if someone told you they know for sure what causes primary hypertension, they probably don't know what they are talking about. Idiopathic/Primary hypertension accounts for over 90% of high blood pressure cases by the way.

With that little introduction, you can conclude that high sodium plasma levels (hypernatremia) do not CAUSE hypertension, but they can contribute to making it worse, so we advise those with hypertension to monitor their salt intake. The mechanism is pretty complicated however, and it is mediated by the Renin-Angiotensin-Aldosterone System.

The R-A-A system is activated in the nephron of the kidney when the "juxtaglomerular apparatus" cells detect a decrease in bloodflow (perfusion). They respond by secreting the enzyme renin. Renin then enters the circulation, where it cleaves the readily available proenzyme angiotensinogen to angiotensin I. Angiotensin I then migrates to the LUNG of all places, where Angiotensin Converting Enzyme converts Angiotenin I into Angiotensin II.

Now this is the important part of this. While increasing intravascular volume is an important step in regulation of blood pressure, it is NOT the most important, as homeostatic mechanisms will simply adjust to volume changes. Angiotensin II itself is a VERY potent vasoconstrictor. So this increase in Total Peripheral Resistance (TPR) mediated by Ang II is VERY important in elevating blood pressure.

Volume changes do of course occur as well. Ang II migrates to the adrenal cortex where it stimulates secretion of aldosterone, which then circulates to the kidney where it acts in tubule cells by improving function of a Na+/K+ pump (among other actions) to increase reuptake of Na+ (and thus WATER to increase plasma volume) and also release of K+ into the urine. Aldo also stimulates thirst receptors in the hypothalamus.

Ang II also mediates volume changes by stimulating release of vasopressin (or anti-diuretic hormone, or simply ADH) by neuroendocrine cells in the posterior pituitary. The ADH then travels to the collecting ducts of the nephron tubules, where it places water channels called aquaporins into the collecting duct lumen, to increase reuptake of water back into the circulation.

This is an area where a lot of old moody is talked. In general, people with high blood pressure improve when they lower their salt intake.

This has led a lot of people to believe that salt has caused the high BP in the first case, which has been demonstrated to be untrue.

So your answer is It doesn't.

It clogs arteries.