The regulation of sodium and water excretion is incredibly complex with many hormonal and neurological reactions. It is our plasma volume that determines the cardiovascular pressures. These pressures control the total body sodium by acting directly on the kidney and stimulating certain reflexes.
The volumes of ECV(effective circulating volume), ECF (extracellular fluid volume), blood pressure and cardiac output increase as sodium levels rise, and decrease as sodium levels fall. The kidneys respond by adjusting the sodium chloride excretion.
For this reason I doubt that dietary potassium can have a great effect on total sodium balance in your body. It is an essential element just like the sodium though that our bodies need. In one study of borderline hypertensive rats, published in JASN in 2006, it was shown that an increased dietary potassium chloride intake did not prevent or reverse salt-sensitive hypertension caused by an increased sodium chloride intake over a period of 12 weeks. By the way, they developed hypertension with the increase in dietary sodium. The increased dietary potassium did not appear to have any protective properties against the sodium intake.
I have been trying to make some sense of these issues because they concern me and I have to learn more about them. I am reading a small book about kidney problems I've had for a while that is written is layman's language. I find that I can sort of understand some of the hormonal mechanisms of fluid & salt regulation-but not very well. I should probably purchase a kidney physiology book for dummies.....