Why is it bad for a person with kidney trouble to have salt?

Answer 1

It is the sodium in salt that is the problem. Your body removes unwanted fluid by filtering your blood through your kidney. This requires a balance of sodium and potassium to pull the water across the wall from the bloodstream into a collecting channel in the kidney. A high salt diet will alter this sodium balance. This causes the kidneys to have reduced function and so less water is removed and this results in higher blood pressure. High blood pressure is not good for you generally but it is especially not good for people with kidney disease.

Answer 2

It "messes" with the law of diffusion, which is how the nephrons in the kidneys use to "push" the blood through the SPM.