Why is it bad for a person with high blood pressure to eat a lot of salt?

Fluid retention.: Salt causes fluid retention because the sodium is higher and it causes the serum osmolality to be higher, which brings in more water to the vessels of the body. This makes the blood pressure higher and the body must excrete more fluid to get it lower. So salt intake must be minimal. It also helps to give a diuretic as this increases urine exretion and so lowers blood pressure.