Why a doctor advises a person suffering from High blood pressure to take less amount of sodium chloride

There is common disease related with it known as edema.In this disease,concentration of water or may be some other related fluid increases in our body cells instead of normally excreating out of our body through the kidneys, after intake of some salty food items(since taking a lot of salty food  is accompanied with the intake of lots of water) .As a result,swelling of body cells  takes place as volume of our blood increases.As a result our heart has to work more and B.P increase.Osmosis is basically behind this phenomenum.