Salt is a hygroscopic substance,that is to say,one of those substances which can absorb moisture i.e. water even from the air and retain it .That's why you must have observed that during rainy season when the air is heavy with moisture, if salt is kept in an open container,it becomes wet.An average adult human being has 5 to 5.5 liters of blood.In a person with normal blood pressure,the kidneys are able to separate out extra amount of water which is then synthesised and thrown out of the body.But in a person with high blood pressure and controlled with medicines,taking excessive salt causes fluid ( water) retension,increasing the blood volume beyond the desired limit,diuretics like frusamide or hydrochlorothiazide is used to bring the blood volume back to the desired levels.Blood Pressure is the pressure exerted on the walls of the vessels it travels through.Extra pressure exerted on the walls of the vessels can burst it at it's weakest point,it can be in the brain,in the eyes or any where in the body.Hence to keep the blood volume under control,extra salt intake has to be avoided.