Why doesn't chloride cause high blood pressure?
Osmotic pressure is not the main mechanism of sodium-induced high blood pressure.  


  high dietary salt raises cerebrospinal fluid [Na(+)]. This leads, via the Na(+)-sensing circumventricular organs of the brain, to increased sympathetic nerve activity (SNA), a major trigger of vasoconstriction


How NaCl raises blood pressure: a new paradigm for the pathogenesis of salt-dependent hypertension.