When the body is dehydrated, it may restrict the flow of blood to non-vital areas and concentrate it instead on the vital organs – this may cause your blood pressure to rise. If you drink enough water, the restricted blood vessels usually begin to open up, lowering blood pressure. Sure, you could take blood pressure medication, but why, when the safest and cheaper solution is usually to drink enough water. Many people have lowered their blood pressure by increasing their water intake. Of course, weight loss, stress reduction and a sensible diet are also important.