From the Mayo Clinic
Your kidneys naturally balance the amount of sodium stored in your body for optimal health. When your body sodium is low, your kidneys essentially hold on to the sodium. When body sodium is high, your kidneys excrete the excess in urine.
But if for some reason your kidneys can't eliminate enough sodium, the sodium starts to build up in your blood. Because sodium attracts and holds water, your blood volume increases, which makes your heart work harder and increases pressure in your arteries. Such diseases as congestive heart failure, cirrhosis and chronic kidney disease can make it hard for your kidneys to keep sodium levels balanced.
Some people's bodies are more sensitive to the effects of sodium than are others. If you're sodium sensitive, you retain sodium more easily, leading to fluid retention and increased blood pressure. If this becomes chronic, it can lead to heart disease, stroke, kidney disease