The body sodium content determines the arterial blood volume.
When dietary salt increases, the body salt content increases, and this in turn increases the arterial blood volume. Blood pressure would rise if the capacity of the arterial space did not increase at the same time.
When dietary salt is reduced, arterial blood volume falls. This would lower blood pressure if the capacity of the arterial space did not contract at the same time.
Thus, the key to maintaining a normal level of BP in the face of changing levels of body salt is reciprocal changes in the capacity of the arterial space.