Relationship between sodium intake and excretion in the steady state

Relation between sodium intake (solid line) and urinary sodium excretion (dashed line) as sodium intake is progressively increased from 10 to 350 meq/day in normal subjects. Although excretion rises in parallel to intake, there is a slight lag in equilibration, and the area under the curve represents the net amount of sodium retained. This volume expansion constitutes the signal to maintain sodium excretion at a high level.