Historically, the paradigm for all maladies was associated with an imbalance of the 4 humors: blood, black bile, yellow bile, and phlegm. Although our understanding of disease has evolved significantly since the time of Hippocrates, a similar cornerstone of inpatient and ambulatory care involves understanding and correcting imbalances of volume. The kidneys are the principal organs controlling extracellular volume, capable of both sensing and altering salt retention through multiple redundant pathways, including the sympathetic nervous system and the renin-angiotensin-aldosterone system.