The ability of the kidneys to regulate extracellular fluid volume by altering sodium excretion is important for maintaining adequate volume within the vascular system. Sodium excretion is controlled by integrating physical, neural, and hormonal regulatory systems. The major systems involved in retaining sodium include the renin-angiotensin-aldosterone and sympathetic systems, whereas natriuretic factors such as atrial natriuretic peptide and nitric oxide are important in promoting sodium excretion. In response to increased sodium intake, the sodium-retaining systems are inhibited and natriuretic hormones are activated. Pressures and flows within the microcirculation of the kidney, in concert with neural and hormonal systems, interact to regulate sodium excretion. The quantitative importance of each of these systems in regulating sodium balance is variable and is determined by the physiological or pathiophysiological condition.