Sodium sensitive and sodium retaining hypertension. The differences between sodium sensitive and sodium retaining hypertension were theoretically considered using a water tank model of body fluid volume-blood pressure regulation. If an outlet valve is attached to a tank with a base area corresponding to the reciprocal of total peripheral resistance (TPR) and water is poured into this tank at a rate corresponding to the amount of Na+ intake, then equilibrium should be achieved at a certain water level, volume and output from the outlet, which represent mean arterial pressure (MAP), cardiac output (CO) and urinary Na+ excretion. The height of the outlet from the tank bottom and the size cross-sectional area, of the outlet correspond to the x-intercept and slope of the renal function (pressure-natriuresis) curve, respectively. In both nonsodium sensitive hypertension, due to the shift of the curve toward a higher blood pressure level (elevated height of the outlet) without change in the slope (size of the outlet), and sodium sensitive hypertension, due to the depressed slope of the curve (reduced outlet size), not only MAP (water level) but also CO (water volume) are increased, resulting in sodium retaining hypertension, if TPR (reciprocal of base area) remained unchanged, while CO is relatively unchanged, resulting in nonsodium retaining hypertension, if TPR is elevated. Thus, the MAP and its sensitivity to sodium intake is determined by the renal function curve. Since body fluid volume is determined by both the renal function curve and TPR, however, changes in TPR during the development of hypertension is a major factor in determining whether or not the body fluid volume has to change only a small amount or a large amount. Therefore, the sodium sensitivity of blood pressure and sodium retention must be considered separately. 