Hypotensive mechanisms of antihypertensives based on the water tank model

Mechanisms of hypotensive action of antihypertensive drugs were theoretically considered using the water tank model on body fluid volume-blood pressure regulation. If a cock 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 cock, which represent mean arterial pressure (MAP), cardiac output (CO), and urinary Na+ excretion rate. The height of the cock from the tank bottom and the size of the cock correspond to the x-intercept and slope of the renal function curve (pressure-natriuresis relationship). Vasodilators lowered the cock height and enlarged the base area (reduced TPR), resulting in an increase in the volume (CO). beta-Blockers not only lowered the height, but also decreased the size of the cock. Because they do not alter TPR (base area), CO (volume) must be reduced. Converting-enzyme inhibitors had the same effects on the height and size of the cock as beta-blockers, but enlarged the base area (reduced TPR), resulting in an increase in the volume (CO). Diuretics increased the cock size without affecting the cock height or the base area, resulting in a decrease in CO. The effects of antihypertensive drugs on the cock (kidney) seemed essential in their hypotensive action, similarly as abnormality of the renal function curve is essential in the genesis of hypertension. These analyses can illustrate schematically the hemodynamic changes induced by antihypertensive drugs.