Roles of the kidneys and fluid volumes in arterial pressure regulation and hypertension.

The goal of this paper has been to show that the renal-fluid volume mechanism for control of the arterial pressure is an extremely powerful one, so powerful that it is either impossible or almost impossible for the arterial pressure ever to be adjusted to any other value besides the level dictated by this mechanism. The reason for the extreme potency of this mechanism for pressure control is that it never stops working until the intake and output of salt and fluid volume come to an exact state of equilibration. This occurs where the salt and water intake line, as illustrated in Figure 3, crosses the renal function curve which expresses the relationship between arterial pressure and fluid output by the kidneys. Some of the factors that make this renal-fluid volume mechanism such an excellent pressure regulator are the following: First, once the baroreceptor mechanism for pressure control has adapted, the systemic arterial pressure is affected greatly by the levels of extracellular fluid volume and blood volume. Second, the blood flow autoregulation mechanism further enhances the effect of slight changes in fluid volume on pressure, increasing the effectiveness of the pressure control system at least five times additional. Third, the renin-angiotensin system also plays a major role in increasing the sensitivity of this pressure control system. When salt intake becomes very great, renal output of renin and subsequent formation of angiotensin both decrease drastically. This shifts the renal function curve back toward a lower pressure level, thus compensating for most of the increase in pressure that would otherwise occur as a result of the increased salt intake. Putting all the above factors together, one finds that very slight changes in the fluid volume level in the body can have drastic effects on pressure. And, conversely, very slight changes in arterial pressure above or below the normal level can initiate large changes in urinary output of salt and water, thus rapidly returning the pressure back to that level at which intake and output of water and salt are in balance.