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Dynamic regulation of mean arterial pressure: special role of renal resistances.

In this paper the general properties of homeostatic variables are discussed, and it is shown that mean state regulation must be defined over some stated epoch and that the variance associated with such regulation can permit maximum/minimum variations of 2:1. Dynamic regulation is then contrasted with (automatic) control, and mean systemic arterial pressure (MSAP) in mammals is shown to be under dynamic regulation in the long run, although it may be under control in the short run. The discussion is next developed around the branching rules for mammalian arterial trees. The heart and lymphatic system are introduced as separate, zero-back-pressure, sump pumps that "ground" central venous pressure and interstitial pressure, respectively. Hydraulic flow arguments, combined with arterial tree branching rules, are used to demonstrate the short-circuit character of the renal circulation, and the peculiar distribution of pressure drops within it. From that peculiar distribution it is proposed that there is a nonanatomic, functional resistance located approximately at the region of efferent arterioles, which adds 15 mm Hg of hydrostatic pressure, upstream, to the central arteries. The chief aim of the paper is to raise certain questions about inconsistencies in data about renal circulation, to suggest a resolution, and to show how MSAP is set at (approximately) 100 mm Hg.