Hemodynamic Homeostasis · §6 · P* = P₀ + ΔP_set

Essential hypertension as a setpoint reset

Essential hypertension is not a stuck operating point but a reset of the defended reference. The integral controller obeys P* = P₀ + ΔP_set, so the attractor moves up by ΔP_set (+20 mmHg, 93 → 113); an operating-point drug (transient −15 mmHg) is opposed back (durable drop 0). Reset shape [V], risk anchor [L], incidence open [O].

Because the renal controller is an infinite-gain integrator, durable hypertension requires resetting its reference, not perturbing the operating point. The defended pressure follows P* = P₀ + ΔP_set and moves up by exactly ΔP_set (+20 mmHg, 93→113); an operating-point push is rejected back. This mirrors the obesity lipostat reset [V]/[L]; absolute incidence is open [O].

The defended attractor moves, and moves by exactly the reference shift

Hypertension is modelled as a reference reset of the slow integrator, which makes a precise quantitative prediction rather than a vague “the body adapts.” The defended pressure obeys P* = P₀ + ΔP_set, so a rightward reset of the pressure-natriuresis curve shifts the attractor up by exactly ΔP_set — here +20 mmHg, from 93 to 113 mmHg (research target RP4), with the predicted and observed shifts equal. This is a defended state, not a passive elevation: the integral controller actively holds the new, higher reference, which is why the elevation is stable rather than drifting, and why it returns after perturbation.

Operating-point drugs are rejected back

An operating-point push does not move the reference, so the integrator opposes it back. A drug that lowers pressure acutely produces a transient drop of 15 mmHg but is rejected toward the reset reference, giving a durable drop of only 0 mmHg — symptomatic relief, not a cure. This is the loop-level explanation for a familiar clinical fact: monotherapy that only dilates or only slows the heart tends to be escaped, while durable control engages the renal/volume arm.

The same logic appears in obesity, which is the point of the parallel: an integral controller (the lipostat) defends a reset body-weight reference, and operating-point pushes such as caloric restriction alone are opposed back. In both systems the fundamental target is the reference itself — here the renal reference — treated in the therapy chapter. The reset shape and the opposed-back behaviour are reproduced [V]; relative risk versus cited salt/BMI cohorts is an anchor [L]; absolute incidence is open [O].