Hemodynamic Homeostasis · §4 · residual = step / (1 + G)

The baroreflex fast buffer and PIEZO knockout

The baroreflex is the seconds-scale negative-feedback buffer. A 20 mmHg step is buffered about 75%, leaving a residual of 5 mmHg, because the closed-loop residual is step/(1 + G) with gain G = 3; settling takes ~1.35 s. Removing the PIEZO sensor abolishes transduction and pressure goes labile. Buffering shape sim-reproduced [V]; gain and latency cited [L].

On the fast track, the baroreceptor–baroreflex arc opposes pressure excursions within seconds. A step is buffered to a residual = step/(1 + G); with G = 3 a 20 mmHg step leaves 5 mmHg (75% buffered). Loss of PIEZO transduction removes the buffer and produces labile pressure; the shape is reproduced [V], absolute gain/latency cited [L].

Fast negative feedback buffers a pressure step

The fast loop is a proportional negative-feedback controller acting on a seconds timescale. Rising pressure raises baroreceptor firing, which travels to the nucleus tractus solitarius and withdraws sympathetic outflow, lowering SVR and heart rate and opposing the excursion (research target RP2). This is the reflex that keeps standing, coughing or a transient surge from translating one-to-one into pressure.

For a step disturbance the steady residual is step/(1 + G). With open-loop gain G = 3 a 20 mmHg step is reduced to 5 mmHg, i.e. about 75% buffered, and settles in roughly 1.35 s. The buffer attenuates but does not eliminate the disturbance: a proportional controller always leaves a finite residual, which is exactly why a second, integral loop is required to drive the long-run steady error to zero. The fast loop buys time; the slow loop sets the reference.

Knock out the sensor and the buffer disappears

The buffer depends on intact mechanotransduction, and removing the sensor is not the same as removing the controller. When the PIEZO1/2 sensor is double-knocked-out, afferent firing no longer tracks pressure and the fast correction vanishes, leaving labile pressure — the model phenotype that matches the cited labile hypertension after baroreceptor PIEZO loss. The downstream arc (NTS, sympathetic outflow, vessels) is intact; there is simply nothing left to read the disturbance.

This cleanly separates a transduction failure from a controller failure, and it is the mirror image of the orthostatic case treated later, where the same buffer loss shows up as a failure to defend against a downward postural step. The buffering shape and the knockout phenotype are reproduced [V]; the absolute open-loop gain and closed-loop latency are cited anchors [L], not derived here, and are anchored to clinical units in the calibration chapter.