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← How it works

02

Autonomic system

The automatic controller of heart rate, blood flow, digestion and temperature.

To the heartTo the lungsTo the gutThe vagus nerveThe pelvicoutflowHeartLungsGutBladder
Fight and flightRest and repairHeartLungsGutBladder

A schematic, not to scale — the places this mechanism works.

Part one — how this normally works

Standing up is a controlled emergency

Standing drops roughly half a litre of blood into the legs and abdomen within seconds. Baroreceptors in the neck and chest sense the fall, sympathetic outflow rises, vessels constrict, heart rate lifts a little, and pressure to the brain is restored in under a minute.

Two branches, one dial

Sympathetic and parasympathetic outflow are not opposites so much as a continuously adjusted balance controlling heart rate, gut motility, sweat, pupils and blood distribution.

Volume is the quiet variable

Blood volume, salt handling and venous tone decide how much reserve the reflex has to work with. Reduce any of the three and the same reflex has to work harder for the same result.

Part two — what varies between bodies

  • Deconditioning, dehydration, heat, illness, poor sleep and the luteal phase all shrink the margin.
  • Some bodies compensate mostly with heart rate, others mostly with vessel constriction.
  • Small nerve fibers carrying autonomic signals are fragile and can be affected by infection and inflammation.

Part three — where it diverges

This is the first place conditions get named

  • When upright posture is compensated for almost entirely with heart rate, the result is a sustained rise of 30 bpm or more without a large drop in pressure — the pattern defined as postural orthostatic tachycardia syndrome, one form of dysautonomia.
  • Post-viral autonomic injury is now the best-described route in, seen widely after Covid-19 infection.
  • Compliant veins and low blood volume push in the same direction, which is why connective tissue differences and autonomic symptoms are so often found in the same person.
  • Mast-cell mediators dilate vessels directly, so immune activation and autonomic instability amplify each other.

What moves this mechanism

  • Volume and salt as first-line, with compression at the waist and abdomen where pooling is largest.
  • Recumbent to upright reconditioning, dosed against symptoms rather than a calendar.
  • Treating the co-travellers: sleep, anemia, mast-cell reactivity, thyroid.
  • Counter-manoeuvres for the moment itself.

Conditions that involve this mechanism