Pressure, Posture, and Heart Rate Variability

Series: Breathing, Body, Consciousness, and the Shifting of the Tensional Selves (Eus Tensionais)

Introduction — Brain Bee (first-person consciousness)

When I squat down, something changes immediately—
before I even think.

My heart speeds up.
My breathing adjusts.
I feel a slight internal shift, as if the body were reorganizing itself to avoid falling.

When I stand up again, everything changes once more.

None of this is emotion.
None of this is choice.
It is the body responding to pressure and gravity.

Pressure as biological information

Blood pressure is not only a clinical number.
It is a continuous signal that tells the body whether it is stable, threatened, or adapting.

Every postural change:

• redistributes blood,

• modifies cerebral perfusion,

• challenges autonomic balance.

The body does not “observe” this.
It responds automatically.

Baroreflex: the silent guardian

The baroreflex is the main mechanism that keeps arterial pressure within functional limits.

Sensors located in the:

• carotid sinus,

• aortic arch,
  detect minimal pressure changes and send signals to the central nervous system.

In response, the body adjusts:

• heart rate,

• vascular tone,

• sympathetic and parasympathetic output.

This adjustment happens within fractions of a second.

Posture and HRV: squatting and standing

When you squat:

• venous return increases,

• central pressure shifts,

• the baroreflex responds.

When you stand up quickly:

• cerebral pressure drops momentarily,

• the sympathetic branch is activated,

• heart rate rises,

• RMSSD tends to fall temporarily.

This is normal.
The issue is not the drop—
it is the inability to recover.

A healthy system:

• reacts quickly,

• stabilizes,

• recovers variability.

Extreme gravity: divers and astronauts

In divers:

• external pressure increases,

• blood redistribution becomes intense,

• the diving reflex emerges,

• vagal tone can increase in certain contexts.

In astronauts:

• microgravity reduces postural challenge,

• the baroreflex becomes deconditioned,

• when returning to Earth, maintaining pressure and HRV can be difficult.

Both cases show:
HRV and RMSSD are trainable through the physical environment.

Heart rate variability as a readout of adaptation

HRV and RMSSD do not measure “calm.”
They measure the capacity to adapt to real physical forces.

When gravity changes, pressure changes.
When pressure changes, the heart changes.
When the heart changes, variability reveals whether the body is prepared.

This is pure physiology.

From heart to brain: direct impact in NIRS/fNIRS

Posture and pressure shifts:

• alter cerebral blood flow,

• modify cortical oxygenation,

• appear clearly in NIRS/fNIRS measures.

In fNIRS, we observe:

• HbO and HbR fluctuations,

• position-dependent changes,

• effects related to baroreflex dynamics and CO₂.

These signals are not “noise.”
They are direct expressions of the body–brain relationship.

APUS, Tekoha, and pressure

Posture (APUS):
defines how gravity runs through the body.

The viscera (Tekoha):
respond to internal pressure redistribution.

Breathing, heart, and brain
reorganize simultaneously.

There is no posture without visceral impact.
There is no pressure without conscious impact.

When the body cannot move

In prolonged immobility:

• the baroreflex loses training,

• HRV becomes impoverished,

• postural adaptation decreases.

This is not psychological.
It is physiology deprived of variability.

The body needs:

• position changes,

• gravitational challenge,

• alternation of pressure.

Recognizing this in your own body

Observe—without correcting:

• How does my body react when I stand up?

• Does my breathing follow the movement?

• Do I feel dizziness, or rapid adaptation?

• Does my heart adjust and then stabilize?

These signals show how your system reads the physical world.

Closing

Pressure and gravity are not enemies of the body.
They are silent teachers.

They train:

• the heart,

• the autonomic system,

• bodily consciousness.

HRV and RMSSD are records of this learning.
And the brain—measured by NIRS/fNIRS—responds to every adjustment.

To live is to adapt to the forces of the world—
with enough variability to remain whole.

This text is part of the series Breathing, Body, Consciousness, and the Shifting of the Tensional Selves (Eus Tensionais), where different aspects of the same living system are approached from complementary angles.

References (post-2020)

La Rovere, M. T., et al. (2020). Baroreflex Sensitivity and Heart Rate Variability. European Heart Journal.
→ Links baroreflex function to direct modulation of HRV and cardiovascular adaptation.

Furlan, R., et al. (2021). Postural Changes and Autonomic Regulation. Autonomic Neuroscience.
→ Analyzes autonomic responses during postural transitions such as squatting and standing.

Hughson, R. L., et al. (2020). Cardiovascular Regulation in Microgravity. Journal of Physiology.
→ Describes adaptations of baroreflex function and HRV in astronauts.

Perini, R., et al. (2021). Diving Reflex and Autonomic Modulation. Frontiers in Physiology.
→ Demonstrates vagal and sympathetic changes in divers.

Tachtsidis, I., & Scholkmann, F. (2021). False Positives and Systemic Physiology in fNIRS. Neurophotonics.
→ Shows how systemic changes in pressure and posture influence fNIRS signals.

Herold, F., et al. (2020). Physical Activity, Cerebral Hemodynamics and fNIRS. Neuroscience & Biobehavioral Reviews.
→ Relates body movement to cerebral hemodynamic variability.

Obrig, H. (2020). NIRS in the Study of Human Brain Function. NeuroImage.
→ Establishes NIRS sensitivity to systemic and postural influences.

Scholkmann, F., et al. (2022). Systemic Physiology in fNIRS Measurements. Neurophotonics.
→ Discusses how blood pressure and posture directly shape fNIRS signals.