WHEN PHYSIOLOGY BECOMES CONSTRAINED

Breathing, CO₂, and the Conditions of Autonomic Regulation

Rethinking Regulation

We often try to influence how we feel by directly targeting the autonomic nervous system — slowing the breath, stimulating the vagus nerve, or applying various relaxation techniques.

But what if the issue is not the lack of stimulation, but the conditions under which regulation is expected to occur?

The autonomic nervous system does not operate as an isolated switch between sympathetic and parasympathetic states. Its expression depends on the physiological conditions in which autonomic regulation takes place.

Rather than asking how to activate or suppress it, it may be more accurate to ask a different question:

Under what conditions can regulation remain effective?

The Body Is Not Neutral

Before considering specific mechanisms, it is important to recognize that the body is never operating from a neutral baseline.

Physiological state is not an abstract concept. It is continuously shaped by patterns of muscular activity, breathing behavior, and the cumulative effects of how the body responds to internal and external demands over time.

Sustained increases in somatic tone do not simply reflect momentary tension. Under certain conditions, they may gradually become part of the body’s baseline organization. Because skeletal muscle represents a large proportion of body mass, even subtle but persistent changes in tone may influence global mechanical conditions within the body.

These changes are not limited to movement. They extend to structures that participate in both voluntary and autonomic processes — most notably, the diaphragm.

In this sense, breathing is not only a function, but also an expression of the underlying physiological state.

The Diaphragm as a Functional Interface

To understand how physiological state translates into regulation, it is necessary to consider the structures through which these processes interact.

The diaphragm occupies a unique position within the body. It is not only the primary muscle of respiration, but also a structure that operates at the intersection of voluntary and autonomic control.

Its activity is influenced by both conscious and unconscious processes — from intentional breathing patterns to reflexive adjustments driven by metabolic demand. At the same time, it plays a central mechanical role in shaping pressure relationships between the thoracic and abdominal cavities.

Because of this dual role, even subtle changes in diaphragmatic tone or mobility may have effects that extend beyond breathing itself.

In this context, the diaphragm can be understood as an interface — a structure through which somatic activity, respiratory mechanics, and autonomic regulation are continuously linked.

How Breathing Works Under Normal Conditions

To understand how these changes develop, it is essential to first clarify how breathing occurs under normal physiological conditions.

During inhalation, the diaphragm contracts and moves downward, flattening from its dome-shaped resting position. This increases the vertical dimension of the thoracic cavity, reduces intrathoracic pressure, and allows air to flow into the lungs. At the same time, the external intercostal muscles assist by elevating and expanding the rib cage, further increasing thoracic volume (De Troyer & Boriek, 2011; Loring & O’Donnell, 2010).

During exhalation, the process reverses. The diaphragm relaxes and moves upward, returning toward its dome-shaped configuration. This reduces thoracic volume and facilitates the passive outflow of air, primarily driven by the elastic recoil of lung tissue and the chest wall (West, 2012).

In a coordinated system, these movements occur rhythmically and with minimal resistance. The diaphragm continuously adapts its position and tension, allowing each breath to begin from a relatively neutral resting state at the end of exhalation.

This end-expiratory phase is particularly important, as it represents the point at which the respiratory system resets before the next inhalation.

When Exhalation No Longer Completes

When the coordinated mechanics of breathing begin to change, these alterations often become most apparent during exhalation.

As described above, normal exhalation allows the diaphragm to relax and return toward its dome-shaped resting position. This upward movement is essential, as it enables the respiratory system to reach its end-expiratory state — the point at which the next breath begins.

When diaphragmatic movement is subtly restricted, this return may become incomplete. Instead of fully relaxing and ascending to its normal dome-shaped position, the diaphragm may remain partially engaged and positioned lower than expected at the end of exhalation.

In practical terms, this means that exhalation does not fully complete. A portion of air remains in the lungs, and the respiratory system does not return to its usual resting configuration before the next inhalation begins.

This shift may be small at first, but when repeated over time, it becomes part of the system’s baseline behavior.

A Shift in the End-Expiratory State

Over time, repeated incomplete exhalation alters the resting configuration of the respiratory system.

Because the diaphragm does not fully return to its relaxed position, each subsequent breath begins from a slightly altered starting point. In this state, a portion of air consistently remains in the lungs at the end of exhalation.

This can be understood as a gradual increase in end-expiratory lung volume (EELV), reflecting a shift in the system’s functional baseline. Similar mechanisms are described in the context of dynamic hyperinflation and altered breathing mechanics (O’Donnell et al., 2001).

Residual volume contributes to this baseline as a structural component, but the primary change occurs within the dynamic portion of the respiratory cycle.

Rather than viewing this as a fixed parameter, it may be more accurate to consider it as an ongoing physiological process — one that reflects how completely the system is able to return to its resting state at the end of each breath. 

In this context, this is not merely a local change in breathing mechanics. It represents a shift in the system’s functional baseline — the state from which each respiratory cycle begins and from which further regulatory responses emerge. 

CO₂ and the Regulation of Breathing

As the end-expiratory lung volume shifts and breathing begins from an altered baseline, the pattern of ventilation also changes.

Breathing may become subtly faster or more shallow — not necessarily as a conscious adaptation, but as a mechanical consequence of the altered starting point. These changes alter the pattern of alveolar ventilation and may influence CO₂ regulation, particularly when accompanied by a tendency toward hyperventilation. 

CO₂ is not merely a by-product of metabolism. It plays a central role in regulating respiratory drive and maintaining acid–base balance, while also serving as a key signal for central and peripheral chemoreceptors (Smith et al., 2006; Dempsey & Smith, 2014). Stable CO₂ levels are essential for maintaining appropriate sensitivity within this regulatory system.

When breathing becomes less efficient or shifts toward habitual over-breathing, CO₂ regulation may become less stable, and CO₂ levels may gradually decrease. Even mild, chronic reductions in CO₂ can alter chemosensitivity and change the conditions under which breathing is regulated.

Autonomic Function as an Expression of State

Changes in CO₂ are not confined to respiration. They extend into the autonomic domain.

Reduced CO₂ levels have been associated with altered autonomic regulation, although these relationships are complex and depend on the broader physiological state, including changes in vagal modulation and cardiorespiratory coupling (Ben-Tal et al., 2012). In this context, the autonomic nervous system does not simply “shift” in response to external stimuli — its range of expression becomes constrained by the underlying physiological state.

Under normal conditions, the autonomic system exhibits dynamic variability, reflected in phenomena such as respiratory sinus arrhythmia and heart rate variability (Shaffer & Ginsberg, 2017). These fluctuations arise from the coordinated interaction between breathing mechanics, gas exchange, and autonomic modulation.

When diaphragmatic movement is restricted, end-expiratory lung volume is altered, and CO₂ regulation becomes less stable, these oscillatory interactions are reduced. The system continues to function, but within a narrower range of variability.

In this sense, the question is not whether the autonomic nervous system is active or inactive, but whether the physiological conditions allow it to remain effective.

From Local Changes to Systemic Constraint

Taken together, these changes form a continuous physiological sequence rather than isolated events.

A persistent increase in somatic tone alters diaphragmatic function. The diaphragm no longer returns fully to its relaxed, dome-shaped position at the end of exhalation. As a result, the end-expiratory lung volume remains elevated relative to its prior functional baseline, and each breath begins from this altered state.

These mechanical changes influence the pattern and efficiency of ventilation, which in turn affect carbon dioxide regulation. As CO₂ stability decreases, chemosensitivity and respiratory control are gradually altered.

Within this environment, autonomic function does not cease, but its expression becomes constrained. The system continues to operate, yet within a narrower range of variability.

In this sense, what appears as an autonomic imbalance may instead reflect a deeper shift in the physiological conditions that support regulation.

Beyond Breathing: A Broader Physiological Context

These mechanical and chemical changes may also extend beyond respiration itself.

Altered intrathoracic pressure can influence venous return, right heart filling, and the dynamics of circulation, while reduced pressure variability may affect the oscillatory interactions between the respiratory and cardiovascular systems.

Such interactions are reflected in phenomena such as respiratory sinus arrhythmia and heart rate variability, suggesting that breathing mechanics, gas exchange, and circulatory dynamics are closely interconnected.

While these aspects extend beyond the scope of this article, they point toward a broader physiological framework in which local mechanical changes may have systemic regulatory consequences.

Returning to the Question

From this perspective, autonomic regulation can be understood not only as a control system, but as a state-dependent phenomenon.

The body does not regulate in the same way under all conditions. When the underlying physiological state is altered — through persistent mechanical tension, changes in breathing patterns, and shifts in CO₂ balance — the capacity for self-regulation is correspondingly affected.

Breathing, in this context, is not the primary driver of regulation, but an expression of these deeper conditions.

Rather than asking how to stimulate the autonomic nervous system, it may be more useful to ask a different question:

Under what conditions can regulation remain effective?

Sometimes, the answer does not lie in adding new inputs, but in recognizing what has gradually limited the system’s ability to return to its functional baseline. 

Regulation does not begin with control. It begins with conditions. 

A related perspective

This text is part of a broader exploration of physiological regulation and the conditions that support it:

https://www.zilvinaskasteckas.lt/post/parasympathetic-nervous-system-not-a-simple-switch

References

1. De Troyer, A., & Boriek, A. M. (2011). Mechanics of the respiratory muscles. Physiological Reviews, 91(4), 1271–1350.https://doi.org/10.1152/physrev.00019.2010 

2. Loring, S. H., & O’Donnell, C. R. (2010). Respiratory mechanics. European Respiratory Review, 19(116), 115–121.https://doi.org/10.1183/09059180.00001510 

3. O’Donnell, D. E., Revill, S. M., & Webb, K. A. (2001). Dynamic hyperinflation and exercise intolerance. American Journal of Respiratory and Critical Care Medicine, 164(5), 770–777.https://doi.org/10.1164/ajrccm.164.5.2103024 

4. Smith, C. A., Rodman, J. R., Chenuel, B. J., Henderson, K. S., & Dempsey, J. A. (2006). Response of the respiratory control system to CO₂. Journal of Applied Physiology, 101(1), 323–332.https://doi.org/10.1152/japplphysiol.01042.2005 

5. Dempsey, J. A., & Smith, C. A. (2014). Pathophysiology of human ventilatory control. European Respiratory Journal, 44(2), 495–512.https://doi.org/10.1183/09031936.00048514

6. Ben-Tal, A., Shamailov, S. S., & Paton, J. F. R. (2012). Central regulation of heart rate and breathing. Philosophical Transactions of the Royal Society B, 367(1592), 1313–1325.https://doi.org/10.1098/rstb.2011.0271 

7. Shaffer, F., & Ginsberg, J. P. (2017). An overview of heart rate variability metrics and norms. Frontiers in Public Health, 5, 258.https://doi.org/10.3389/fpubh.2017.00258

8. West, J. B. (2012). Respiratory Physiology: The Essentials. Lippincott Williams & Wilkins.

Lithuanian version of this text:

https://www.zilvinaskasteckas.lt/post/kai-fiziologija-apribota