PHYSIOLOGICAL VARIABILITY AND THE DYNAMIC OF AUTOREGULATION
Living organisms are never truly static.
The heart does not beat with mechanical precision. Breathing is never exactly the same from one moment to the next. Blood pressure fluctuates, vascular tone changes, neural activity shifts, hormones are released in dynamic patterns, attention moves, and even muscle tone is constantly being adjusted.
For many years, physiology focused primarily on stability. Small fluctuations in biological signals were often regarded as random noise or minor deviations from an ideal steady state. Yet the deeper researchers looked into living systems, the clearer it became that biological regulation is inherently dynamic rather than static.
Over the past few decades, heart rate variability (HRV) has become one of the most widely studied indicators of autonomic regulation and physiological adaptation. Yet HRV is only one expression of a much broader principle.
Variability is a fundamental characteristic of every living regulatory system. It is present in breathing, blood pressure, chemoregulation, vascular tone, emotional responses, attention, and countless other physiological processes.
This perspective invites a different way of thinking about health. Rather than representing a fixed and unchanging state, health may be understood as the capacity to preserve function through continuous adaptive change. From this viewpoint, variability is not the goal itself, but one of the most visible signs of effective autoregulation.
Variability is not merely a feature of biological systems. It is one of the signatures of living autoregulation.
Heart rate variability: one expression of a broader principle
Many people assume that a healthy heart should beat with perfect regularity, almost like a metronome. Physiologically, however, such precision would not reflect vitality. Even at rest, the intervals between consecutive heartbeats change continuously. One beat may occur after 0.82 seconds, the next after 0.79, then another after 0.85. These subtle fluctuations are known as heart rate variability (HRV).
HRV is often misunderstood as a measure of heart rate. In reality, it describes the variation in time between consecutive heartbeats, typically assessed as changes in RR intervals on the electrocardiogram. These fluctuations are not measurement errors or random irregularities. Rather, they emerge from the continuous interaction between breathing, autonomic regulation, baroreflexes, chemoregulation, and central neural control.
During the second half of the twentieth century, growing evidence showed that reduced HRV after myocardial infarction was associated with an increased risk of sudden cardiac death. Since then, HRV has become one of the most widely used markers for studying autonomic function, stress, sleep, physical training, and chronic disease. Yet an equally important realization gradually emerged: HRV cannot be reduced to a simple measure of parasympathetic activity, despite how it is often portrayed in popular discussions. As emphasized by Thayer and Lane, autonomic regulation is better understood as a dynamic network linking the brain, the heart, respiration, and emotional processes rather than as the isolated activity of a single branch of the autonomic nervous system.
Breathing is one of the most important contributors to this variability. During inspiration, heart rate usually accelerates slightly; during expiration, it slows. This phenomenon, known as respiratory sinus arrhythmia (RSA), is not a pathological arrhythmia despite its name. On the contrary, it is generally regarded as a normal expression of coordinated autonomic regulation.
Breathing and cardiac rhythm should therefore not be viewed as independent physiological processes. Every breath changes intrathoracic pressure, venous return, baroreceptor activity, and autonomic modulation. With each respiratory cycle, the cardiovascular and respiratory systems continually adapt to one another. Variability, in this sense, is not generated by the heart alone. It emerges from the dynamic interaction of multiple regulatory systems.
Living systems remain stable not because they resist change, but because they continuously adapt without losing coherence.
Beyond isolated variables - autoregulation and variability
Breathing occupies a unique position in physiology because it is both automatic and voluntary, mechanical and emotional, chemical and neural. Few physiological processes integrate so many regulatory systems simultaneously. Changes in breathing therefore often provide one of the earliest windows into the broader state of physiological regulation.
For many years, variability was studied in a fragmented way. Researchers examined heart rate variability, blood pressure oscillations, respiratory rhythms, hormonal cycles, and neural activity as largely independent phenomena. More recent developments in nonlinear physiology and complexity science, however, have shifted this perspective. Living systems are increasingly understood as networks of interacting feedback loops rather than collections of isolated variables.
Breathing influences heart rhythm. Heart rhythm affects blood pressure. Blood pressure activates the baroreflex. Chemoreceptors modify breathing. Emotional states alter muscle tone, while muscle tone changes the mechanics of breathing. Those mechanical changes, in turn, reshape autonomic regulation.
What initially appears to be a sequence of separate physiological events gradually reveals itself as a continuously interacting regulatory network.
This systems perspective has become increasingly important in contemporary physiology. Goldberger and Lipsitz described physiological complexity as the capacity of living systems to maintain stability not through constancy, but through coordinated variability. Stability, in other words, is not the absence of change. It is the successful organization of change.
Living regulation emerges from coordination rather than from the isolated activity of individual physiological systems.
Rethinking autonomic regulation
From this perspective, the familiar description of the autonomic nervous system as a simple balance between "sympathetic" and "parasympathetic" activity begins to appear incomplete. The physiology is considerably more dynamic.
The autonomic nervous system does not operate as a switch that can simply be turned on or off. Instead, it continuously adjusts its activity in response to breathing, blood pressure, metabolism, posture, emotional state, sleep, and countless other internal and external influences.
For this reason, attempts to "activate the parasympathetic nervous system" may sometimes oversimplify a far more complex process. If physiological regulation has become chronically constrained—if breathing has grown shallow, muscle tone remains persistently elevated, expiration has lost part of its natural range, and adaptive flexibility has diminished—the central issue may not be insufficient parasympathetic activity. It may be a reduction in the regulatory freedom of the system as a whole.
This idea resonates with McEwen's concept of allostatic load. Living organisms are remarkably capable of adapting to stress, but adaptation is not without cost. Over time, the regulatory baseline itself may shift, leaving the organism functioning from a progressively narrower physiological range. Sterling later extended this view by describing the organism as a predictive system that continuously anticipates and adjusts to its environment rather than merely defending a fixed internal equilibrium.
A different perspective on health
This broader understanding of variability gradually changes the way we think about health.
Health is often imagined as a stable condition in which physiological variables remain within predefined limits. Yet living organisms do not maintain health by remaining unchanged. They preserve it by continuously adjusting to changing internal and external conditions while maintaining functional coherence.
In this sense, variability is not the opposite of stability. It is one of the mechanisms through which stability is achieved.
This perspective also changes how physiological measurements are interpreted. Whether we observe heart rate variability, breathing patterns, blood pressure oscillations, or other biological rhythms, these parameters should not be viewed simply as isolated numbers. They are expressions of an integrated regulatory system adapting moment by moment to the demands placed upon it.
For this reason, an important clinical question may not always be How can variability be increased? A more fundamental question is What prevents physiological regulation from expressing its natural variability?
The answer is unlikely to be the same for every individual. Aging, disease, medication, genetic factors, chronic stress, sleep deprivation, and persistent physiological overload may all contribute through different mechanisms. Sometimes the answer lies not in the regulatory system itself, but in the conditions under which it is expected to operate.
Persistent increases in skeletal muscle tone may become one of the factors that progressively constrain breathing mechanics and the adaptive flexibility of the regulatory network. The organism often continues to function remarkably well, but it does so within an increasingly narrower regulatory range.
When health improves, physiological variability often becomes more apparent. Clinical interventions may therefore be better understood as restoring the conditions in which healthy variability can emerge, rather than directly increasing variability itself.
Sometimes this requires no new techniques or increasingly sophisticated interventions. What matters most may simply be creating the conditions that allow the organism's intrinsic autoregulatory processes to unfold. Time, patience, careful observation, and the gradual removal of unnecessary physiological constraints often become just as important as any specific therapeutic method.
Living systems do not remain healthy because they resist change. They remain healthy because they retain the capacity to adapt.
Conclusion
Physiological variability is far more than a collection of fluctuating biological measurements. It reflects the dynamic organization of living systems.
The heart, lungs, blood vessels, nervous system, endocrine system, and countless other regulatory mechanisms do not function independently. They operate as an integrated network in which continuous adaptation is essential for maintaining physiological balance.
Heart rate variability represents one important window into this process—but it is only one window.
The broader principle extends throughout physiology. Perhaps the most important question is therefore not how to increase variability, but how to understand the conditions that allow it to arise.
Variability should not be interpreted as biological imperfection or noise. Rather, it represents one of the clearest observable expressions of living autoregulation.
References
Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability: standards of measurement, physiological interpretation and clinical use. Circulation. 1996;93(5):1043–1065. https://doi.org/10.1161/01.CIR.93.5.1043
Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Front Public Health. 2017;5:258. https://doi.org/10.3389/fpubh.2017.00258
Thayer JF, Lane RD. A model of neurovisceral integration in emotion regulation and dysregulation. J Affect Disord. 2000;61(3):201–216. https://doi.org/10.1016/S0165-0327(00)00338-4
Thayer JF, Lane RD. Claude Bernard and the heart–brain connection: further elaboration of a model of neurovisceral integration. Neurosci Biobehav Rev. 2009;33(2):81–88. https://doi.org/10.1016/j.neubiorev.2008.08.004
Ben-Tal A, Shamailov SS, Paton JFR. Central regulation of heart rate and the appearance of respiratory sinus arrhythmia: new insights from mathematical modeling. Math Biosci. 2014;255:71–82. https://pubmed.ncbi.nlm.nih.gov/25004397/
Lipsitz LA, Goldberger AL. Loss of “complexity” and aging. JAMA. 1992;267(13):1806–1809. https://doi.org/10.1001/jama.1992.03480130122036
Goldberger AL, Peng CK, Lipsitz LA. What is physiologic complexity and how does it change with aging and disease? Neurobiol Aging. 2002;23(1):23–26. https://doi.org/10.1016/S0197-4580(01)00266-4
Costa M, Goldberger AL, Peng CK. Multiscale entropy analysis of biological signals. Phys Rev E Stat Nonlin Soft Matter Phys. 2005;71(2 Pt 1):021906. https://doi.org/10.1103/PhysRevE.71.021906
McEwen BS. Stress, adaptation, and disease: allostasis and allostatic load. Ann N Y Acad Sci. 1998;840:33–44. https://doi.org/10.1111/j.1749-6632.1998.tb09546.x
Sterling P. Allostasis: a model of predictive regulation. Physiol Behav. 2012;106(1):5–15. https://doi.org/10.1016/j.physbeh.2011.06.004
De Troyer A, Boriek AM. Mechanics of the respiratory muscles. Physiol Rev. 2011;91(4):1273–1300. https://doi.org/10.1152/physrev.00019.2010
Dempsey JA, Smith CA. Pathophysiology of human ventilatory control. Eur Respir J. 2014;44(2):495–512. https://doi.org/10.1183/09031936.00048514
Original Lithuanian version:
Comments