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Nature repeatedly reinvented certain control strategies shared among different body systems to maintain our physiological machinery. Each strategy not only works in a generic way and it also can fail in a generic way. Understanding these universal mechanisms can help to infer from symptoms the underlying pathology.
The two previous posts What is physiology? and Physiology organized by major body systems lay the basis. However, you don’t have to read through these posts to understand this one.
Their distilled version is that in physiology we mainly study functioning of major human body systems. Like, for example, the respiratory system, and the blood and circulatory system. Just to name those systems that will also be mentioned in this post. Of course, there are several more human body systems and there is also non-human physiology.
These posts (initially I planned three, this being the last, but there will be more) were written during my preparation for a course “Dynamical Diseases”. In this one, we will mainly learn that there are typical patterns how biological regulation systems work—and fail.
Physiology is the science of biological regulation, in technical terms, it is about closed-loop control systems. Pathophysiology is about the failure of such control systems. That’s the 30,000-foot view. This was correctly diagnosed in another blog. The question is, what can we see from far distance? Is it relevant for clinical reasoning?
In the previously reviewed editorial of Acta Physiologica we read [1]:
[T]here is […] an underlying necessity for thorough knowledge and understanding of the basic sciences, like medical physiology. It is well appreciated that basic sciences support sound clinical reasoning and are indispensable for understanding pathological mechanisms […] I am convinced that this 30,000-foot view provides exactly this: an indispensable viewpoint for understanding certain pathological mechanisms, because it focuses on the overarching theme how physiological systems work and fail.
By their very nature, physiological processes are dynamic processes! The aim of these processes is to preserve a certain stable physical and chemical environment for the organs. Stable can mean constant, e.g., think about your body temperature. But more often it means a stable periodic oscillation, like a beating heart or a breathing pattern.
Sumo Wrestler Asash?ry? fighting against Kotoshogiku at the January Tournament 2008, Creative Commons Attribution 3.0 Unported license. Even a seemingly stable situation is dynamic. Photo: thanks to Eckhard Pecher
Physiological control mechanisms keep the physiological quantities (e.g. body temperature, heart rate, etc) in a certain range. Let it be the amplitude range and/or frequency range. Furthermore, some quantities (e.g. body temperature) must stay within that range even if outer body conditions change, while others (e.g. heart rate) must adapt to changing conditions (e.g. running).
There are thousands physiological quantities that must be controlled within the human body. Yet their respective control strategies narrow down to far fewer. In other words, nature repeatedly reinvented generic control strategies and implemented these in various different body systems.
Even more important is the next point. A specific control method, say one that achieves a constant but adaptive period, for example, one that controls the heart rate, usually fails in only a few generic ways. The physiological target state can become unstable only in certain generic bifurcations. If, to stay with this example, the physiological heart rate becomes unstable, is is attracted to a new stable pathophysiological state that could cause fibrillation. An example for a failure due to a bifurcation in another major body system is the Cheyne-Stokes respiration, an abnormal crescendo-decrescendo breathing pattern.
This is what the concept of a dynamical disease is about. It is a top down description on the level of bifurcations, that is, rather abrupt transitions from a healthy into a diseased state.
Not all diseases are dynamical diseases. A physiological target state can drift, maybe because an antagonistic interaction is weakened, without causing an instability to occur. Usually, you can infer from the dynamics of the symptoms and physiological measurements whether the underlying disease is a dynamical one or not.
The details have to be worked out. This requires some knowledge in nonlinear dynamics. This is what my course at the TU Berlin is about, but it is clearly beyond a blog post.
(Just to be clear on this, knowledge in nonlinear dynamics is required to work this out—you don’t need this to infer from symptoms the diseased state and the corresponding best therapy. I do not claim that all medical students must master complex mathematical tools. Quite the contrast, I prefer physicians who are extremely experienced and open minded. Usually I don’t care about their math skills. Yet in medical research math matters.)
I will give later more examples, in particular, cardiac arrhythmias , the already mentioned Cheyne-Stokes respiration, seizure activity, migraine with aura, and others.
Furthermore, I will review the historical context and argue that the industrial revolution in the nineteenth century was the midwife when this mechanical view on the physiological machinery saw the light of day.
[1] Luc Snoeckx, Minimum standard and learning outcomes in physiology required by the Bologna process: the Federation of European Physiological Societies end-terms of physiology in a medical curriculum, Acta Physiologica, 200:1-2, 2010.
In German we should translate the word control as Steuerung or Regelung and not merely as Kontrolle.