Homeostasis (biology)

Referring to animal systems, pioneering 20th century physiologist Walter Cannon,   who coined the word homeostasis in 1926, defined homeostasis as follows:

Walter Bradford Cannon Template:Photo

The coordinated physiological reactions which maintain most of the steady states in the body are so complex, and are so peculiar to the living organism, that it has been suggested (Cannon, 1929) that a specific designation for these states be employed — homeostasis.

Cannon recognized that "living being[s] " function as 'open' systems (see Life), having many "relations" with their surroundings — for example, interchange of materials through airways, gastrointestinal tract and skin. He noted that the surroundings could perturb the system, dislocating the states of activity of its key internal components or subsystems to states of activity outside of their relatively stable and optimal ranges — the "steady states" he referred to in his definition. A change in outside temperature, for example, might perturb the dynamic stability of internal biochemical processes to the detriment of the organism. If temperature of the brain exceeds a certain value it malfunctions globally. The organism reacts to such potentially adverse effects of its surroundings with physiological adjustments that tend to maintain steady-state, i.e., to maintain 'homeostasis'.

This article will explore the concept of homeostasis from an early 21st century biological perspective, exemplify 'homeostatic' (i.e., homeostasis-maintaining) mechanisms, and relate homeostasis to metabolism, physiology, cybernetics, systems biology, and the concepts of cellular and organismic adaptability, robustness, growth, development and reproduction.

Terminology and examples of usage

Note that Cannon applied the term 'homeostasis' to the "steady states in the body", not specifically to the physiological mechanisms maintaining them.

In common usage, 'homeostasis' refers not only to a living system’s internal stability, but also to the system’s ability or tendency to maintain that stability, or to its process of maintaining that stability. As to the latter, some biologists interpret Walter Cannon as defining 'homeostasis' "…to describe the regulation of [the] internal environment [emphasis added].] Thus the nuances of 'homeostasis' and the vagaries, or flexibility, of language.

Use of the adjectival form, homeostatic, adds a modicum of coherence. Typically biologists speak of 'homeostatic mechanisms', namely mechanisms or processes that achieve, or tend to achieve, internal stability of a particular internal state. For example, the behavior of a thirsty land vertebrate seeking and drinking water qualifies as a homeostatic mechanism adjustive to the organism’s internal state of dehydration. Biologists do not typically speak of homeostatic mechanisms as mechanisms that endow an organism with the ability to make the adjustive physiological changes required to maintain homeostasis — a subtle but important distinction. To explain the foundation that underpins a living system’s ability to self-regulate its internal environment requires investigation of the foundation that underpins the very activity of living itself, including evolutionary forces enabling self-organization and autonomy (see Life).

In applying a narrow definition of homeostatic mechanisms in living systems, we can say they operate as built-in autonomous molecular physiological processes, goal-directed to maintain, within an optimal range, the properties, functions or behaviors of the system’s key components (structures, metabolic pathways, networks, or other subsystems) when any key component deviates from its optimal ‘set-point’, or often more precisely, ‘set-range’. The metrics (volume, concentration, output rate, shape, etc.) of that range depend on the nature of the regulated component.

For a cell, homeostatic mechanisms operate, for example, to maintain its internal concentration (or more precisely, chemical activity) of hydrogen ion within its optimal range, the failure of which would, among other adverse consequences, perturb catalytic activity of enzymes necessary to maintain cell organization for ‘healthy’ living. For a multicellular multi-organ organism like a land vertebrate, homeostatic mechanisms operate, for example, to maintain within optimal ranges the supplies of oxygen to organs, the failure of which could lead to organ dysfunction and cascading deleterious effects on other organs, leading to organ system and organism dysfunction. Sometimes, for want of a nail a kingdom tumbles.

In applying a broader definition of homeostatic mechanisms to living systems, we can say they operate as the totality of those molecular and physiological adjustive processes that maintain the system's dynamic organization in a state that sustains its activity of living. Homeostatic mechanisms operate widely in biological nature, including all living systems, and those biological systems whose parts consist of living systems, e.g., a community of populations of many different species (an ecosystem). The Gaia hypothesis entertains the proposition that homeostatic mechanisms operate in sustaining the biopshere itself.

Living systems have a natural history, growing in time and space in a self-programmed process, behaving 'healthfully' differently in different environments, and sustaining their living processes by manufacturing living things like themselves. Because of that natural history, set-range optimality values do not remain fixed for some system components. Homeostatic mechanisms themselves adjust their goals to accommodate the system’s natural history, indicating their adaptability. Optimality ranges do not remain constant, though remaining optimal, for some systems.

‘Optimal’ here does not intend to imply that organic evolution optimizes living systems and subsystems for anything more, ultimately, than reproductive success. Moreover, when cultural evolution emerged in one species of land vertebrate, Homo sapiens, the determination of ‘optimal’ became individualized, and conscious choice could determine criteria for reproductive success, reject reproduction, or strategize to maximize fecundity.

We can view homeostasis in a somewhat narrow sense of keeping a molecular network in a cell operating within a determined range, or more broadly, of keeping a living system thriving, regardless of level of organization from cells to communities, thriving. In any case, homeostasis emerges from a self-organized process.

History of concept of homeostasis

Claude Bernard (1813-1878)

The French physiologist, Claude Bernard (1813-1878), introduced the concept of the internal environment of an organism, upon which the concept of homeostasis was built. Claude Bernard was a staunch advocate of experimental verification in physiology, and a prolific experimental physiologist. He postulated, initially blood, as a milieu interieur, or internal environment, surrounding the interior cells of higher animals. He first recognized that mechanisms operated to maintain relatively constant the temperature and glucose concentration of the blood, and the importance of those stabilities to the health of the organism. From those and many other observations, he developed the concept of the fixity or constancy of the internal environment as essential to the vital processes of the body.

Although not the first to have inklings of physiological homeostasis, and not the first to coin the term, physiologists would not dispute that the impact of his researches and ideas merit him the title of ‘father of homeostasis’.

In his book on Claude Bernard’s place in the history of ideas, historian and Professor of French, Reino Virtanen, writes:

“…there is one basic insight we owe to Bernard which has continued to exert a seminal influence on contemporary science. It is the concept of the milieu intérieur. Yet although he stated it quite early and repeatedly, its full potentialities were not generally realized until decades after his death. The basic significance is suggested by the words of the historian of physiology John F. Fulton: "One can again approach the human body as a single functional entity. The first great step toward this goal was taken in 1878 by Claude Bernard who enunciated the conception..."

And Virtanen writes further to indicate how Claude Bernard’s idea of the fixity of the internal environment help lead physiologists to consider the broader organizational character of the organism, the “coordination of physiological processes”:

It is with the researches of [respiratory physiologist, Fellow of the Royal Society] J. S. Haldane [1860-1936] and [biochemist and physiologist] Lawrence J. Henderson [1878-1942] that Bernard's teaching really began to show its suggestive value...Henderson himself has more than once acknowledged the impact of Bernard's ideas. In his valuable introduction to H. C. Greene's translation of the Introduction to the Study of Experimental Medicine, Henderson suggests a reason for the tardy recognition of the concept: "The theory of the internal environment . . . we owe almost entirely to Claude Bernard himself. . . . Today with the aid of a physical chemistry unknown to the contemporaries of Claude Bernard, it is fulfilling the promise which he alone could clearly see."…J. S. Haldane’s study of Respiration was another sign of the times. But neither Haldane nor Henderson was content with the verifiable implications of the concept. The inner medium involves the whole question of the coordination of physiological processes. This becomes their springboard into speculations on the philosophy of the organism.

Neuroscientist Charles Gross quotes Bernard directly, from Bernard's Lectures on the Phenomena of Life Common to Animals and Plants.:

The fixity of the milieu supposes a perfection of the organism such that the external variations are at each instant compensated for and equilibrated .... All of the vital mechanisms, however varied they may be, the uniformity of the conditions of life in the internal environment.... The stability of the internal environment is the condition for the free and independent life…

Many physiologists, and students of physiology, who credit Claude Bernard and Walter Cannon as ‘fathers’ of the concept of homeostasis tend to ignore other scholars who had adumbrations of the concept, some of whom may have known of Claude Bernard's ideas. Walter Cannon did not ignore them. In his 1932 book, Wisdom of the Body, Cannon mentions earlier thinkers:

• Hippocrates (ca. 460 – ca. 377 B.C.E.): Cannon points out that Hippocrates recognized the role of “natures helping hand”, otherwise referred to as the doctrine of vis medicatrix naturae (the healing power of nature).  

• Cannon recognized that Hippocrates' healing power of nature “...implies the existence of agencies which are ready to operate correctively when the normal state of the organism is upset.”

• Eduard F. W. Pfluger (1829-1910), German physiologist: Cannon quotes Pfluger as having written in 1877:

• “"The cause of every need of a living being is also the cause of the satisfaction of the need", in the context of natural adjustments maintaining the stability of the organism.

• Leon Fredericq (1851-1935), Belgium physiologist: Cannon quotes Fredericq as having written in 1885:

• “The living being is an agency of such sort that each disturbing influence induces by itself the calling forth of compensatory activity to neutralize or repair the disturbance…[It] tend[s] to free the organism completely from the unfavorable influences and changes occurring in the environment.”

• Charles Richet (1850-1935) French physiologist: Cannon quotes Richet as having written in 1900, remarkably:

• "The living being is stable…"It must be so in order not to be destroyed, dissolved or disintegrated by the colossal forces, often adverse, which surround it. By an apparent contradiction it maintains its stability only if it is excitable and capable of modifying itself according to external stimuli and adjusting its response to the stimulation. In a sense it is stable because it is modifiable — the slight instability is the necessary condition for the true stability of the organism."

The work of Walter Cannon

Fortunately we have online access to two key works of Walter Cannon that summarize his work and ideas:

• A 1929 extensive review article, "Organization for physiological homeostasis" 
• A 1932 famous and influential book, The Wisdom of the Body 

It may serve to emphasize the Cannon's concept of 'homeostasis' to quote from the latter work:

The constant conditions which are maintained in the body might be termed equilibria. That word, however, has come to have fairly exact meaning as applied to relatively simple physico-chemical states, in closed systems, where known forces are balanced. The coordinated physiological processes which maintain most of the steady states in the organism are so complex and so peculiar to living beings — involving, as they may, the brain and nerves, the heart, lungs, kidneys and spleen, all working cooperatively — that I have suggested a special designation for these states, homeostasis. The word does not imply something set and immobile, a stagnation. It means a condition — a condition which may vary, but which is relatively constant.

The quote also serves to indicate Cannon's view that homeostasis does not mean the "conditions of the body" will not vary. Though he sets the limits of variation as "relatively constant", he does not take into consideration the great variations in "conditions of the body" in the organism as it develops, in the case of mammals, from a single fertilized cell to a mature adult.

Homeostasis and longevity

Cell homeostasis, tissue homeostasis, and organ homeostasis determine organismic homeostasis . Therefore the 'efficiency' with which cells, tissues and organs in maintain homeostasis would likely influence the longevity of the emergent organism.

To quantify the homeostasis efficiency of a complex system, even one low in hierarchy, like a eukaryotic cell, one might try valuating the degree/promptness of homeostasis of its major subsystems in response to a perturbation spectrum. But that could only quantify efficiency under the environmental conditions of the studies. Each different environmental condition might affect efficiency differently, and variably differently, in the various subsystems. Because an enormous number of environmental conditions test homeostasis-maintaining ability of the organism during a lifespan, one would need to obtain and integrate too much detail of human subsystems’ properties for any valuation of efficiency of homeostasis to have practical value in controlling human lifespan.

The property of 'lifespan' in the human system emerges only when organismic homeostasis fails completely and death results. A model that could predict lifespan long in advance of death, even one that age-modified the prediction, might lend itself to teaching how to treat the system to improve the efficiency of homeostasis of its subsystems.

What form would such a model take? For personal benefit — a major goal of aging research — the model would seem to require itself to extensively interrogate the individual human system before running its lifespan-predicting algorithm. And do such interrogation time after time as time goes by. One would want the model’s systems readout, however implemented and interpreted in relation to previous readouts, followed by a prediction of lifespan as well as a prescription of steps to take to reverse damage and improve homeostasis-maintaining ability. A massive-load-capable information-gathering-and-processing method, abstract, computational: a cyber-smart doctor, distributed geographically or miniaturized.

But that ideal model allows control of lifespan for extreme longevity, as opposed to merely extending it substantially beyond present norms. Yet, learning to extend lifespan substantially may crucially underpin any model that permits control of lifespan for extreme longevity. Minimized energy consumption in the form of food extends lifespan in diverse genera. That would seem to have potential for obese humans, but not necessarily for non-obese humans. We do not know whether calorie minimization, ceteris paribus, extends lifespans in non-obese humans. If it did, we might want to revise our quantitative criteria for obesity to retain its connotation of poor health. We have no firm idea what body mass indexes, or percent body fat, however adjusted for other anthropomorphic variables, associate with human lifespans substantially greater than current norms.

Depending on how extreme the possible longevity, achieving it may require the complex task of controlling the entire human environment, the biosphere at minimum. Hopefully, but likely, all humans will require a large core-biosphere-set of common conditions, however geo-regional, for super-efficient organismic homeostasis. In recognizing that, the motivation of individuals for youthful longevity may impel them to interact in ways to achieve that common set of conditions. Sacrifices might involve opposing nature’s algorithmic drive to reproduce. Doing that would step us closer to the question of optimal sustainable population size, and if one can be determined satisfactory, how to achieve that ethically.

The property of lifespan has interest because the desirer of longevity wants a long healthy mental life, a long-lived kingdom of the mind. Why? Because as one’s knowledge increases so do the number of paths for curiosity to pursue — and a healthy youthful mind dictates the exercise of curiosity. Often one has ambitions and goals that require many prolonged stages. Those who do not believe in ‘afterlife’ feel they should get the greatest possible satisfaction from living before dying. Living longer increases the chances of participating in breakthroughs to extreme longevity.

Though some suggest the possibility that someday supercomputers, perhaps quantum computers, will have the ability to simulate the processes that generate conscious and self-conscious experience in simulated humans living in a simulated biosphere . For all we know, we live as a simulation in a simulated world, as an experiment, perhaps an iterative run of a model program developed by model-building systems scientists beyond our ken.