The Local Control of Blood Flow: An Illustration of Homeostasis

There is a principle found in any general chemistry textbook which states: "If a stress, such as a change in concentration, pressure or temperature, is applied to a system in equilibrium, the equilibrium is shifted in a way that tends to undo the effect of the stress." (Nebergall et al., 1963). This is known as Le Chatelier's principle and, as is obvious, applies to chemical reactions in equilibrium states. Now if one stops to think about it, isn't this almost what Claude Bernard (1878) was saying when he wrote: "It is the fixity of the 'milieu interieur' which is the condition of free and independent life. All the vital mechanisms, however varied they may be, have only one object, that of preserving constant the conditions of life in the internal environment." And when he said, "Admitting that vital phenomena rest upon physico-chemical activities, which is the truth, the essence of the problem is not thereby cleared up; for it is no chance encounter of physico-chemical phenomena which constructs each being according to a pre-existing plan, and produces the admirable subordination and the harmonious concert of organic activity. "There is an arrangement in the living being, a kind of regulated activity, which must never be neglected, because it is in truth the most striking characteristic of living beings .... "Vital phenomena possess indeed their rigorously determined physicochemical conditions, but, at the same time, they subordinate themselves and succeed one another in a pattern and according to a law which pre-exists; they repeat themselves with order, regularity, constancy, and they harmonized in such manner as to bring about the organization and growth of the individual, animal or plant" (Bernard, 1878). We find that this thought of constancy within the organism so aptly termed "the fixity of the 'milieu interieur' " was not an isolated thought of one scientist with much vision, for we find the following statement made by Pfluger in 1877, a year before Bernard wrote the above, "The cause of every need of a living being is also the cause of the satisfaction of the need." Again in 1885 one finds a restatement of the same thought by Leon Fredericq: "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 disturbances. The higher in the scale of living beings, the more numerous, the more perfect and the more complicated do these regulatory agencies become. They tend to free the organism completely from the unfavorable influences and changes occurring in the environment." This was followed in 1900 by Charles Richet's observation: "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." What really have these great men been trying to say? What very fundamental law of living and surviving have they in their times observed and written down as a guide to all in delving into the phenomena of life? In 1929, Walter B. Cannon supplied us with the answer to this with his invention of the term homeostasis. He states: "The coordinated physiological processes which maintain most of the steady states in the organism are so complex and so peculiar to living beingsinvolving, as they may, the brain and nerves, the heart, lungs, kidneys, and spleen, all working co-operatively 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." Cannon here is referring to the whole organism with its constancy, its state of dynamic equilibrium between internal and external environment. But in view of this it is reasonable to believe that if the whole organism has this attribute of homeostasis that the organs and organ systems, which make up the whole organism, have within them also this property of maintaining their own environment in equilibrium with that external to them, that of the whole. Based on this assumption, much work has been done over the years to uncover such mechanisms as might be operating to keep at all times and under all conditions of stress a constant supply of nutrients moving from the external to the internal environment, and to allow for movement from internal to external environment of end products and waste materials. Krogh (1924) states it thus: ". . we want to find out the meaning, part played by the reactions in the delicate regulations by which the organism and the organs are adapted to the ever changing environmental conditions."