The Local Control of Blood Flow: An Illustration of Homeostasis

Abstract

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, equilibrium is shifted in a way that tends to undo effect of 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: is fixity of 'milieu interieur' which is condition of free and independent life. All vital mechanisms, however varied they may be, have only one object, that of preserving constant conditions of life in internal And when he said, Admitting that vital phenomena rest upon physico-chemical activities, which is truth, essence of 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 admirable subordination and harmonious concert of organic activity. There is an arrangement in living being, a kind of regulated activity, which must never be neglected, because it is in truth most striking characteristic of living beings .... Vital phenomena possess indeed their rigorously determined physicochemical conditions, but, at 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 organization and growth of individual, animal or plant (Bernard, 1878). We find that this thought of constancy within organism so aptly termed the fixity of 'milieu interieur' was not an isolated thought of one scientist with much vision, for we find following statement made by Pfluger in 1877, a year before Bernard wrote above, cause of every need of a living being is also cause of satisfaction of need. Again in 1885 one finds a restatement of same thought by Leon Fredericq: living being is an agency of such sort that each disturbing influence induces by itself calling forth of compensatory activity to neutralize or repair disturbances. The higher in scale of living beings, more numerous, more perfect and more complicated do these regulatory agencies become. They tend to free organism completely from unfavorable influences and changes occurring in This was followed in 1900 by Charles Richet's observation: living being is stable. It must be so in order not to be destroyed, dissolved or disintegrated by 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 stimulation. In a sense it is stable because it is modifiable slight instability is necessary condition for true stability of 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 phenomena of life? In 1929, Walter B. Cannon supplied us with answer to this with his invention of term homeostasis. He states: coordinated physiological processes which maintain most of steady states in organism are so complex and so peculiar to living beingsinvolving, as they may, brain and nerves, 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 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 whole organism has this attribute of homeostasis that organs and organ systems, which make up whole organism, have within them also this property of maintaining their own environment in equilibrium with that external to them, that of whole. Based on this assumption, much work has been done over 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 external to 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 meaning, part played by reactions in delicate regulations by which organism and organs are adapted to ever changing environmental conditions.