PHYSIOLOGY. - From the Greek φύσις = nature and λογέω = I investigate, physiology is the science of the functioning of the living organism. Being eminently experimental in nature, it has attained great development only recently.
The origin of physiology in history must be sought at the point when, from the simple description of living organisms, man passed to the attempt to explain to himself the meaning of the structures observed. Galen (b. at Pergamon in 129 A.D.) certainly posed these problems; he began vivisection for experimental purposes, and to him are owed the first observations on the function of the nervous system and of the circulatory system. The definitive explanation of the mechanism of the circulation, however, came much later, with the contribution of numerous observers (R. Colombo, 1559; F. D'Acquaperedente, 1574; W. Harvey, 1628; M. Malpighi, 1661; L. Spallanzani, 1773) and was one of the first conquests of modern physiology (v. CIRCOLAZIONE).
With the use of ever-new experimental methods this science has developed enormously, occupying one of the foremost places among the biological sciences.
Vivisection, often preceding the most important conquests of human surgery, enormously facilitated by the discovery of narcotics and anesthetics, has made possible the realization of an infinite number of experiments capable of enhancing or diminishing the activities of organs so as to be able to study their function. The possibility, moreover, of keeping organs and tissues alive in isolation from the organism has permitted the direct study of functional performance, under conditions that can be precisely limited by artifice and without interference from other organs or tissues. With an ever finer and more exact technique of recording, it has become possible to follow in time and in space not only the gross mechanical performances of organs, but also movements imperceptible to the naked eye, and performances of another nature (electrical, thermal, chemical), enormously magnifying our capacity for analysis of physiological phenomena. With the valuable aid of chemical technique, moreover, the complicated chemical mechanism of the transformation of chemical energy into other energies (mechanical, electrical, thermal) in living organisms has been brought to light, along with the validity of the laws of thermodynamics even in these transformations.
From a scholastic point of view one can distinguish a physiology of the organs of vegetative life and a physiology of the organs of relational life.
The first comprises the physiology of the circulatory apparatus, of the respiratory apparatus, of the digestive apparatus, of the urogenital apparatus and of the glands of internal secretion. The second comprises essentially the physiology of the central and peripheral nervous system and of the sense organs (sensibility) and of the muscular and skeletal system (motility).
A distinction responding instead to practical necessities, based on the different methodologies followed, is the following: chemical physiology is the name given to that physiological science which makes use of chemical methods and which therefore tends toward the solution of questions that can be resolved only by chemical methods, such as the problem of nutrition, the chemistry of respiration, intermediate chemical transformations, overall energy expenditure, etc.; chemical physiology has then given origin to further branches further specialized such as enzymology, vitaminology, etc. By the appellation of physiology properly so called is meant, instead, that part of physiology which is based on the classical techniques of physiology, vivisection, recording of mechanical, thermal, electrical performances; within physiology too, specialized branches have developed such as electrophysiology, endocrinology, etc.
The present state of our knowledge and the questions that beset modern physiology are the reflection of the present state of experimental technique and of its possibilities of development. Every progress of technique has marked a progress in physiological knowledge, channeling research toward those problems whose solution was made possible by the new acquisitions.
In the field of chemical physiology it has been demonstrated in a precise manner that the material and energetic transformations in living organisms (metabolism) obey the law of conservation of matter and energy; it has been demonstrated that organisms function as chemical machines and not as thermal machines, and obtain the necessary energy by oxidizing the carbon and hydrogen of foods into carbon dioxide and water.
The efficiency of the transformation of chemical energy into mechanical energy generally does not exceed 25-30%.
The adult living organism can be considered as a stationary system with periodic and aperiodic accelerations of its metabolism.
The applicability of physical and chemical laws to living organisms has revived the question of the definition of living beings with respect to non-living ones. In this regard it must be said to be incontestable (and admitted by most) that a finalistic ordering must belong to the formation and functioning of living beings; this is not in contradiction with the validity, even in living beings, of the laws of the inanimate world. The study of the chemical mechanism of energetic transformations has brought to light the fact that chemical energy is released little by little through an infinity of intermediate reactions, whose speed is regulated by innumerable catalysts (ferments); in particular the chemistry of muscular function is well known, although to this day it is unknown how the transformation of chemical energy into mechanical energy occurs; this is a problem subjected today to assiduous research.
The necessity of foods, understood initially as a simple necessity of energy, is now understood, besides as covering the energy requirement, also as reintegration of particular substances (nitrogenous substances, fatty substances, vitamins) that the organism necessarily consumes and is not capable of synthesizing; this chapter too is still open.
In the field of physiology properly so called, the mechanical, electrical, thermal and other phenomena of the various organs and tissues have been examined in depth, along with the correlations among these, through the nervous pathways, or through the circulation, by means of substances elaborated by special glands (glands of internal secretion). Nervous correlation, that is, with the interposition of separate nervous pathways between organ and organ, makes possible a very fine regulation in intensity, in time and in space, which is not possible with correlation through the interposition of the products of the glands of internal secretion (humoral correlation). It is still a debated question, at least for certain sectors, whether the transmission of the nervous impulse in the nervous pathways is of an electrical nature (diffusion of potential) or of a chemical nature (diffusion of a substance).
Sensibility to environmental stimuli is ensured by organs specialized for the reception and analysis of these stimuli: the cutaneous nerve endings (tactile, thermal, pain) and the sense organs (sight, hearing, taste, smell); the excitation that is determined in these organs is transmitted by means of nervous pathways (sensory pathways) to the nervous centers (neuraxes) where the motor or secretory response is elaborated, which is transmitted to the effectors, that is, to the organs that must carry it out (muscles, glands) by means of other nervous pathways (motor pathways). The entire mechanism which, as it appears, governs all our relations with the external world, is called a reflex.
As regards the relations between the energy of environmental stimuli and the intensity of sensory excitation, the stage of empirical laws (Weber's and Fechner's law) has by now been surpassed, and an attempt is now being made to frame these relations with quantum-statistical concepts.
The phenomena of consciousness and of knowledge remain completely beyond the possibility of a physiological explanation, even though they presuppose, for their outward manifestation, the presence of particularly differentiated nerve centers (the human cerebral cortex).
BIBLI, R. Hoeber, Phytophthische Chemie der Zelle und der Gewebe, 6ª ed., Lipsia 1926; A. P. Matthews, Physiological Chemistry, 5ª ed., Londra 1930; H. Rein, Physiologie des Menschen, 5ª ed., Berlino 1940; C. H. Best-N, B. Taylor, The Physiological basis of Medical Practice, 3ª ed., Baltimore 1943; M. Flockan, Biochimie humaine, Parigi e Lieut 1944; C. Lovatt Evans, Principles of Human Physiology, 9ª ed., Londra 1945; W. R. Fearon, An Introduction to Biochemistry, 3ª ed., ivi 1946; L. V. Heilbrunn, An Outline of General Physiology, 2ª ed., Filadelfia e Londra 1947; R. Margaria, Principi di fizico-chimica e chimica-fisiologica, 6ª ed., Milano 1947; R. Margaria-L. De Caro, Principi di F. umana, ivi 1948.