Physiology (from the greek φύσις, physis, ‘nature’, and λόγος, logos, ‘speech’, then ‘study of natural phenomena’) is the branch of biology that studies the functioning of living organisms, analyzing the chemical and physical principles of the functioning of living beings, whether single or multi-cellular, animals or plants.
A “physiological condition” is a state in which normal bodily functions occur, while a pathological condition is characterized by abnormalities that result in disease. Given the breadth of the field of study, physiology is divided into animal physiology, plant physiology, cellular physiology, and microbial, bacterial, and viral physiology, among others.
It is divided into: plant physiology, which deals with plants; comparative physiology, which studies the various functions comparatively between various animal organisms of different phyla; human physiology, which is a part of medicine and studies the functioning of healthy organs; physiopathology, which studies the functioning of diseased organs.
Physiology is strictly based on the experimental method: only by removing, altering and otherwise acting on a particular organ is it possible most often to understand its function. For this purpose we use animals chosen according to the study in progress and since many fundamental processes (respiration, cell division, heredity, reproduction) are common to all or almost all organisms, the result of the tests can be extended to a wide range of living beings. To explain many behaviors of organisms, physiology uses physical, chemical, and even mechanical models compatible with the anatomical structures of the organs called into question, since the same laws applicable on those models can be referred to biological processes.
Given the articulation of studies, included in physiology have arisen specializations, each dealing with a single organ or apparatus. Thus, neurophysiology studies the central and peripheral nervous systems, cardiovascular physiology the blood circulation and hemodynamics, the science of nutrition the mechanisms of digestion, the physiology of work the energy consumption of a working organism, etc.
In sociology, part of the discipline that, by analogy with human physiology, studies the functioning and relationships among the various “organs” of society. The expression was codified by the French Ph. Buchez who, while affirming the usefulness of the analogy between human physiology and social physiology, reiterated the differences: while the human being does not change physically except in very long times, society is subject to continuous changes. Even with this precaution, a large part of nineteenth-century sociology, especially positivist sociology, profitably used this analogy, making its own the distinction between the “anatomical” point of view (the conditions of existence of society) and the “physiological” one (social movements and processes). In this sense, for example, A. Comte divided sociology into static and dynamic.
The first interpretations of organic functions were speculative, as found in the texts of Alcmeon of Croton, Empedocles and Democritus. In the Hippocratic texts one encounters the first important theoretical synthesis of the functions of the organism, whose normal development is traced back to the balance of the four humors (blood, yellow bile, black bile and phlegm) and directly influenced, as well as by environmental conditions, by an internal heat, with center in the heart, and produced in the encounter between nourishment and air (pneuma) absorbed in the lungs.
According to Aristotle (4th century BC) the soul forms and directs the various organs and is located in the heart; to the brain he attributed only the function of cooling the blood. The more precise recognition of the functions of this organ, as well as nerves and muscles, was the work of the school of Alexandria during the fourth and third centuries BC, especially through the anatomical studies of Europhilus and Erasistratus. Galen (sec. II), to whom we owe the lasting synthesis of ancient medicine and physiology, identified the soul, the organizing principle of bodily functions, with the spirit or pneuma to which he attributed almost material nature according to three distinct forms: natural, located in the liver, vital, located in the heart, and animal, acting in the brain.
According to Galenic physiology, vital processes, governed by strict laws, are always aimed at specific purposes and there is nothing superfluous or lacking in them: blood, composed of the four humors already identified by Hippocrates and produced in the liver, reaches the right side of the heart and here, through the partition or septum, considered perforated, passes to the left side (ventricle), where it is mixed with the pneuma coming from the lungs and heated; its impurities are exhaled through the lungs with the breath. The vital spirit arriving with the blood in the brain is transformed into animal spirit that performs psychic functions and, traveling through the nerves, supposed to be hollow, reaches the muscles causing their movements.
Throughout the Middle Ages physiology did not deviate significantly from the teaching of Galen and only with the anatomical research carried out since the sixteenth century there were new developments. First A. Vesalio denied that the interventricular septum that separates the two parts of the heart is permeable to blood, while M. Serveto and R. Colombo suggested that to circulate from the right receptacle of the heart to the left one, the blood must pass through the lungs (small circle). The complete demonstration of the circulation of blood was in Exercitatio anatomica de motu cordis et sanguinis (1628) by W. Harvey, which marked the beginning of modern physiology.
Harvey stated that the blood does not make a movement of flow and reflux from the center to the periphery through the vessels, as Galen had argued, but is expelled from the left ventricle, which contracts with each pulse, and through the arteries comes to all organs where, according to the methods later clarified with the discovery of capillary vessels by Malpighi, passes through the veins and reaches the right atrium of the heart (great circle). This revolutionary discovery, based on observations, vivisection of animals and the calculation of the amount of blood in motion, opened the way to the so-called animate anatomy, which studied the function of organs starting from their structure; moreover, the consideration of the heart as a pump and vessels as hydraulic conduits was in line with the concept, theorized mainly by Descartes, that living organisms were machines. Among the supporters of this address: S. Santorio, who used the balance to calculate the balance of substances absorbed and expelled from the body, and G. A. Borelli who applied the laws of mechanics to the study of locomotion in animals and flight in birds. To this school iatromechanics was opposed in the seventeenth century that iatrochemical.
Paracelsus, in the previous century, had already argued that the human body is essentially a chemical system composed of mercury, sulfur and salt, and J. B. van Helmont, his follower, indicated with the term gas the ferments that govern the vital functions; these, according to F. Sylvius, are based on the balance of acid and alkaline substances. T. Willis supposed that muscular contraction was due to a sort of explosion of sulphurous and nitric particles in the blood, while R. Lower and J. Mayow believed that respiration is not intended to cool the blood but to draw from the air a substance essential to life.
In the early eighteenth century, great theoretical syntheses were elaborated based on the concept that the organism is governed by a single center. For F. Hoffmann, mechanist, the functions are regulated by a spirit derived from the cosmic ether and acting as a subtle matter through the nervous system. For G. E. Stahl, vitalist, everything is governed by the immaterial soul that saves the body until death from chemical decomposition.
Towards the middle of the eighteenth century was established the idea of spontaneous activity of matter, its ability to produce life. For J. O. La Mettrie not only the animal but also man is considered a machine, but not as a set of hypothetical craft devices but as organized matter so that heart and muscles move even if isolated. This concept was developed by the medical school of Montpellier, in particular by T. Bordeu, who believed that organs have a life of their own and harmonize by mutual consent or sympathy. This pluralistic or federative conception of the organism led to a fusion of vitalism with materialism through the recognition of life as a peculiar property of matter and not as an effect of the soul.
Also around the middle of the century opened a new chapter in modern physiology with the studies of A. Haller on irritability and sensitivity that were recognized as properties of precise anatomical structures (muscles and nerves) and resulting from the application of different stimuli, to which the organism reacts. At the dawn of the nineteenth century physiology drew new impetus from the study of anatomy: X. Bichat decomposed the organs into their respective tissues considered the carriers of vital properties; F. Magendie distinguished sensory nerves from motor nerves. Visual sensation in its subjective aspects was investigated by J. Müller who theorized the specific energy of nerves for which they react in a constant way to different stimuli (color, sound, etc.). Abandoning vitalism, his students H. Helmholtz, E. Du Bois-Reymond, E. Brücke formed, towards the middle of the century, the new mechanistic school destined to triumph, even after the resumption of microscopic studies. The observation of cells, recognized as the elementary units of life, led R. Virchow to conceive the organism as the sum of cellular individuals that remain in the greatest possible autonomy.
A decisive importance for the development of physiology assumed in this century the research in chemistry. Lavoisier had established that respiration and combustion are oxidation processes producing heat; J. Berzelius made the first attempts to establish the composition of organic substances from carbon, oxygen, hydrogen and nitrogen; for J. Liebig a vital force operates the synthesis of these substances. After 1850, with the establishment of the principle of conservation of energy, this no longer appeared sustainable. Investigations were oriented to determine what chemicals are absorbed or excreted and how they are transformed in the body that now appears as a thermal machine that produces work through a slow combustion that does not occur in the lungs or blood, but in the cells. Other challenging fields of research were muscle contraction and the nature of enzymes.
The reduction of physiology to physical-chemical investigation often neglected the peculiar aspects of organisms, favoring some ephemeral return to vitalism. It was recognized, however, more and more the importance of the processes of interaction between organs and regulation of functions that ensure the conservation and adaptation to the environment. Already C. Bernard had already considered the blood as an internal means of stabilization of functions with respect to the changing environment.
With the discovery of the secretion of hormones by the endocrine glands resulted a new network of interaction and regulation that is integrated with that of the nervous system. For the latter, which had always been one of the most difficult fields of investigation of physiology, the greatest contributions were made by: P. Flourens, who ascertained that the brain is the seat of higher psychic functions (it also came to locate in its cortex sensory and motor centers); M. Hall, who identified the reflex action as the basis of involuntary movements related to the spinal cord; H. Jackson, who conceived the nervous system as a sensorimotor machine with three levels of organization, the reflex, the intermediate centers and the voluntary. The latter was unequivocally linked to the cortex, which was also studied according to the theory of conditioned reflexes formulated by I. P. Pavlov. These functions are analyzed from extremely different and specialized points of view and methodologies, from cybernetics to molecular biology, to the analysis of animal behavior. These addresses, although open to a fruitful interdisciplinary encounter, have not reached adequate theoretical synthesis.
Plant physiology as a science has recent origins; however, observations on the vital phenomena of plants are already found among the ancients. Aristotle and his disciple Theophrastus, author of De causis plantarum, believed that plants derive nutrients from the earth already in a directly assimilable form. This conception, partly resumed by Cesalpino, practically dominated until the sixteenth century, when, taking advantage of the progress of chemistry and physics, were started the first studies on experimental basis on the process of nutrition of plants, the circulation of sap and the transport of substances in the form of solution. J. B. van Helmont, based on his experiences, believed that the various constituents of plants were manufactured by the plants themselves.
E. Mariotte tried to trace the nutrition and growth of plants to chemical or physical processes. Fundamental for the development of plant physiology was the work of S. Hales, who carried out a series of systematic experiments on the problem of lymph circulation, highlighting the role played by the transpiration of leaves and discovering the root pressure. The function of chlorophyll was highlighted towards the end of the XVIII century by J. Priestley, who demonstrated that green plants are capable of emitting “vital air”, that is oxygen. G. Ingenhousz completed this observation by noting that only the green parts of plants and only in the light develop “vital air” and then gave the exact interpretation of the phenomenon using the discoveries of A.-L. Lavoisier.
The set of gaseous exchanges that take place in plants were further clarified by J. Sebenier, who recognized that carbon dioxide is decomposed by plants, under the influence of light, with the emission of oxygen. At the beginning of the XIX century T. de Saussure performed precise experiments that allowed to evaluate in quantitative terms the phenomena that occur during nutrition. H. Dutrochet discovered the fundamental laws that regulate the permeability and osmosis phenomena, later specified by W. Pfeffer.
The plant physiology drew new impetus from the studies of J. Sachs that, in a group of works published between 1860 and 1865, recognized the general significance of photosynthetic processes and clarified how and where the organization of carbon dioxide occurs until the formation of starch. Research on mineral nutrition were initiated by J. Liebig and later by J.-B. Boussingault, who demonstrated the importance of nitrogenous substances, and especially nitrates, for plant life. In 1887 H. Helbriegel and H. Wilfarth discovered the biological fixation of nitrogen.
With the XX century begins a new period for plant physiology: they discovered common phenomena that had gone unnoticed (photoperiodicity) as well as the existence of hereditary physiological characters; they isolated substances that regulate the development; they discovered many enzymes and, using the technique of marked elements and chromatography, they clarified the process of photosynthesis. However, it is difficult to make a separation between plant physiology and biochemistry, sciences that, together with genetics, have increasingly contributed to the emergence of molecular biology, while the links between plant physiology and ecology become closer.