Human physiology

The earliest interpretations of organ functions were speculative in nature, as found in the texts of Alcmeon of Croton, Empedocles, and Democritus. In the Hippocratic texts we find the first important theoretical synthesis of the functions of the organism, the normal course of which is traced back to the balance of the four humors (blood, yellow bile, black bile and phlegm) and is directly influenced, in addition to environmental conditions, by an internal heat centered in the heart and produced by the encounter between food and air (pneuma) absorbed in the lungs.

According to Aristotle (4th century B.C.), the soul forms and directs the various organs and is located in the heart; he attributed to the brain only the function of cooling the blood. The more precise recognition of the functions of this organ, as well as of the nerves and muscles, was the work of the school of Alexandria during the 4th and 3rd centuries B.C., especially through the anatomical studies of Europhilus and Erasistratus. Galen (2nd century), 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 quasi-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 Galen’s physiology, the vital processes, which are governed by strict laws, are always directed to specific ends and nothing in them is superfluous or lacking: the 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, which is considered perforated, it passes to the left side (ventricle), where it is mixed with the pneuma coming from the lungs and heated; its impurities are expelled through the lungs with the breath. The vital spirit, which arrives in the brain with the blood, is transformed into animal spirit, which performs psychic functions and, traveling through the nerves, which are supposedly hollow, reaches the muscles, causing their movements.

Throughout the Middle Ages, physiology did not deviate significantly from Galen’s teachings, and it was not until the anatomical researches of the 16th century that new developments took place. First, A. Vesalius denied that the interventricular septum, which separates the two parts of the heart, is permeable to blood, while M. Serveto and R. Colombo proposed that in order to circulate from the right ventricle to the left ventricle, the blood must pass through the lungs (small circle). The complete demonstration of blood circulation came in W. Harvey’s Exercitatio anatomica de motu cordis et sanguinis (1628), which marked the beginning of modern physiology. Harvey claimed that the blood did not travel through the vessels from the center to the periphery in an ebb and flow motion, as Galen had argued, but that it was expelled from the left ventricle, which contracts with every pulse, through the arteries to all the organs, where, in the manner later clarified by Malpighi’s discovery of the capillaries, it passes through the veins and reaches the right atrium of the heart (great circle). This revolutionary discovery, based on observations, vivisection of animals and calculation of the volume of moving blood, paved the way for the so-called animate anatomy, which studied the function of organs from their structure; moreover, the consideration of the heart as a pump and the vessels as hydraulic conduits was in line with the idea, theorized above all by Descartes, that living organisms are machines.

Among the advocates of this direction: S. Santorio, who used scales to calculate the balance of substances absorbed and excreted by the organism, and G. A. Borelli, who applied the laws of mechanics to the study of locomotion in animals and flight in birds. This iatromechanical school was opposed in the seventeenth century by the iatrochemical school. Already in the previous century, Paracelsus had argued that the human body is essentially a chemical system composed of mercury, sulphur and salt, and J. B. van Helmont, his follower, used the term gas to describe the fermentations that govern the vital functions, which, according to F. Sylvius, are based on the balance of acid and alkaline substances.

T. Willis believed that muscle contraction was due to a kind of explosion of sulfur and nitrogen particles in the blood, while R. Lower and J. Mayow believed that respiration was not intended to cool the blood, but to draw from the air a substance essential to life. At the beginning of the nineteenth century, great theoretical syntheses were elaborated based on the idea that the organism is controlled by a single center. For F. Hoffmann, a mechanist, functions are regulated by a spirit derived from the cosmic ether and acting as subtle matter through the nervous system. For G. E. Stahl, a vitalist, everything is controlled by the immaterial soul, which protects the body from chemical decomposition until death.

In the middle of the 18th century, the idea of the spontaneous activity of matter, its ability to produce life, became established. For J. O. La Mettrie, not only the animal but also the human being is considered a machine, not as a collection of hypothetical tools, but as matter organized in such a way that heart and muscles move even when isolated. This conception 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 agreement or sympathy. This pluralistic or federative conception of the organism led to a fusion of vitalism with materialism by recognizing life as a peculiar property of matter and not as an effect of the soul.

Also around the middle of the century, a new chapter in modern physiology opened with A. Haller’s studies on irritability and sensitivity, which were recognized as properties of precise anatomical structures (muscles and nerves) and resulting from the application of various stimuli to which the organism reacts. At the beginning of the 19th century, physiology received new impulses from the study of anatomy: X. Bichat decomposed the organs into their respective tissues, considered as carriers of vital properties; F. Magendie distinguished sensory nerves from motor nerves. The visual sensation in its subjective aspects was studied by J. Muller, who theorized the specific energy of the nerves, making them react consistently to the most diverse stimuli (color, sound, etc.).

His disciples, H. Helmholtz, E. Du Bois-Reymond and E. Brücke, abandoned vitalism towards the middle of the century and founded the new mechanistic school, which was to triumph after the revival of microscopic studies. The observation of cells, recognized as the elementary units of life, led R. Virchow to conceive of the organism as the sum of cellular individuals that persist in as much autonomy as possible. The development of physiology in this century was decisively influenced by research in chemistry. Lavoisier had established that respiration and combustion are heat-producing oxidation processes; J. Berzelius made the first attempts to determine the composition of organic substances from carbon, oxygen, hydrogen, and nitrogen; for J. Liebig, a vital force operates the synthesis of such substances.

After 1850, with the establishment of the principle of conservation of energy, this no longer seemed tenable. Investigations were directed at determining which chemicals were absorbed or excreted and how they were transformed in the organism, which now appeared as a thermal machine producing work by slow combustion that took place not in the lungs or blood but in the cells. Other challenging areas of research were muscle contraction and the nature of enzymes.

The reduction of physiology to physicochemical inquiry often neglected the peculiar aspects of organisms, favoring a certain ephemeral return to vitalism. However, the importance of the processes of interaction between organs and the regulation of functions that ensure maintenance and adaptation to the environment was increasingly recognized. C. Bernard had already considered the blood as an internal means of stabilizing functions with respect to the changing environment.

With the discovery of the secretion of hormones by the endocrine glands, a new network of interactions and regulation emerged to complement that of the nervous system. For the latter, which had always been one of the most difficult areas of study in physiology, major contributions were made by P. Flourens, who established the brain as the seat of higher psychic functions (they even came to locate sensory and motor centers in its cortex); M. Hall, who identified reflex action as the basis of involuntary movements related to the spinal cord; H. Jackson, who conceived of the nervous system as a sensorimotor machine with three levels of organization: reflex, intermediate centers, and voluntary. The latter was clearly linked to the cortex, which was also studied on the basis of the theory of conditioned reflexes formulated by I. P. Pavlov. These functions are analyzed from extremely diverse and specialized points of view and methodologies, from cybernetics to molecular biology to the analysis of animal behavior. These directions, although open to fruitful interdisciplinary encounters, have not yet arrived at adequate theoretical syntheses.

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