PHYSIOLOGY

study of the functioning of living organisms, animal or plant, and of the functioning of their constituent tissues or cells. The word physiology was first used by the Greeks around 600 BC to describe a philosophical inquiry into the nature of things. The use of the term with specific reference to vital activities of healthy humans, which began in the 16th century, also is applicable to many current aspects of physiology. In the 19th century, curiosity, medical necessity, and economic interest stimulated research concerning the physiology of all living organisms. Discoveries of unity of structure and functions common to all living things resulted in the development of the concept of general physiology, in which general principles and concepts applicable to all living things are sought. Since the mid-19th century, therefore, the word physiology has implied the utilization of experimental methods, as well as techniques and concepts of the physical sciences, to investigate causes and mechanisms of the activities of all living things. study of the functioning of living organisms, animal or plant, or of the functioning of their constituent tissues or cells. Although historically physiology was usually considered separately from anatomy, with the development of high-powered microscopes, and in particular the electron microscope, structure and function at the cellular and even the molecular level came to be understood as inseparable. By the same token, while biochemistry is now a separate science, an understanding of biochemistry is fundamental to physiology. A major feature of physiology is its dynamic state. Cells change their function in response to changes in the composition of their local environment, while the organism responds to alterations in both the internal and the external environment. Thus, many physiological reactions are aimed at preserving a constant physical and chemical internal environment, which Claude Bernard defined as the milieu intrieur. This property, known as homeostasis, a term introduced by American physiologist Walter Bradford Cannon, operates in animals through a range of sensory receptors for the detection of change in either the internal or external environment of the organism. These receptors initiate specific and appropriate responses from effector organs, such as the muscles, kidneys, liver, and endocrine glands. Much of the current understanding of physiology has resulted from studying the responses of cells and tissues to imposed modifications in their environment. This has been done both in the living animal or plant (called in vivo studies) and in cells and tissues removed from the animal or plant and maintained alive in culture (called in vitro studies). For example, a lack of oxygen in arterial blood is sensed by receptors in small structures associated with the carotid arteries and the aorta called carotid and aortic bodies. By a variety of experimental procedures, both in vivo and in vitro, it has been shown that these receptors respond to reductions in the oxygen tension of blood perfusing through these structures and that they generate nerve impulses in afferent nerves that drive a reflex increase in the frequency and depth of breathing. This response tends to correct the reduction in oxygen tension. There are many similar examples of homeostasise.g., the control of bodily metabolism by hormones. The release of insulin from the endocrine tissue of the pancreas is stimulated by glucose, among other factors, and this hormone then acts on muscle and fat cells to facilitate the transport of glucose and other substances into these cells. Thus, the level of glucose circulating in blood is continuously sensed and is also constantly being adjusted through the release of insulin and in other ways. There are physiological controls of almost all tissues and organs in the body. They form the basis for the understanding of clinical diseases and disorders in both human and veterinary medicine. The advent of new techniques has significantly extended the boundaries of physiology; e.g., radioactive isotopes have enabled the measurement of amounts and fluxes of substances present at low concentrations inside cells and in extracellular fluids. Also, by the application of techniques based on immunology, it has been possible to identify and trace the functions of minute quantities of hormones and other chemical agents that are important to understanding the responses of tissues to environmental stimuli. Parallel advances in the application of the physical and chemical sciences to physiology have promoted new understanding; such techniques as X-ray crystallography, nuclear magnetic resonance, and advanced forms of chromatography have enabled the molecular limits of cell physiology to be probed. Organic chemistry has assisted in the creation of new molecules similar in structure to natural molecules (called analogues), which have considerably advanced the understanding of tissue reactions to hormones and other substances and which form the basis of modern pharmacology. Efforts are now being made to integrate all the information obtained from single cells, tissues, and organ systems into an understanding of the way in which the whole organism responds to its environment. Thus, the wide span of knowledge from subcellular molecular biology, including the biochemical basis of gene expression, to the behaviour of the whole animal is now appropriately encompassed by the term physiology. Additional reading American Physiological Society, Handbook of Physiology (1959 ), is a series of separately titled volumes summarizing in detail the status of research in many different specialized areas of physiology. Edward F. Adolph, Origins of Physiological Regulations (1968), contains a broad survey of regulatory physiology. R.M. Case (ed.), Variations in Human Physiology (1985), emphasizes the physiological changes that occur in the human body in response to different human conditions, environmental exposure, stress, and trauma. These topics are dealt with for the animal and plant worlds also by Peter W. Hochachka, Living Without Oxygen: Closed and Open Systems in Hypoxia Tolerance (1980); Peter W. Hochachka and George N. Somero, Strategies of Biochemical Adaptation (1973); and Guido Di Prisco (ed.), Life Under Extreme Conditions: Biochemical Adaptation (1991).Books on function in animals include H.H. Dukes, Dukes' Physiology of Domestic Animals, ed. by Melvin J. Swenson and William O. Reece, 11th ed. (1993); Roger Eckert and David Randall, Animal Physiology, 3rd ed. rev. in part by George Augustine (1988), with coverage extending from the cellular to the organ systems level; C. Ladd Prosser (ed.), Environmental and Metabolic Animal Physiology (1991), and Neural and Integrative Animal Physiology (1991), valuable sources on function in a wide variety of organisms, with extensive bibliographies; and Kurt Schmidt-Neilsen, Animal Physiology: Adaptation and Environment, 4th ed. (1990).Treatments of the history of physiology are found in John Farquhar Fulton and Leonard G. Wilson (eds.), Selected Readings in the History of Physiology, 2nd ed. rev. and enlarged (1966), excerpts from classical publications; James L. Larson, Interpreting Nature: The Science of Living Form from Linnaeus to Kant (1994), a discussion of physiological thought in the 18th century; and W. Bruce Fye, The Development of American Physiology: Scientific Medicine in the Nineteenth Century (1987), examining the growth of the discipline in the United States. Gerald L. Geison, Michael Foster and the Cambridge School of Physiology: The Scientific Enterprise in Late Victorian Society (1978), is a classic study of Foster and the creation of modern physiology.David T. Dennis and David H. Turpin (eds.), Plant Physiology, Biochemistry, and Molecular Biology (1990), provides an introduction to several aspects of plant growth and metabolism using a variety of perspectives. Specialized adaptations of organisms are particularly addressed in Richard E. Lee, Jr., and David L. Denlinger (eds.), Insects at Low Temperature (1991); and Peter W. Hochachka and Michael Guppy, Metabolic Arrest and the Control of Biological Time (1987). Detailed information on the flow of energy in living systems is provided in Harold J. Morowitz, Energy Flow in Biology: Biological Organization as a Problem in Thermal Physics (1968, reissued 1979), still a useful account; Ewald R. Weibel, The Pathway for Oxygen: Structure and Function in the Mammalian Respiratory System (1984); Franklin M. Harold, The Vital Force: A Study of Bioenergetics (1986); and W.A. Cramer and D.B. Knaff, Energy Transduction in Biological Membranes: A Textbook of Bioenergetics (1990).Communication between different organisms, often involving chemical signals, is the subject of William C. Agosta, Chemical Communication: The Language of Pheromones (1992); J.B. Harborne, Introduction to Ecological Biochemistry, 4th ed. (1993); and Michael M. Martin, Invertebrate-Microbial Interactions: Ingested Fungal Enzymes in Arthropod Biology (1987), on complex symbiotic interactions between certain groups of organisms, such as leaf-cutting ants and the fungi whose growth they nurture. Bradley Titus Scheer The Editors of the Encyclopdia Britannica