physiology, study of the normal functioning of animals and plants during life and of the activities by which life is maintained and transmitted. It is based fundamentally on the activities of protoplasm. The study of function is usually undertaken along with a study of structure (see anatomy), the two being intimately related. Since the discovery of the cell structure of tissues, the science of physiology has undergone rapid development. It includes the study of vital activities in cells, tissues, and organs—of processes such as contractility of muscle tissue, coordination through the nervous system, feeding, digestion, excretion, respiration, circulation, reproduction, and secretion. Virtually every specialized field in the biological sciences (e.g., embryology, pathology, botany, zoology) involves a consideration of the physiological aspects of its subject. The study of human physiology was stimulated by the development of medicine, and it embraces many chemical and physical principles. Plant physiology includes also the study of photosynthesis and transpiration. A separate and specialized branch, plant physiology arose from attempts to apply the findings of animal physiology to plants and in its turn contributed to the development of general physiology, especially in the study of cells.

Study of the functioning of living organisms or their constituent tissues or cells. Physiology was usually considered separately from anatomy until the development of high-powered microscopes made it clear that structure and function were inseparable at the cellular and molecular level. An understanding of biochemistry is fundamental to physiology. Physiological processes are dynamic; cells change their function in response to changes in the composition of their local environment, and the organism responds to alterations in both its internal and external environment. Many physiological reactions are aimed at preserving a constant physical and chemical internal environment (homeostasis). Seealso cytology.

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