Physiology is the scientific study of how living organisms, organs, tissues, cells and biological systems function. Human physiology examines the mechanisms that allow the body to maintain life, respond to its environment and coordinate activities across multiple levels of organisation. It includes processes such as electrical signalling, muscle contraction, gas exchange, circulation, digestion, renal filtration, hormone action, immune responses and reproduction. A central concept is homeostasis, the regulation of internal conditions within ranges compatible with cellular and organismal function.
Physiological mechanisms operate through interacting feedback systems rather than independently, so changes in one process can influence many others. Physiology differs from anatomy primarily in emphasis: anatomy describes structure and spatial relationships, whereas physiology investigates mechanisms and functions. In practice, the two are inseparable because the properties of a structure constrain what it can do, while physiological activity can modify structural organisation over time.
Physiological regulation commonly involves sensors, integrating mechanisms and effectors operating through feedback. Negative feedback counteracts deviations from a regulated condition and is widespread in biological control. For example, changes in blood glucose can stimulate hormonal responses that alter glucose uptake, production or storage, thereby tending to restore concentrations towards a regulated range.
Positive feedback instead amplifies a process and is less common because it can drive a system rapidly towards a defined endpoint, as occurs during aspects of childbirth or blood clot formation. Homeostasis does not mean that physiological variables remain perfectly constant; rather, they fluctuate within regulated limits and can shift according to circumstances. The concept of allostasis is used to describe adaptive changes in regulatory states that help an organism respond to changing demands. Physiological regulation therefore involves dynamic control rather than mechanical constancy.
Human physiology is organised across multiple levels. Cellular physiology examines membrane transport, ion gradients, signalling, metabolism and electrical activity. Tissue physiology examines coordinated properties of specialised cell populations, while organ physiology investigates integrated functions such as cardiac pumping or renal filtration. Systems physiology considers interactions among organ systems, including relationships between respiratory and cardiovascular function or between endocrine and nervous regulation. Integrative physiology studies how these levels operate together in the intact organism.
The nervous system can alter heart rate and vascular tone within seconds, endocrine signals can modify metabolism over longer periods, and local cellular mechanisms can independently regulate tissue activity. Physiological responses therefore occur across different spatial and temporal scales, with rapid electrical and chemical signalling interacting with slower changes in gene expression, protein abundance and tissue structure.
The major areas of human physiology correspond broadly to major functional systems. Cardiovascular physiology examines cardiac electrical activity, contraction, blood flow and vascular regulation. Respiratory physiology investigates ventilation, pulmonary gas exchange and oxygen and carbon dioxide transport. Renal physiology concerns filtration, tubular transport, fluid balance, electrolytes and acid-base regulation. Gastrointestinal physiology covers digestion, absorption and motility. Neurophysiology studies neuronal signalling, sensory processing and motor control. Endocrine physiology examines hormones and their effects on target tissues. Muscle physiology investigates excitation-contraction coupling and force generation. Reproductive physiology concerns gametogenesis, hormonal regulation, fertilisation, pregnancy and reproductive cycles. Each area has specialised mechanisms, but normal function depends on continuous interaction among them.
Physiological processes are based on physical and chemical principles. Membrane potentials arise from ion concentration gradients and selective membrane permeability. Osmosis and diffusion influence movement of water and solutes, while active transport uses energy to move substances against electrochemical gradients. Enzymes accelerate biochemical reactions, receptors detect signalling molecules, and intracellular pathways translate extracellular information into cellular responses. Blood flow is influenced by pressure gradients, vascular resistance and vessel properties, while gas exchange depends on partial-pressure gradients and membrane characteristics. These mechanisms illustrate why physiology cannot be understood solely by memorising organ functions: physiological outcomes emerge from measurable interactions among physical forces, chemical reactions, cellular structures and regulatory systems. Quantitative approaches, mathematical models and experimental measurements are therefore important components of advanced physiology.
Physiology also explains adaptation to changing conditions. Exercise increases cardiovascular output, ventilation and oxygen delivery while altering metabolic demand and temperature regulation. Exposure to high altitude produces responses involving ventilation, circulation and red-blood-cell production. Fasting changes hormonal signalling and shifts energy metabolism towards stored substrates, while prolonged nutritional deprivation can produce further metabolic adaptations. During ageing, many physiological capacities change because of alterations in cellular function, tissue structure, endocrine regulation and nervous-system control.
Disease can disrupt regulatory mechanisms at virtually any level, producing pathological physiology. For example, impaired insulin signalling can alter glucose regulation, while reduced renal filtration can disturb fluid, electrolyte and acid-base balance. Pathophysiology therefore uses physiological principles to explain how normal mechanisms become altered in disease.
Experimental physiology has developed from direct observation and measurement into a highly quantitative discipline. Early physiological research used anatomical observation, animal experimentation and measurement of variables such as pulse, respiration and blood pressure. Modern methods include electrophysiology, molecular assays, imaging, metabolic measurements, computational modelling and controlled clinical experiments. Measurements can be made at scales ranging from individual ion channels to whole-organism oxygen consumption.
Physiological experiments must account for variables such as temperature, age, activity, nutrition, circadian rhythms, stress and medications because these can alter measured responses. Modern physiology also integrates findings from genetics, molecular biology, neuroscience, immunology and systems biology. This interdisciplinary approach allows physiological mechanisms to be traced from molecular events through cellular behaviour and organ-level activity to observable characteristics of an organism.
Physiology provides a functional framework connecting many areas of biology. It explains how anatomical structures operate, how cells communicate, how biochemical reactions support cellular activity and how organ systems maintain coordinated internal conditions. Physiological concepts are used in medicine, pharmacology, sports science, nutrition, biomedical engineering and environmental biology. Pharmacological effects can often be understood as modifications of physiological signalling pathways, while medical diagnosis frequently involves measuring physiological variables such as blood pressure, electrical activity, respiratory function or hormone concentrations.
A complete account of physiology therefore includes normal mechanisms, regulatory principles, adaptive responses, quantitative relationships and disturbances caused by disease. Its central significance is that biological structure becomes meaningful through the processes it enables, while those processes themselves depend on molecular and cellular mechanisms operating within organised anatomical systems.
