What Is Physiology?

Physiology is the branch of biology concerned with the normal functions of living organisms and their component parts, including cells, tissues, organs, and organ systems. It addresses the mechanical, physical, bioelectrical, and biochemical processes that sustain life, maintain internal homeostasis, and enable organisms to respond to their environment. Physiology draws on chemistry, physics, and molecular biology as foundational disciplines and is closely linked to anatomy, which describes structure, since form and function are inseparable in biological systems. The field spans organisms from single-celled prokaryotes to complex vertebrates, with human physiology occupying the largest share of biomedical and engineering research.

Human physiology is organized around the study of interacting organ systems, each performing specialized regulatory and metabolic roles that together sustain the organism. Major systems include the cardiovascular, respiratory, nervous, endocrine, musculoskeletal, renal, digestive, immune, and reproductive systems. Understanding their individual functions and their cross-system regulatory interactions is a prerequisite for designing medical devices, developing therapeutics, and interpreting clinical measurements.

Organ System Function

Each organ system performs a set of specialized physiological tasks coordinated with the others through hormonal and neural signaling. The cardiovascular system circulates blood to deliver oxygen and nutrients to tissues and to remove metabolic waste, operating under the control of cardiac electrical pacemaker cells and regulated by baroreceptor feedback loops. The respiratory system facilitates gas exchange across the alveolar membrane, with ventilation rate modulated by chemoreceptors sensitive to arterial carbon dioxide and oxygen partial pressures. The renal system filters plasma, recovers solutes through active transport mechanisms, and excretes waste as urine, maintaining plasma osmolarity, pH, and volume within narrow ranges. The nervous system integrates sensory inputs and generates motor outputs through electrochemical signaling along axons and across synaptic junctions, operating on time scales from milliseconds for reflex arcs to days for hormonal modulation. The OpenStax Anatomy and Physiology 2e textbook provides a widely used open-access reference for the structural organization underlying these functions.

Cellular and Molecular Physiology

At the cellular level, physiology examines ion channel behavior, membrane transport, signal transduction cascades, and the metabolic pathways that convert nutrients to adenosine triphosphate. The resting membrane potential of excitable cells, maintained by sodium-potassium ATPase pumps and selective ion permeability, is the electrical basis for action potentials in neurons and muscle fibers. Receptor-ligand binding at the cell surface triggers intracellular signaling through G-protein-coupled receptors, receptor tyrosine kinases, and second messengers such as cyclic AMP, which regulate gene expression, metabolism, and cytoskeletal dynamics. The study of insect physiology, which falls within entomology, examines analogous cellular mechanisms in arthropods, including the tracheal respiratory system, the open circulatory hemolymph, and the hormone ecdysone, which regulates molting. These comparisons illuminate the evolutionary conservation and variation of fundamental physiological mechanisms across animal phyla. The Kenhub overview of human physiological systems surveys these cellular mechanisms in the context of whole-organ function.

Integrative and Systems Physiology

Integrative physiology examines how multiple systems interact to produce coordinated whole-organism responses to stimuli such as exercise, temperature change, injury, and disease. During aerobic exercise, cardiac output increases, skeletal muscle blood flow is redistributed away from visceral organs, ventilation rate rises in proportion to metabolic demand, and the adrenal glands release epinephrine to mobilize glucose reserves. Mathematical modeling of these integrated responses, as described in the Stanford Online Systems Physiology course, uses control-theoretic frameworks including feedback, feedforward, and proportional-integral regulation to describe homeostatic mechanisms quantitatively, connecting physiology directly to engineering system analysis.

Applications

Physiology has applications in a range of fields, including:

• Biomedical device design, including pacemakers, ventilators, and dialysis machines
• Pharmacology and drug development targeting specific receptor and transporter systems
• Sports science and exercise physiology for performance and rehabilitation
• Insect and agricultural pest management through applied entomological physiology
• Aerospace and occupational medicine addressing physiological limits under extreme conditions