Physiology is the branch of biology concerned with how living organisms and their parts function. It investigates mechanisms that sustain life, including the exchange of materials, communication between cells, regulation of internal conditions, and responses to environmental change. Its explanations connect molecular events with the activity of cells, tissues, organs, and entire organisms, making the integration of different levels of organization a defining feature of the discipline. (physoc.org)
Scope and relationship to other sciences
Whereas anatomy primarily describes biological structures, physiology explains their activities and the mechanisms underlying them. The two disciplines are closely connected because structure constrains function: the arrangement of a tissue, the shape of an organ, and the properties of its constituent cells affect what it can do. Human physiology includes specialized fields such as neurophysiology, cardiovascular physiology, respiratory physiology, and renal physiology. (openstax.org)
Physiology overlaps with biochemistry, which examines biological substances and reactions, and with molecular biology, which studies molecular mechanisms within living systems. Physiological investigation often asks how such mechanisms combine to produce an observable function, such as a change in heart rate or the detection of light. It therefore includes both detailed analysis of individual components and study of their interactions in an intact organism. (physoc.org)
Levels of organization and cellular mechanisms
In multicellular organisms, a cell is a basic functional unit. Cells form tissues, different tissues constitute organs, and organs cooperate within organ systems. Physiological explanations move between these scales: an organ’s performance depends on cellular processes, but cells also depend on conditions maintained by other organs. For example, delivery of oxygen and nutrients supports cellular activity throughout the body. (openstax.org)
The cell membrane separates intracellular and extracellular environments and selectively regulates exchange between them. Diffusion allows substances to move down concentration gradients, while osmosis describes water movement across a selectively permeable membrane. Transport also depends on membrane proteins, including channels, carriers, and pumps. Primary active transport uses adenosine triphosphate directly; secondary active transport uses gradients established by other transport processes. (openstax.org)
Differences in ion concentrations and selective membrane permeability contribute to electrical potentials across membranes. In excitable cells, changes in ion channel activity produce an action potential, a transient electrical signal. These processes connect molecular transport with communication in nerves and activity in muscle, illustrating how a cellular mechanism can support functions at larger scales. (openstax.org)
Homeostasis and regulation
Homeostasis is the maintenance of relatively stable internal conditions despite disturbances. It does not mean complete constancy: physiological variables fluctuate within ranges, and regulatory processes continually adjust them. Examples include the control of body temperature, blood pressure, and nutrient concentrations. A simple regulatory model distinguishes a sensor that detects a condition, a control center that processes information, and an effector that changes the condition. (openstax.org)
Negative feedback counteracts a deviation. In thermoregulation, excessive heat can trigger increased skin blood flow and sweating, promoting heat loss. Regulation of blood glucose provides another example: pancreatic cells respond to elevated glucose by releasing the hormone insulin, which influences glucose uptake and storage. Positive feedback instead reinforces an ongoing change; during childbirth, uterine contractions and oxytocin release form an amplifying loop that ends with delivery. (openstax.org)
Organ systems and comparative physiology
Human organ systems perform distinct but interdependent functions. The cardiovascular system circulates blood, the respiratory system exchanges gases, and the urinary system removes wastes and helps regulate fluid composition. Digestion and absorption supply nutrients, while nervous and endocrine signaling coordinate responses. The immune system contributes defense, and the reproductive system enables production of offspring. Dividing these functions into systems is useful for study, but their operation remains interconnected. (openstax.org)
Physiology is not restricted to humans. Plant physiology examines functions such as photosynthesis, growth, and transport. In vascular plants, xylem carries water and mineral nutrients, while phloem distributes organic products of photosynthesis. Water movement depends on differences in water potential, and transpiration links transport within a plant to exchange with the atmosphere. Stomatal regulation influences both water loss and gas exchange. (openstax.org)
Historical development
Experimental investigation helped establish physiology as a distinct scientific discipline. In 1628, William Harvey published his account of blood circulation, describing the heart’s role in driving blood around the body. In the nineteenth century, Claude Bernard developed the concept of the milieu intérieur, or internal environment. Walter B. Cannon subsequently introduced the term “homeostasis” in 1926, extending this tradition of explaining physiological stability through coordinated regulation. (physiology.org)
Twentieth-century electrophysiology clarified the ionic mechanisms of nerve signaling. Alan Hodgkin and Andrew Huxley demonstrated the roles of sodium and potassium movement in nerve membrane excitation. They shared the 1963 Nobel Prize in Physiology or Medicine with John Eccles for discoveries concerning excitation and inhibition in nerve cells. (nobelprize.org)
Experimental methods and applications
Physiological research uses measurements and controlled interventions to investigate function. Methods include cellular imaging, biochemical measurements, voltage clamping, and patch clamping, which records membrane currents. Different preparations allow investigators to examine whole-cell currents or the activity of individual membrane channels. The choice of technique determines which processes can be observed and what limitations affect interpretation. (physiology.org)
Physiology provides foundations for medicine by explaining normal function and identifying mechanisms that change in disease. Animal research is also shaped by the principles of replacement, reduction, and refinement: replacing animal use where possible, using appropriately designed studies that minimize numbers, and reducing pain, distress, and lasting harm. These principles connect experimental practice with animal welfare and the reliability of physiological observations. (physoc.org)