Physiology is the study of how living organisms function, from the chemical reactions inside a single cell to the coordinated work of entire organ systems. It covers every process that keeps you alive: how your heart pumps blood, how your lungs exchange oxygen, how your brain interprets a conversation, and how your body maintains a stable internal environment despite constantly changing conditions. The field spans multiple levels of complexity, and understanding those levels is the easiest way to see what physiologists actually spend their time investigating.
Levels of Organization
Physiology organizes the body into a hierarchy, starting small and building upward. At the chemical level, scientists study atoms and molecules, the building blocks of every body structure. One step up, the cell is the smallest independently functioning unit of a living organism, and all physiological functions are either performed in cells or initiated by them.
Cells of similar type group together into tissues, which carry out specific functions like contraction or insulation. Two or more tissue types form an organ, such as the stomach or the heart, each performing one or more physiological jobs. Organs that cooperate toward a shared goal make up an organ system, like the cardiovascular or respiratory system. At the top sits the organism itself: a living being capable of independently performing every function necessary for life. Physiologists work at every one of these levels, sometimes zooming in on a single ion channel in a cell membrane, sometimes studying how multiple organ systems coordinate during exercise or stress.
Homeostasis: The Central Concept
If physiology has one unifying idea, it’s homeostasis, the body’s ability to maintain stable internal conditions. The concept traces back to Claude Bernard, a 19th-century French scientist widely considered the founder of modern physiology. Bernard observed that animals have two environments: the external one they live in, and an internal one where their tissues operate. He argued that “the fixity of the internal environment is the condition of free and independent life,” a principle so foundational that his original French term, “milieu intérieur,” still appears untranslated in English-language textbooks.
Homeostasis works through feedback loops. A negative feedback loop detects a change and produces the opposite response to correct it. If your blood sugar rises after a meal, your body releases a hormone that brings it back down. If your core temperature drops, your muscles shiver to generate heat. Five components drive this process: a sensor that detects the change, a set point representing the ideal value, an error detector that compares the two, a controller that decides what to do, and an effector that carries out the correction.
Positive feedback loops are less common but equally important. During childbirth, the release of oxytocin stimulates uterine contractions, which push the baby’s head against the cervix, which triggers more oxytocin release. The cycle intensifies until delivery is complete. Unlike negative feedback, positive feedback amplifies a stimulus rather than opposing it.
The Major Organ Systems
A large portion of physiology courses and research focuses on how ten major organ systems function and interact:
- Cardiovascular system: pumps blood to deliver oxygen and nutrients and remove waste
- Respiratory system: exchanges oxygen and carbon dioxide between your cells and the outside air
- Nervous system: detects changes in the internal and external environment and coordinates responses
- Endocrine system: uses hormones to regulate growth, metabolism, reproduction, and mood
- Muscular system: produces movement through the contraction of muscle fibers
- Skeletal system: provides structural support, protects organs, and stores minerals
- Digestive system: breaks complex foods into simple molecules that can be absorbed into the blood
- Urinary system: filters blood and removes metabolic waste products
- Lymphatic system: drains excess fluid from tissues and plays a central role in immune defense
- Reproductive system: produces gametes and supports the development of offspring
Each system is studied both in isolation and in terms of how it communicates with others. Your cardiovascular and respiratory systems, for example, are so tightly linked during physical activity that they’re often studied together under the label “cardiorespiratory physiology.”
What Happens at the Cellular Level
Cellular physiology examines the processes that occur within and between individual cells. A major focus is membrane transport, the movement of ions and molecules across the cell’s outer boundary. This movement creates the electrical and chemical gradients that make nerve signaling, muscle contraction, and hormone secretion possible. Chloride transport in lung cells, for instance, regulates the volume of fluid in your airways. Hydrogen ion transporters fine-tune the pH inside cells, which affects everything from gene expression to cell growth.
Cells also communicate through signal transduction, a chain of molecular events that translates an external signal (like a hormone arriving at a cell surface) into an internal response (like releasing stored energy or dividing). Immune cells use membrane transport to sense chemical signals in their environment and mount a defense. Because so many diseases involve disruptions in these processes, membrane transporters have become important targets for treatments in cardiovascular disease, diabetes, dementia, and cancer.
Life Processes Physiology Explains
Beyond organ systems and cells, physiology addresses a set of fundamental life processes that apply to the body as a whole. Metabolism encompasses every chemical reaction in the body, from breaking down food for energy to building new tissue. Responsiveness is the ability to detect a change and react to it, whether that’s pulling your hand from a hot surface or adjusting your heart rate during a sprint. Growth involves increases in cell number or cell size. Differentiation is the process by which unspecialized cells become specialized, developing the distinct structures and functions of, say, a neuron versus a red blood cell. Reproduction transmits life from one generation to the next. Excretion removes waste products of digestion and metabolism. At every level of organization, there’s a division of labor, and physiology maps out exactly how that labor is distributed.
Specialized Branches
The field branches into numerous specialties. Neurophysiology focuses on how the brain and nerves generate electrical signals, process information, and control behavior. Cardiovascular physiology studies the mechanics of blood flow, heart rhythm, and blood pressure regulation. Exercise physiology examines how the body adapts to physical activity, from changes in muscle fiber composition to shifts in oxygen delivery. Endocrinology investigates hormone production and its effects on nearly every tissue in the body. Comparative physiology looks at how different species solve the same biological problems, like staying warm or conserving water, often revealing principles that apply to humans as well. Reproductive physiology covers everything from gamete development to the hormonal orchestration of pregnancy.
Graduate programs in physiology, such as the one at UC Davis, typically offer general training in cellular, molecular, and systemic physiology alongside these more specialized tracks, reflecting the field’s breadth.
From Normal Function to Disease
Understanding how the body works when healthy is the foundation for understanding what goes wrong in disease. Pathophysiology, the study of disordered function, depends entirely on first knowing what normal function looks like. One striking example comes from gastroenterology. Basic physiological research identified the specific molecular pump responsible for secreting acid in the stomach. That discovery led directly to the development of proton pump inhibitors, a class of medication that revolutionized the treatment of peptic ulcers and acid reflux after its launch in the late 1980s. Before that, acid-related disease was one of the most common reasons for hospitalization in gastrointestinal wards and had no effective treatment.
Today, physiologists are tackling increasingly ambitious questions: the basic biology of learning and language, the mechanisms behind cardiovascular disease, pancreatic cancer, and dementia. In each case, the approach is the same. Map the normal process first, then identify where it breaks down.
Modern Tools and Approaches
Traditional physiology relied on direct measurement: recording electrical activity in the heart, tracking oxygen consumption, measuring blood pressure under different conditions. Those tools remain central, but the field has expanded dramatically. Electroencephalography (EEG) and magnetoencephalography (MEG) allow researchers to noninvasively track brain activity in real time. Imaging techniques based on blood flow and oxygen use reveal which brain regions are active during specific tasks.
At the molecular level, a set of approaches collectively called “omics” has transformed how physiologists study complex systems. Genomics examines the full set of genes in an organism. Transcriptomics looks at which genes are actively being read at a given moment. Proteomics catalogs the proteins a cell is producing, and metabolomics tracks the small molecules generated by metabolism. Combining these data sets gives a far more complete picture of what’s happening inside a cell or tissue than any single technique could provide. Systems biology, an interdisciplinary field drawing on chemistry, biology, mathematics, physics, and engineering, uses computational models to integrate all of this molecular data and predict how a biological system will respond to a given change, including potential treatments for disease.