Physiology is the branch of biology that studies how living things work. It covers everything from how a single cell produces energy to how your heart, lungs, and brain coordinate to keep you alive. While anatomy describes what body structures look like, physiology explains what those structures actually do and how they do it together.
The field spans multiple levels of organization: molecules, cells, tissues, organs, organ systems, and the whole organism. That emphasis on integration, understanding how all these levels connect, is what sets physiology apart from other life sciences.
How Physiology Differs From Anatomy
Anatomy and physiology are closely linked but focus on different questions. Anatomy asks “what does it look like?” while physiology asks “how does it function?” Your lungs have a tree-like branching structure (anatomy) that maximizes surface area for gas exchange (physiology). The two fields are so intertwined that studying one without the other can be frustrating. The shape of a structure almost always reveals something about its job. A heart valve’s flap-like design, for instance, only makes sense once you understand it needs to prevent blood from flowing backward.
Homeostasis: The Central Concept
If physiology has one organizing principle, it’s homeostasis: the body’s constant effort to keep its internal conditions stable. Your temperature, blood sugar, hydration, and dozens of other variables all need to stay within a narrow range for your cells to survive. The concept traces back to the French physiologist Claude Bernard, who wrote in 1865 that “the fixity of the internal environment is the condition of free and independent life.” The term “homeostasis” came later, but the idea that your body actively maintains a stable inner world remains the foundation of the field.
Most homeostatic control works through negative feedback loops. These have a simple logic: a sensor detects a change, a control center processes the signal, and an effector reverses the change. Blood sugar regulation is a classic example. When glucose levels rise after a meal, cells in the pancreas detect the increase and release a hormone that signals muscle, fat, and liver cells to absorb the excess sugar, bringing levels back down. The system works like a thermostat: detect a deviation, correct it, return to normal.
Positive feedback loops are rarer and work in the opposite direction, amplifying a change instead of reversing it. Childbirth is the most familiar example. Early contractions push the baby toward the cervix, which triggers stronger contractions, which push harder, and so on until delivery. Positive feedback always has a definite endpoint. Without one, the escalating response would be dangerous.
How Cells Power the Body
At the most basic level, physiology starts with the cell. Every function your body performs, from thinking to digesting food, depends on cells extracting energy from nutrients and using it to do work. The cell membrane acts as a selective barrier, letting specific molecules in through specialized proteins while keeping others out. Some of this transport requires energy, some doesn’t.
Once nutrients enter a cell, they’re broken down in stages rather than all at once. Cells convert the energy stored in food molecules into small, portable energy carriers, the most important being ATP. The bulk of ATP production happens inside mitochondria through a process that shuttles electrons along a chain of proteins embedded in the mitochondrial membrane. This creates a gradient of hydrogen ions that the cell harnesses to generate ATP. The full process yields roughly 15 times more energy per glucose molecule than simpler methods like fermentation, which is why oxygen-dependent energy production is so critical to complex life.
Major Organ Systems and Their Roles
Physiologists study how the body’s organ systems carry out their individual jobs and coordinate with each other. The human body has about 11 major systems, each with a distinct role:
- Cardiovascular: pumps blood throughout the body, delivering oxygen and nutrients to cells while removing carbon dioxide and waste
- Respiratory: brings air into the body, adds oxygen to the blood, and removes carbon dioxide
- Nervous: directs voluntary and automatic actions, enables thought, self-awareness, and emotion
- Endocrine: produces hormones that travel through the blood to regulate other organ systems
- Digestive: extracts nutrients from food and excretes solid waste
- Musculoskeletal: provides structural support and allows movement
- Urinary: filters waste from the blood and excretes it as urine
- Integumentary (skin): forms a protective barrier and helps regulate body temperature
- Reproductive: enables reproduction through the production of sperm or eggs and, in females, supports fetal development
No system works in isolation. Your nervous and endocrine systems constantly communicate to coordinate the others. When you exercise, your cardiovascular, respiratory, musculoskeletal, and nervous systems all adjust simultaneously. Physiology is fundamentally about understanding these interactions.
Why Physiology Matters for Medicine
Understanding how the body normally works is the starting point for understanding disease. Pathophysiology, a closely related field, studies the functional and biochemical changes that occur when something goes wrong. It examines the body’s molecular, cellular, and systemic responses during disease development and progression. A doctor diagnosing diabetes, for example, relies on knowledge of normal glucose regulation to identify where the process has broken down.
The Nobel Prize in Physiology or Medicine, one of the most prestigious awards in science, is given annually for discoveries in this space. The 2025 prize went to three researchers for their work on how the immune system learns to tolerate the body’s own tissues, a process called peripheral immune tolerance. Failures in this process underlie autoimmune diseases, where the immune system mistakenly attacks healthy cells.
Branches of the Field
Physiology is broad enough to encompass very different kinds of research. Cell physiology focuses on how individual cells function, including how proteins behave and how signals pass between cells. Systems physiology looks at how organs and organ systems work together. Exercise physiology studies how the body responds to physical activity. Ecophysiology examines how whole organisms adapt to their environments, connecting body function to ecology and evolution.
Some physiologists work at the molecular level, investigating single proteins within a cell. Others study how cells interact within tissues, or how multiple organ systems integrate to control a complex organism. Modern research tools include techniques like body-surface electrical mapping to study heart rhythms, optical mapping to investigate how electrical signals travel through cardiac tissue, and imaging methods that combine electrical recordings with 3D anatomical data. These approaches let researchers observe physiology in action rather than piecing it together from static snapshots.