Physiology is the study of how living systems function. Scientists who study physiology are called physiologists. Physiologists attempt to describe biological phenomena in physical and chemical terms. Physiologists come to the field with extremely diverse backgrounds and apply their specialized skills and knowledge to understanding the function of living systems. Physiologists study living systems from the subcellular level (molecules and organelles) all the way to the level of the whole organism and how organisms adapt to vastly different environmental conditions such as hot, cold, dry, humid, or high altitude. In their attempt to elucidate the mechanisms operating in living systems, physiologists make use of an enormous array of experimental techniques. Techniques of cell and molecular biology, genetics, genomics, bioinformatics, chemistry, physics, and engineering, all find applications in the study of living systems by physiologists.

Physiological investigations range from examining the molecular basis of life to understanding the integrative functions of biological systems. For example, whether one is interested in the function of soluble enzymes, membrane proteins, mitochondria, the heart, or the integrated function of the nervous system, the work can be categorized as physiology. The physiologist, therefore, is interested not only in the molecular function of individual players (e.g., enzymes, membrane transporters, etc.), but also in how these molecules interact within a network of proteins to serve the needs of the cell, tissue, organ, organ-system, and ultimately the organism. Therefore, physiologists wish to know more than just how molecules function in isolation. They also want to understand the cellular and physiological context in which molecules operate.

Physiology is the basis for all of the biomedical sciences, and forms the bridge between all of the other biomedical sciences. Physiology enables information gained by molecular biology, cell biology, genetics, pharmacology, biophysics, biomathematics, and biochemistry to be described in an integrated manner that can be applied to human medicine. Much of clinical medicine is based on a sound understanding of molecular, cellular, and organ-system physiology.

Human physiology, a branch of general physiology, is concerned with how the human body works. It is common to approach the study of human physiology through an organ-system approach. Organ-systems are collections of cells, tissues and organs, which have dedicated functions in the body. In the human body, the organ-systems are the nervous system, endocrine system, cardiovascular system, respiratory system, urinary system, musculoskeletal system, integumentary system, reproductive system, digestive system, and immune system. While the proper function of each organ-system is essential, it is the collective and integrative functions of all organ-systems that contribute to a healthy state. A thorough study of human physiology includes an understanding of the organ-systems as well as the underlying tissue, cellular, and molecular principles. Unless otherwise indicated, the information provided on this web site relates to human physiology. Yet, it is important to keep in mind that many physiological principles share common features among different organisms and, therefore, are not unique to human physiology. Indeed, it is very common to utilize model organisms or cell lines to study a physiological process. In many cases, the findings can be extended to other organisms.

Homeostasis means staying in a similar state. It refers to the various physiological parameters in the human body that are constantly maintained within very narrow limits even in the face of constantly changing external environmental conditions. For example, physiological parameters such as core body temperature, plasma concentrations of glucose and electrolytes (Na+, K+, Cl-) and many others are tightly regulated to remain within allowed limits. The French physiologist, Claude Bernard, considered to be the father of modern experimental physiology, was the first to take notice of this maintenance of the internal environment. Later, the American physiologist, Walter B. Cannon coined the term homeostasis. Although homeostasis implies constancy, it is in fact a state of dynamic constancy, meaning that physiological parameters are not constant, but fluctuate around a set point. For example, while the human core body temperature is said to be 37 °C (98.6 °F), it is known that core body temperature normally fluctuates above and below this set point by up to ~1 °C (1.8 °F) in either direction. Thus, the set point per se is only an average of all the points that fall within the normal range.