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Physiology is the study of how various biological components work independently and together to enable organisms, from animals to microbes, to function. This scientific discipline covers a wide variety of functions from the cellular and subcellular level to the interaction of organ systems that keep more complex biological machines, like humans, running.

Physiological studies are aimed at answering many questions. For instance, physiologists investigate why plants grow or bacteria divide, how food is processed in various organisms, and how thought processes occur in the brain (a branch of this discipline known as neurophysiology). It is often physiology-related investigations that uncover the origins of diseases.

Human (or mammalian) physiology is the oldest branch of this science dating back to at least 420 B.C. and the time of Hippocrates, the father of medicine. Modern physiology first appeared in the seventeenth century when scientific methods of observation and experimentation were used to study blood movement, or circulation, in the body. In 1929, American physiologist W. B. Cannon coined the term homeostasis to describe one of the most basic concerns of physiology: how the varied components of living things adjust to maintain a constant internal environment conducive to optimal functioning.

With the steady advance of scientific technology-from the simple microscope to ultra high-tech computerized scanning devices-the field of physiology grew in scope. No longer confined to investigating the functioning components of life that could be observed with the naked eye, physiologists began to delve into the most basic life forms, like bacteria. They could also study organisms' basic molecular functions, like the electrical potentials in cells that help control the heart beat.

The branches of physiology are almost as varied as the countless life forms that inhabit the earth. Viral physiology, for example, focuses on how these minute life forms feed, grow, reproduce, and excrete by-products. However, the more complex an organism, the more avenues of research open to the physiologist. Human physiology, for instance, is concerned with the functioning of organs, like the heart and liver, and how the senses, like sight and smell, work.

Physiologists also observe and analyze how certain body systems, like the circulatory, respiratory, and nervous systems, work independently and in concert to maintain life. This branch of physiology is known as comparative physiology. Ecological physiology, on the other hand, studies how animals developed or evolved specific biological mechanisms to cope with a particular environment. An example is dark skin, which provides protection against harmful rays of the sun for humans who live in tropical clients. Cellular physiology, or cell biology, focuses on the structures and functions of the cell. Like the term cell biology, many branches of physiology are better known by other names including bio-chemistry, biophysics, and endocrinology (the study of secreting tissues).

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Science EncyclopediaScience & Philosophy: Philosophy of Mind - Early Ideas to Planck length