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Why was Claude Bernard’s idea of the internal environment important to physiology?

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Diagram explaining ATP's role in powering cellular activities, with arrows showing ATP produced by mitochondria, chloroplasts, and glycolysis, and used for making biomolecules, transporting substances, and driving energy-requiring processes; includes a structural diagram of ATP with labeled adenine, ribose, and phosphate groups. — Physiology Adenosine triphosphate (ATP) is the power source of many biochemical reactions. It is produced in the cell structures and system listed at the left to energize the important life processes listed on the right. An abbreviated chemical formula of the structure of ATP is also shown. The two high-energy P―O―P bonds are responsible for its power.

Claude Bernard’s concept of the milieu intérieur, or "internal environment," was a seminal contribution to physiology because it provided the theoretical foundation for the modern understanding of homeostasis.12

By defining the internal environment as the fluid medium in which the body's cells exist, Bernard moved physiology beyond the vague notions of "vital force" that had previously dominated the field. His work demonstrated that life processes are mechanistically determined by physico-chemical forces, establishing that organisms maintain stability through the self-regulation of vital processes despite external changes.231

This insight shifted the focus of biological research toward how systems integrate and coordinate functions—such as those mediated by the nervous system and hormonal systems—to sustain a dynamic equilibrium. Ultimately, Bernard’s framework allowed scientists to view the organism as a system that actively resists outside forces of change to preserve the conditions necessary for survival, a principle that remains central to contemporary biology and medicine.412