The term "milieu intérieur," coined by the French physiologist Claude Bernard, refers to the internal environment of an organism. It encompasses the fluid surrounding cells, including blood and interstitial fluid, which provides a stable environment for cellular functions. This concept is crucial for maintaining homeostasis, allowing organisms to regulate their internal conditions despite external changes. Maintaining a stable milieu intérieur is essential for the proper functioning of organs and systems within the body. It involves various physiological processes, such as temperature regulation, pH balance, and electrolyte levels. Disruptions in this internal environment can lead to health issues, highlighting its importance in biology and medicine.