Introduction: Homeostasis and cellular physiology

Homeostasis enables the body to survive in diverse environments

Humans are independent, free-living animals who can move about and survive in vastly diverse physical environments. Thus we find human habitats ranging from the frozen tundra of Siberia and the mountains of Nepal ^a

a The adaptability of humans is remarkable: humans can survive on Mount Everest, which, at 29,028 feet above sea level, is at the cruising altitude of jet airplanes. At the summit the temperature is approximately −40° Celsius (same as −40° Fahrenheit), the thin atmosphere supplies only about one third of the oxygen at sea level, and the relative humidity is zero.

to the jungles of the Amazon and the deserts of the Middle East. Nevertheless, the elemental constituents of the body are cells, whose survival and function are possible only within a narrow range of physical and chemical conditions, such as temperature, oxygen concentration, osmolarity, and pH. Therefore the whole body can survive under diverse external conditions only by maintaining the conditions around its constituent cells within narrow limits. In this sense the body has an internal environment, which is kept constant to ensure survival and proper functioning of the body’s cellular constituents. The process whereby the body maintains constancy of this internal environment is referred to as homeostasis. ^b

b The concept of the internal environment was first advanced by the 19th-century pioneer of physiology Claude Bernard, who discussed it in his book, Introduction à l’étude de la médecine expérimentale in 1865. Bernard’s often-quoted dictum is: “The constancy of the internal environment is the prerequisite for a free life.” ( “La fixeté du milieu intérieur est la condition de la vie libre.” from Leçons sur les phénomènes de la vie communs aux animaux et aux végétaux, 1878.) The term “homeostasis” was introduced by Walter B. Cannon in his physiology text, The Wisdom of the Body (1932).

When homeostatic mechanisms are severely impaired, as in a patient in an intensive care unit, artificial life support systems become necessary for maintaining the internal environment.

Achieving homeostasis requires various component physiological systems in the body to function coordinately. The musculoskeletal system enables the body to be motile and to acquire food and water. The gastrointestinal system extracts nutrients (sources of both chemical energy, such as sugars, and essential minerals, such as sodium, potassium, and calcium) from food. The respiratory (pulmonary) system absorbs oxygen, which is required in oxidative metabolic processes that “burn” food to release energy. The circulatory system transports nutrients and oxygen to cells while carrying metabolic waste away from cells. Metabolic waste products are eliminated from the body by the renal and respiratory systems. The complex operations of all the component systems of the body are coordinated and regulated through biochemical signals released by the endocrine system and disseminated by the circulation, as well as through electrical signals generated by the nervous system.

The body is an ensemble of functionally and spatially distinct compartments

The organization of the body may be viewed hierarchically ( Fig. 1.1 ). The various systems of the body not only constitute functionally distinct entities, but also comprise spatially and structurally distinct compartments. Thus the lungs, the kidneys, the various endocrine glands, the blood, and so on are distinct compartments within the body. Each compartment has its own local environment that is maintained homeostatically to permit optimal performance of different physiological functions.

Compartmentation is an organizing principle that applies not just to macroscopic structures in the body, but to the constituent cells as well. Each cell is a compartment distinct from the extracellular environment and separated from that environment by a membrane (the plasma membrane ). The intracellular space of each cell is further divided into subcellular compartments (cytoplasm, mitochondria, endoplasmic reticulum, etc.). Each of these subcellular compartments is encompassed within its own membrane, and each has a different microscopic internal environment to allow different cellular functions to be carried out optimally (e.g., protein synthesis in the cytoplasm and oxidative metabolism in the mitochondria).