Unicellular organisms rely on exchanges with their environment to maintain their volume and composition. Similarly, the human body's billions of cells face the challenge of maintaining their volume and composition, but they have a more complex task. French physiologist Claude Bernard first explained this challenge and its solution. He recognized that cells within the body cannot maintain their volume and composition through exchanges with the external environment, but they can do so through exchanges with the fluid environment surrounding them, known as the extracellular fluid or "milieu intérieur." Bernard also understood that the body's organ systems are designed to maintain a constant internal environment, allowing all cells to maintain their volume and composition through exchanges with the extracellular fluid via membrane transport.

The gastrointestinal tract, kidneys, and lungs serve as the body's interface with the external environment, controlling the intake and excretion of various substances and water through transport by the epithelial cells. The cardiovascular system delivers nutrients to cells and tissues while removing waste products and ensuring proper mixing of the extracellular fluid. The nervous and endocrine systems regulate and integrate these crucial functions.

To provide a foundation for studying organ systems, this chapter provides an overview of the normal volume and composition of body fluid compartments and explains how cells maintain their intracellular composition and volume. It also discusses the generation and maintenance of membrane potential, which is essential for understanding the function of excitable cells like neurons and muscle cells. Additionally, the chapter reviews the principles of solute and water transport by epithelial cells, given their central role in regulating the volume and composition of body fluids.

The body is composed of approximately 60% water, with variations based on adipose tissue content. Adipose tissue contains less water than other tissues, so increased adipose tissue decreases the fraction of water in the total body weight. The percentage of body weight attributed to water also varies with age, being around 75% in newborns and reaching the adult value of 60% by the age of 1 year.

Total body water is distributed between two major compartments divided by the cell membrane. The intracellular fluid (ICF) compartment is larger, containing about two-thirds of the total body water, while the remaining third is held in the extracellular fluid (ECF) compartment. Expressed as percentages of body weight, the volumes of total body water, ICF, and ECF are as follows:
- Total body water = 0.6 x (body weight)
- ICF = 0.4 x (body weight)
- ECF = 0.2 x (body weight)

The ECF compartment is further divided into interstitial fluid and plasma. In addition to these, the ECF includes fluid within bone, dense connective tissue, and cerebrospinal fluid. Interstitial fluid surrounds cells in various tissues and accounts for three-fourths of the ECF volume. Plasma is contained within the vascular compartment and represents the remaining fourth of the ECF. In certain pathological conditions, excess fluid may accumulate in what is referred to as a third space. These third-space collections of fluid are considered part of the ECF.
Unicellular organisms maintain their volume and composition through exchanges with the environment they inhabit (e.g., sea water). The billions of cells that constitute the human body must maintain their volume and composition as well, but their task is much more difficult. This challenge, as well as its solution, was first articulated by the French physiologist Claude Bernard (1813-1878). He recognized that although cells within the body cannot maintain their volume and composition through exchanges with the environment, they can do so through exchanges with the fluid environment that surrounds them (i.e., the extracellular fluid). Bernard referred to the extracellular fluid as the milieu intérieur (“the environment within”). He also recognized that the organ systems of the body are designed and function to maintain a constant milieu interieur or a “constant internal environment.” This in turn allows all cells to maintain their volume and composition through exchanges with the extracellular fluid as a result of membrane transport (see
Transport by the epithelial cells of the gastrointestinal tract, kidneys, and lungs are the body’s interface with the
CHAPTER 2 Homeostasis: Volume and Composition of Body Fluid Compartments external environment and control both the intake and excretion of numerous substances, as well as water. The cardiovascular system delivers nutrients to and removes waste products from the cells and tissues and keeps the extracellular fluid well mixed. Finally, the nervous and endocrine systems provide regulation and integration of these important functions.
To provide background for the study of all organ systems, this chapter presents an overview of the normal volume and composition of the body fluid compartments and describes how cells maintain their intracellular composition and volume. Included is a presentation on how cells generate and maintain a membrane potential, which is fundamental for understanding the function of excitable cells (e.g., neurons and muscle cells). Finally, because epithelial cells are so central to the process of regulating the volume and composition of the body fluids, the principles of solute and water transport by epithelial cells are also reviewed.
Definition and Volumes of Body Fluid Compartments
Water makes up approximately 60% of the body’s weight; variability among individuals is a function of the amount of adipose tissue. Because the water content of adipose tissue is lower than that of other tissue, increased amounts of adipose tissue reduce the fraction of water in the total body weight. The percentage of body weight attributed to water also varies with age. In newborns, it is approximately 75%. This decreases to the adult value of 60% by the age of 1 year.
Fig. 2.2, total body water is distributed between two major compartments, which are divided by the cell membrane.
The intracellular fluid (ICF) compartment is the larger compartment, and contains approximately two thirds of the total body water. The remaining third is contained in the extracellular fluid (ECF) compartment. Expressed as percentages of body weight, the volumes of total body water, ICF, and ECF are as follows:
Total body water = 0 6. × (body weight) ICF = 0 4. × (body weight) ECF = 0 2. × (body weight)
The ECF compartment is further subdivided into interstitial fluid and plasma. The ECF also includes fluid contained within bone and dense connective tissue, as well as the cerebrospinal fluid. The interstitial fluid surrounds the cells in the various tissues of the body and makes up three fourths of the ECF volume. Plasma is contained within the vascular compartment and represents the remaining fourth of the ECF. In some pathological conditions, additional fluid may accumulate in what is referred to as a third space. Third-space collections of fluid are part of the ECF; an aIn these and all subsequent calculations, it is assumed that 1 L of fluid (e.g., ICF and ECF) has a mass of 1 kg. Although, 1 L of the ICF and ECF has a mass of slightly more than 1 kg, this simplification allows conversion from measurements of body weight to volume of body fluids.
Total body water (TBW) 0.6 x body weight 42 L Extracellular fluid (ECF) 0.2 x body weight 14 L Intracellular fluid (ICF) 0.4 x body weight 28 L Cell membrane