Billions of years ago, life developed in the form of small, single-celled organisms in a large primal ocean. Their aqueous (watery) environment was marked by a milieu of constant composition. Nutrients were plentiful, and waste products were instantly diluted effectively to infinity. In a similar way, the cells of a multicellular organism live in an aqueous environment that contains all the salts and nutrients required for the sustenance of the cell. Compared to the primal ocean, however, this fluid has a much smaller volume, and there is a much greater danger of short-term changes in its composition.
Of all the chemical compounds in the organism, water (H2O) forms the largest percentage part. Thus the body of an adult contains about 60% water, which is distributed in two distinct compartments: the intracellular space (total volume enclosed in all the cells) and the extracellular space (total volume present outside the cells). About two-thirds of the total body fluids are located inside the cells (intracellular fluid) and the remaining third (about 14 liters in a person weighing 70 kg) bathes the exterior of the cells. Of the 14 liters of extracellular (interstitial) fluid, three-quarter are contained in the tiny spaces that separate the cells from each other, and one-quarter in the vascular systems (arteries, veins, capillaries, and lymphatic vessels), where it forms the aqueous part of the plasma and the lymphatic fluid.
a Unicellular organism: Interaction between the first cells and their environment—the primal ocean, a milieu distinguished by its constant composition
b Human: Cells in a multicellular organism are bathed in extracellular fluid, the volume of which is distinctly smaller than that of the fluid inside the cell. This “internal milieu” would change its composition very quickly if the space between the cells (interstitial space, intercellular space) were not connected to organs such as the lung, the kidney, or the digestive tract by the vascular bed, which take up fresh nutrients and eliminate metabolic waste products.
The water content of the body is kept constant with great precision. This is necessary in order not to endanger the equilibrium of the numerous substances dissolved in the body fluids. For instance, physiological water losses (e. g., the production of urine, the secretion of sweat, and loss by humidification of expired air) must be balanced by fluid intake.
Keeping the “internal milieu” constant (homeostasis) is a life-preserving precondition for the optimal functioning of every cell in the body. Since the most diverse substances reach the extracellular space as a result of respiration, the intake of nutrients, and the metabolic activity of cells, maintaining homeostasis is one of the most important tasks of the organism. Besides the activity of the lungs, the intestines, and the kidneys, certain specific transport processes are of importance; these processes (e. g., diffusion, osmosis, active transport) serve to exchange solids and fluids between the cell and its environment (see Solid and Fluid Transport below). Transport of substances over greater distances within the body (e. g., nutrients taken up in the intestines and oxygen taken up in the lungs) is accomplished in the blood vessels. Similarly, transport by the lymphatics, passage through the intestines, and the emptying of the gallbladder accomplish rapid distribution of solids and fluids.