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About 60 per cent of the adult human body is fluid, mainly a water solution of ions and other substances. Although most of this fluid is inside the cells and is called intracellular fluid, about one third is in the spaces outside the cells and is called extracellular fluid. This extracellular fluid is in constant motion throughout the body. It is transported rapidly in the circulating blood and then mixed between the blood and the tissue fluids by diffusion through the capillary walls.
In the extracellular fluid are the ions and nutrients needed by the cells to maintain cell life. Thus, all cells live in essentially the same environment—the extra-cellular fluid. For this reason, the extracellular fluid is also called the internal environment of the body, or the milieu intérieur, a term introduced more than 100 yearsago by the great 19th-century French physiologist Claude Bernard.
Cells are capable of living, growing, and performing their special functions as long as the proper concen-trations of oxygen, glucose, different ions, amino acids, fatty substances, and other constituents are available in this internal environment.
The extracellular fluid contains large amounts of sodium, chloride, and bicarbonate ions plus nutrientsfor the cells, such as oxygen, glucose, fatty acids, and amino acids. It also contains carbon dioxide that isbeing transported from the cells to the lungs to be excreted, plus other cellular waste products that are being transported to the kidneys for excretion.
The intracellular fluid differs significantly from the extracellular fluid; specifically, it contains large amounts of potassium, magnesium, and phosphate ions instead of the sodium and chloride ions found in the extracellular fluid. Special mechanisms for transport-ing ions through the cell membranes maintain the ion concentration differences between the extracellular and intracellular fluids.
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