Sodium is a cation, i.e., a positively charged ion; its distribution and physiological effects are fairly independent of the negative ions (anions) that originally accompanied its ingestion though they may affect its absorption and excretion. Most sodium is in ECF (Table 1), kept there by the sodium pump, an enzyme system, Na+/K+-exchanging ATPase, which uses substantial amounts of energy (adenosine tri-phosphate; ATP) in maintaining a low intracellular sodium concentration and a high intracellular potassium (K+) concentration. Sodium transport is a central issue in the physiology of sodium for a number of reasons:

1. It helps to maintain the ionic environment of ICF and the volume of ECF.

2. It prevents cell swelling (the Na+ efflux exceeds the K+ influx).

3. It establishes gradients which, in various tissues, allow transport of other cations in exchange, other anions in parallel or organic solutes -these are often cotransported with sodium down concentration gradients which are secondary to the low sodium environment created by the pump.

4. It establishes the membrane voltages on which excitability and secretory activities frequently depend.

5. The energy expenditure of the pump is a substantial portion of total metabolic activity and contributes to thermogenesis.

6. Sodium transport is not only a key factor in the retention and loss of sodium in the kidney, gut, salivary, and sweat glands but also influences the

Table 1 Summary of sodium (Na) distribution and requirements

Typical plasma Na concentration (mmolT1)

145 (130-160)

Typical body Na content (mmol kg-1)

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