Sodium (intracellular)

The main cation of the extracellular space. It regulates fluid balance, blood pressure and nerve impulses. The Na/K-ATPase is one of the cell's largest consumers of energy.

What sodium does at the cell

Sodium is about ten times more concentrated outside the cells than inside. This gradient is maintained by the sodium-potassium pump, which uses ATP to move three sodium ions out of the cell and two potassium ions in; it needs magnesium for this. The gradient is an energy store: when sodium flows back into the cell, it takes glucose and amino acids along via transporters, and the sodium-calcium exchanger moves calcium out in the process. When sodium channels open in nerve and muscle cells, the rapid influx of sodium triggers an electrical impulse. Because water follows sodium, sodium outside the cells also determines how much water stays in the cells.

Low sodium – what happens in the body?

A low sodium content inside the cell is the desired state: the pump is working sufficiently, and the gradient is available for transport and signalling. If, on the other hand, sodium outside the cells falls, water flows into the cells and they swell. The brain is particularly sensitive to this, responding with headache, nausea, difficulty concentrating and confusion; muscles become weak and prone to cramps. Causes include very large fluid intake, an excess of the water-retaining hormone ADH, diuretic medications and losses through vomiting or diarrhoea.

Raised sodium in the cell – what happens in the body?

When sodium inside the cell rises, the gradient becomes flatter. This happens when the sodium-potassium pump is not performing enough – for example with an energy shortage in the cell, with magnesium deficiency or with cardiac glycosides, which inhibit the pump. With the flatter gradient, the sodium-calcium exchanger moves less calcium out; calcium accumulates in the cell, and smooth muscle cells in the vessel wall contract more strongly. At the same time the cell takes up water and swells. In studies, a raised sodium content in red blood cells has been observed in high blood pressure.

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