Biological Importance of Sodium and Potassium

A typical 70 kg adult carries roughly 90 g of sodium and 170 g of potassium in the body — far more than the 5 g of iron or 0.06 g of copper also present — underlining just how central these two ions are to normal physiology.

Sodium ions are concentrated mainly outside cells, in the blood plasma and the interstitial fluid that bathes them. In that role they help transmit nerve impulses, regulate the movement of water across cell membranes, and assist the transport of sugars and amino acids into cells. Potassium, despite being chemically so similar to sodium, behaves quite differently at a biological membrane: the two ions differ in how readily they cross cell membranes, in the transport machinery that moves them, and in how efficiently each activates particular enzymes. Potassium is in fact the dominant cation inside cells, where it activates numerous enzymes, participates in the oxidative breakdown of glucose to generate ATP, and — together with sodium — underlies the transmission of nerve signals. …