Sodium is a metallic element designated by the symbol Na, from Latin natrium. It is a soft whitish alkali metal that is extremely reactive; like its close cousin potassium it oxidizes quickly and reacts violently with water, liberating potentially flammable hydrogen and forming hydroxide. Hence sodium must be stored away from air and water and handled with care.
Because of its reactivity, sodium is never found uncombined in nature, and Humphry Davy is generally credited with having discovered it because he successfully separated it from its hydroxide in 1807. Sodium compounds, though, are very common, and have been known since antiquity. Sodium has many uses. It is used for those yellowish glowing street lights; pure or in an alloy with potassium, it's a heat-transfer liquid. As for the compounds, they are numerous and ubiquitous, and so I will confine myself to some common household examples: sodium chloride (NaCl) is common table salt; sodium bicarbonate (NaHCO3) is baking soda; sodium carbonate (Na2CO3) is washing soda; sodium hydroxide (NaOH) is lye, used in making soap; sodium nitrate (NaNO3) is saltpeter; and of course monosodium glutamate contains sodium.
In the human body, sodium is an electrolyte that helps maintain fluid balance in the body. Most sodium is found in the bones and in the extracellular fluids of the body. By contrast, most potassium is found inside the cells. The cells of the body pump sodium out and potassium in; if this didn't happen, water would accumulate in the cells, causing them to swell and burst. The potassium-sodium pump also helps to maintain the electrical charge within the cell, which is particularly important to muscle and nerve cells. During muscle contraction, potassium exits and sodium enters, reversing the electrical charge and causing muscle contractions or nerve impulses.
Moderate sodium intake helps increase resistance to heat cramps and heat stroke, especially during times of excessive sweating and hence increased fluid loss.
Symptoms of sodium deficiency include intestinal gas, weight loss, poor memory, short attention span, vomiting, low blood sugar, heart palpitations, and muscle weakness; prolonged deficiency can lead to arthritis, rheumatism, and neuralgia. But sodium deficiency is very rare because most foods contain some sodium; the only way to really suffer from a deficiency is to literally starve or lose all your nutrients through prolonged vomiting or diarrhea resulting in excessive fluid loss. If replacing lost sodium in such extreme circumstances, it is essential to replace the fluid at the same time as the sodium because in the absence of liquid, sodium can't be absorbed.
Much more common is sodium excess, which occurs because sodium is found in almost all foods and because processed foods contain large amounts of added sodium, primarily in the form of salt. Too much sodium is bad for people with high blood pressure. High sodium intake increases the risk of liver, heart, and kidney disease and causes edema and dizziness. Sodim and potassium ratios must be in balance in order for both to function properly; too much sodium disrupts potassium's action, and vice versa.
The daily recommended intake of sodium is the subject of some debate, but generally falls somewhere between 1000 and 3000 mg per day; Americans apparently eat on average 4000 to 5000 mg per day. 1/2 teaspoon (2-1/2 ml) of salt contains about 1000 mg of sodium, so cutting down on salt is one way to control sodium intake. Be careful of processed foods, too. The American Heart Association recommends that for every 1000 calories of food eaten, the amount of sodium should be under 1000 mg, certainly no more than 3000 mg. Foods labelled "sodium-free" contain less than 5 mg of sodium per serving; "very low-sodium", less than 35 mg; "low-sodium" no more than 140 mg of sodium per serving.