nitrogen Four-fifths (79%) of the air we breathe consists of nitrogen, nearly all the rest being oxygen. It was known in the late seventeenth century that breathing air with its oxygen removed resulted in death, but only in 1772 did Rutherford isolate nitrogen; soon after, Lavoisier showed that pure nitrogen could not support life, although he misnamed it mephitic or ‘smelly’ air. It is odourless.

The nitrogen we breathe is chemically inert and takes no part in the chemical or metabolic reactions in the body. In this respect it resembles the ‘inert gases’ such as argon and neon which are a small part of the atmosphere. Nitrogen is poorly soluble in water and body liquids, and there is virtually no exchange between the nitrogen we breathe into the lungs and the body itself. However the chemical combinations of nitrogen are crucial for life. It is a definitive component of proteins and their constituents, amino acids. It is present in innumerable other essential chemical components of the body, from vitamins to hormones to enzymes and many other vital molecules; in recent years the ubiquitous importance of nitric oxide (NO) in physiological function has been recognized. It is no exaggeration to say that life only became possible by the creation of nitrogen-containing chemicals. But these chemicals reach their sites in the human body not from inhaled nitrogen, but from ingested plant and animal materials. Only plants (including some bacteria) can convert atmospheric nitrogen to the organic compounds needed for animal life, so plants are the ultimate source of all nitrogenous chemicals in the body.

In proteins nitrogen occurs mostly in amino- (-NH2) groups. During metabolic breakdown of these and other nitrogen-containing substances the nitrogen is not converted to its gaseous form for excretion in the lungs, but forms mainly urea, a small molecular-weight waste product that, as its name implies, is excreted in the urine. Although the metabolism of proteins provides some energy for the body, this is normally far smaller than that due to burning carbohydrates (that contain no nitrogen), and fatty substances (most of which contain no nitrogen). Rather, the amino acids derived from the dietary proteins are taken up by body cells for use in the turnover of their own proteins, which they need to synthesize continually: for their growth and repair, cellular enzymes, secretions and so forth. In health, the necessary daily intake of nitrogen to balance inevitable losses is estimated at about 12 g, which would be contained in about 75 g of protein. In starvation or in the aftermath of serious injury or infection requiring rebuilding of tissues, protein is depleted, mainly from muscle, and is used for the production of glucose by the liver; only adequate nutritional supplements can avoid a state of ‘negative nitrogen balance’, with wasting and weakness.

Although nitrogen is poorly soluble in water, so that little is normally dissolved in body liquids, it is more soluble in fats, which accounts for its role in ‘bends’ or decompression sickness, seen when deep-sea divers breathing air ascend too rapidly to the surface. After a significant time underwater the nitrogen will first have dissolved in the blood, since its pressure is high in the lungs, then passed into the tissues, particularly fat; on rapid ascent (‘decompression’) it comes out of solution to form bubbles in nerves and round joints, causing the characteristic pain of the bends. In practice air is nowadays never used in deep diving, the nitrogen being replaced by helium, which is far less soluble in fat.

Nitrogen under pressure will also cause psychological and neurological disturbances. This condition is called nitrogen narcosis, but long before actual narcosis (sedation and anaesthesia) occurs the nitrogen exerts toxic effects. These include euphoria, fixed and complacent ideas, uncontrollable laughter, and neuromuscular incoordination. Scuba divers may suffer from this, and it has been called the ‘rapture of the depths’. It is not due to any chemical reaction of the nitrogen, since it can also be caused by ‘inert’ gases such as argon, but probably by the solution of the pressurized nitrogen in fatty substances such as the membranes of nerve cells in the brain. Possibly also the nitrogen attracts water to form hydrated forms which disrupt brain cell function. Although nitrogen narcosis may have the same physicochemical basis as decompression sickness, its clinical manifestations are quite different. In many respects it resembles the psychological and neurological effects of acute lack of oxygen, but the mechanisms are probably very dissimiliar.

John Widdicombe

See also amino acids; decompression sickness; diving; gases in the body; proteins.