oxygen

oxygen is the most common of all chemical elements on earth, being found in water, air, and most mineral and organic substances, including most compounds in the human body. It combines with almost all other elements, and is so reactive that it was given the Greek name ‘oxygen’, meaning acid-forming. However, most of the compounds it forms are not acids. Its chemical reactions usually form heat (as in the animal body) and sometimes light (as in candles).

It has always been known that animals cannot live without air, but in 1674 Mayow showed that only one part of the air, about one-fifth, is essential for life, and named it ‘vital air’. A hundred years later Priestley isolated this part, oxygen; Lavoisier purified oxygen and its properties began to be studied.

Atmospheric air contains 21% oxygen, at a pressure of about 150 mm Hg varying with barometric pressure and to a small extent with humidity. It enters the lungs during breathing and is absorbed into the blood passively by diffusion, combining with haemoglobin and being carried in the bloodstream to all parts of the body. There it is used to metabolize or ‘burn’ foodstuffs in the cells, especially fats and carbohydrates, providing heat and creating new chemical compounds, water, and the waste product carbon dioxide. Tissues and organs vary in the length of time they can survive without oxygen, according to their provision for anaerobic metabolism. The brain cannot survive without oxygen; the cessation of breathing will cause unconsciousness in a few minutes, and death soon afterwards. Other tissues such as skeletal muscle can continue to work for a limited time, when glycogen stores are broken down without oxygen to provide energy; lactic acid is a by-product that leaks into the blood and makes it more acid, but can be recycled into carbohydrate stores in the liver.

In quiet breathing at rest we absorb about 0.2–0.3 litres/min of oxygen (depending on body size), but in vigorous exercise this can go up to over 2 litres/min. This increase is accomplished by increased breathing (which supplies oxygen to the lungs at a greater rate), increased cardiac output and flow of blood to the muscles, and greater extraction of oxygen from the blood by the muscles. If the oxygen supply to the muscles is inadequate then the anaerobic threshold is passed and anaerobic metabolism takes place, with production of lactic acid. After the exercise additional oxygen is needed to convert the lactic acid back to glycogen, and breathing remains enhanced while the oxygen debt is repaid.

The supply of oxygen to the body depends not on the percentage in the air breathed but on its tension or pressure. At high altitude, say 5000 metres above sea level, the percentage of oxygen is still 21%, but because atmospheric pressure is halved, the oxygen pressure is half that at sea level — 75 mm Hg rather than 150 mm Hg. A person may as a result suffer from hypoxia — a lack of oxygen.

High oxygen pressures can be harmful and cause oxygen poisoning, including lung damage and brain dysfunction. In nature high oxygen pressures only exist in deep water diving, and mankind has not had to evolve ways of combating them. Once scientists had purified oxygen it became possible to administer it to patients; this has life-saving possibilities, but care has to be taken not to exceed the toxic level.

Some compounds rich in oxygen, such as the pollutant ozone (itself a molecular form of oxygen), and hydrogen peroxide, can react with cells to produce strongly reactive forms of oxygen. Superoxide anions and unstable oxygen free radicals (such as hydroxyl and hydroperoxy radicals) can be toxic to cells, by way of excess lipid peroxidation. These are implicated, for example, in damage following the restoration of blood flow (reperfusion) after the blockage which causes heart attacks or strokes, and in a variety of other disease processes. However the body does have inherent enzymatic defences against free radical accumulation, and there are antioxidants, such as uric acid, ascorbate, and glutathione, which can inactivate them. Free radicals are likely to contribute also to the ageing process: the very substance by which we live may itself limit our lifespan. Thus oxygen, like most good things, can be dangerous in excess.

Mankind evolved to live close to sea level. Climbing mountains (causing hypoxia) and deep-sea diving (causing nitrogen narcosis or oxygen poisoning) can both be dangerous, in the absence of the right precautions.

John Widdicombe

See also breathing; diving; free radicals; gases in the body; hypoxia; respiration.