A colourless and odourless gas constituting just under 21 per cent of the earth's atmosphere. As a medical gas, it is supplied in the UK compressed at high pressure (13,600 kilopascals (KPa)) in cylinders which are black with white shoulders. In hospitals, oxygen is often stored as a liquid in insulated tanks and controlled evaporation allows the gas to be supplied via a pipeline at a much lower pressure.

Oxygen is essential for life. It is absorbed via the lungs (see RESPIRATION) and is transported by HAEMOGLOBIN within the ERYTHROCYTES to the tissues. Within the individual cell it is involved in the production of adenosine triphosphate (ATP), a compound that stores chemical energy for muscle cells, by the oxidative metabolism of fats and carbohydrates. HYPOXIA causes anaerobic metabolism with a resulting build-up in LACTIC ACID, the result of muscle cell activity. If severe enough, the lack of ATP causes a breakdown in cellular function and the death of the individual.

Oxygen lack (hypoxia) may be corrected by giving extra oxygen. This is usually given via a face mask or nasal cannula, or, in severe cases, during ARTIFICIAL VENTILATION OF THE LUNGS. Some indications for oxygen therapy are high altitude, ventilatory failure, heart failure, ANAEMIA, PULMONARY HYPERTENSION, CARBON MONOXIDE (CO) poisoning, anaesthesia and post-operative recovery. In some conditions – e.g. severe infections with anaerobic bacteria and carbon monoxide poisoning – hyperbaric oxygen therapy has been used.