ABC of oxygen

doi:10.1136/bmj.317.7164.996

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This article has a correction

Please see: ABC of oxygen: diving and oxygen

Please see: ABC of oxygen: Diving and oxygen

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Peter Wilmshurst

All organisms require oxygen for metabolism, but the oxygen in water is unavailable to mammals. Divers (and diving mammals such as whales and seals) are entirely dependent on the oxygen carried in the air in their lungs or their gas supply. Divers also have a paradoxical problem with oxygen. At higher partial pressures oxygen causes acute toxicity leading to convulsions. To understand the diver's narrow knife edge between fatal hypoxia and fatal hyperoxia we need to recall some of the physical properties of gases.

A dive to 30 m for 20 minutes puts the scuba diver at risk of nitrogen narcosis and decompression illness. The elephant seal can dive to 1 km for 1 hour without risk of either condition

Physics

At sea level atmospheric pressure is 1 bar absolute (1 standard atmosphere =101 kPa=1.013 bars). The weight of the atmosphere exerts a pressure which will support a column of water 10 m high; 10 m under water the pressure on a diver is 200 kPa. The volume of gas in an early diving bell full of air at sea level is halved at 10 m according to Boyle's law; at 20 m pressure is 300 kPa absolute and the gas is compressed into one third the volume.

The pressure on a diver increases by 100 kPa for every 10 m he or she descends

Dry air is composed of roughly 21% oxygen, 78% nitrogen, and 1% other gases. According to Dalton's law the partial pressure of oxygen at any depth will be 21% of the total pressure exerted by the air and the partial pressure of nitrogen will be 78% of total pressure.

Effect of depth on partial pressures of nitrogen and oxygen

Gases dissolve in the liquid with which they are in contact. Nitrogen is fat soluble and at …