It soon becomes clear to any student of physiology that there are many systems of units and forms of terminology. For example, respiratory physiologists measure pressures in centimetres of water and cardiovascular physiologists use millimetres of mercury. As the study of any single branch of physiology becomes increasingly sophisticated, more and more use is made of other disciplines in science. As a result, the range of units has increased to such an extent that conversion between systems takes time and can easily cause confusion and mistakes.

We see also frequent misuse of terminology which can only confuse; for example, the partial pressure of oxygen in blood is often referred to as the ‘oxygen tension’, when in reality tension means a tensile force and is hardly the appropriate word to use.

In order to combat a situation which is deteriorating, considerable effort is being made to reorganize and unify the systems of nomenclature and units as employed in physiology. For any agreed procedure to be of value, it must be self-consistent and widely applicable. Therefore, it has to be based upon a proper understanding of mathematical principles and the laws of physics.

The system of units which has been adopted throughout the world and is now in use in most branches of science is known as the Système International or SI (see p. 28).