1. An extensive study of the functional and dimensional components of the oxygen-transporting system in relation to age and the indirect estimation of maximum aerobic power (V̇o2max.) has been conducted on eighty healthy male subjects aged 20–50 years. Measurements were made of ventilation, cardiac frequency and blood lactic acid expressed in terms of oxygen intake of 1·51/min (V̇e, 1·5; fh, 1·5 and LA, 1·5 respectively) maximum blood lactic acid, ventilation and oxygen intake, thigh (TM) and calf (CM) and total body muscle estimated from soft tissue radiographs, whole body potassium (40K), blood volume (BV), heart volume (HV), and tests of lung function.
2. In the younger subjects it was shown that the functional and dimensional capacities of the O2-transporting system were closely integrated and the performance of organs in relation to size was essentially similar. In older subjects there was a gradual decline in the overall capacity to transport oxygen which was closely associated with a decrease in the functional rather than the dimensional aspects of the cardiovascular system. In terms of a given body weight the dimensional capacities of the older subjects, BV, THb (total body haemoglobin), 40K, TM CM, were shown to be independent of age.
3. The best single guide to V̇o2max. was given by BV in subjects under 35 years of age and by HV in older subjects. By using a multiple regression on either V̇e, 1·5, fh, 1·5 and leg muscle or FVC, HV, TM and age, V̇o2max. could be estimated with coefficient of variation of ∼9%. This is ∼2% improvement on conventional methods of determining V̇o2max. from measurements of fh and V̇o2 taken at four levels of work and in addition when using the latter equation there is no need for the subject to exercise. The use of these equations and their clinical application to the testing of V̇o2max. in large community studies of physical working capacity are discussed.