Some Observations on the Testing of Pulmonary Function

In pathological conditions involving destruction or impairment of lung tissue the vital capacity is reduced; owing to the adaptability of the vascular system in the lungs there is not necessarily any direct relationship between the area of impaired tissue, or change in air-containing volume of the lungs, and the amount of blood that is exposed for aeration. The capacity of the pulmonary vascular bed may be adequate for a resting cardiac output of about five litres a minute, but even moderate exercise more than doubles this output of venous blood into the lungs, and the remaining "vascular reserve" may not be adequate to deal with the increased volume efficiently, in which case the CO₂ tension of the blood increases. This increase can be measured by a simple method, and provides for investigation a means of observing the efficiency of pulmonary function.