The pulmonary circulation: the radiology of adaptation.

In this George Simon Lecture, I have tried to present a simplistic yet philosophical approach to the physiological processes of adaptation of circulatory and respiratory function in response to pathophysiological challenge. The objective of the body is to achieve a normal ventilation-perfusion ratio of 1 in order to optimise blood-gas exchange in a wide variety of pathological conditions. It is all a question of the right ingredients (oxygen in atmospheric air and reduced haemoglobin in desaturated blood), in the right copious amounts (100 ml/s), in the right proportion (1:1) and in the right place (alveolar capillary membrane). In order to demonstrate the above concepts, a variety of well known conditions, congenital and acquired, have been discussed in the developing infant lung and the adult mature lung; some primarily cardiovascular, others primarily respiratory; some conditions resulting in permanent and others resulting in temporary and reversible major readjustments of the blood supply to the lung. The emphasis has been to illustrate that simple, unsophisticated radiological techniques may provide a substantial amount of valuable information concerning the remarkable capacity of the pulmonary circulation to adapt to varying physiological or pathological demands in order to conserve our precious respiratory resources. George Simon, like all good doctors, had a tremendous respect for the ability of the body to respond to these challenges. He always emphasised the important basic principles rather than fine minutiae. It is hoped that this lecture in his honour follows these principles which he preached so well.