SummaryThe actual methods of investigation of the cardio-pulmonary function and especially the cardiac catheterization, have allowed to realize great advances in the study of the pulmonary circulation. Accurate notions have been obtained concerning gaseous exchanges, blood flow through the lung and pulmonaries pressures; however the problem of the intra-thoracic blood volume remains vet much discussed.The pressures of the pulmonary circulation are very different from those of the systemic circulation : the arterial pressure is lower and the venous pressure is higher in the first than in the second case; this depends and the anatomical and physiological characteristics of the pulmonary circulation and of the left atrio-venous system.The pressure in the pulmonary artery may be increased secondarily to an augmentation of the pressure in the left auricle, or may increase after a change of the arteriolar resistance. The latter appears either for a considerable increase of the blood flow or for a reduction of the capacity of the vascular bed. This reduction is due to anatomical lesions or to a vasoconstriction caused by anoxemia, hypercapnia, intervention of the nervous system or to the action of some drugs.The vasoconstriction of the pulmonary vascular bed seems very likely, but the importance of the factors considered and their mechanism remain discussed.If we consider the different clinical conditions that are accompanied with pulmonary arterial hypertension, we see that for some of them, the mechanical factor (left auricular hypertension) is preponderant, though the modifications of the blood volume, of the cardiac flow and of the arteriolar tonus are also important; this is the case in left ventricular insufficiency, mitral stenosis and constrictive pericarditis. In chronic lung diseases, the part played by the functional factors is probably much more important, but must not be exaggerated; especially in pulmonary emphysema, the respective parts played in the development of the hypertension, by the anatomical lesions and by the physiopathological troubles consecutive to anoxemia, still remain discussed. Finally, in case of«primary»pulmonary arterial hypertension, isolated or associated to congenital cardiovascular malformations, it is perhaps a residuum of the foetal physiology, but the exact mechanism of the increase of the arterial resistance still remains to be precised, as well as the histological characteristics of this affection.At the present time it is possible to establish a nosological list of the types of pulmonary arterial hypertension based on the causal disease or on the more important pathological mechanism. (List 7.)But we must recognize that, in each case, the factors are numerous and that the part played by each of them is difficult to precise; these mechanisms are often associated, the existence of some among them remains discussed and the way of action of others is not yet determined.