RESUMO
Interpretation of chest X-ray relies not only on morphological criterion but also on physiological bases. Vascular opacities represent 80% of visible structures in the lung area; the contour of the heart and large vessels are well recognized. In the first part we review the current knowledge about the physiology of the pulmonary circulation and its factors of regulation. In the second part we present the reading principals of a chest X-ray obtained in an erect patient at the end of inspiration. A check list should be verify in every patient: pulmonary flow distribution (ratio apex/basis); size of small vessels in the periphery of both lungs; Simon's line; shape of vessels; ratio of the artery/bronchus diameter; size and shape of the heart; diameters of aorta, right pulmonary artery, and azygos vein. In the last part, we present the main pathological patterns: diffuse increase of pulmonary flow; pulmonary venous hypertension; pulmonary arterial hypertension; localized changes of the pulmonary blood flow in which the role of expiratory X-ray is emphasized. Many pitfalls are identified: egalisation of the ratio apex/basis; enlargement of the vascular pedicle; enlargement of the azygos vein.