ABSTRACT
The lung contains two anatomically and functionally distinct circulatory systems: the pulmonary and the bronchial circulation. The pulmonary circulation serves to exchange gas with the alveoli, while the bronchial circulation supplies the parenchyma of the lung itself. The pulmonary vascular bed is a high flow, low pressure circuit with blood passing from the main pulmonary artery through precapillary resistance vessels to the alveolar gas exchange units, where endothelial cells are exposed to the highest oxygen concentrations in the body and are in close proximity to surfactant producing epithelial cells. Recruitment of so-called reserve capillaries occurs during exercise in a system designed to maximize gas exchange and to avoid airspace fluid flooding. The alveolar capillary network then drains into the left atrium of the heart via the pulmonary veins (Fig. 1). Lung endothelium possesses properties that are unique to this organ. Moreover, endothelial cell phenotypes differ within the lung itself, according to the blood vessel type and location. The biological plausibility of these differences is supported by functional and developmental considerations. This chapter will provide information regarding site-specific properties of lung endothelial cells in health and in pulmonary hypertension (PH).
