Pulmonary vascular disease: pulmonary thromboembolism and pulmonary hypertension

The alveolar–capillary barrier, comprising capillary endothelium, extracellular matrix and alveolar epithelium, is a thin membrane that provides a large surface area for diffusive gas exchange. In human health, the pulmonary circulation is completely separate from the systemic circulation, which allows the maintenance of a high-flow, low-pressure circuit to ensure optimal gas exchange even with increasing physiological demand. Increases in pressure within this circulation either acutely, for example secondary to pulmonary embolism, or over time secondary to vascular remodelling, can detrimentally affect ventilatory efficiency and also lead to excessive afterload being placed on the right ventricle.