Alveolar capillary dysplasia and pulmonary capillary hemangiomatosis The pulmonary capillaries form a dense sheet-like meshwork composed of short interconnected capillary segments. The capillary meshes are wrapped over the alveoli, with only a single sheet of capillaries between adjacent alveoli on the same alveolar duct. Impaired development of this vascular network can be caused by genetic defects, prematurity or injury. Aberrant angiogenesis documented in pediatric patients include mainly alveolar capillary dysplasia, and pulmonary capillary hemangiomatosis [178] . Alveolar capillary dysplasia is a rare disorder, presenting with persistent pulmonary hypertension of the newborn [179] . The strongest diagnostic features are poor capillary apposition and density, allied with medial arterial hypertrophy and misalignment of pulmonary vessels [180] . Pulmonary capillary hemangiomatosis is also a rare disease that is characterized by proliferation of capillary-sized vessels within the alveolar walls of the lung [181] . Intimal thickening and medial hypertrophy of the small muscular pulmonary arteries are present resulting in elevated pulmonary vascular resistance. Most cases appear sporadic. Chest imaging shows nodular pulmonary infiltrates and septal lines. A definitive diagnosis can be made only by histologic examination. Interestingly, capillary proliferation in the alveolar wall has been reported in hereditary haemorrhagic telangiectasia [182] . Lymphatic disorders Alveolar structure formation is characterized by refinement of the gas exchange unit and functional adaptation of endothelial cells into vessels including pulmonary lymphatics. The pulmonary lymphatic network promotes efficient gas exchange through maintaining interstitial fluid balance. Lymphatic disorders can be classified as primary or secondary.