The lung has a unique vascular structure and function; it has low pressure, low resistance circulation with a highly compliant system which accommodates the same amount of flow as the systemic circulation. In addition, pulmonary and systemic vasculatures have divergent responses to various stimuli. For example, pulmonary arteries constrict in the setting of hypoxia, while systemic circulation dilates. This is due to distinctive developmental characteristics, anatomic and histological structure, as well as physiological properties. These properties stem from a particular array of molecular and cellular mediators which favor vasodilation and maintenance of thinwalled and pliable pulmonary resistance vessels
