This article focuses on reviewing the basics of the lung circulation (pulmonary and bronchial), normal development and transition at birth and vasoregulation. Infection and inflammation compromise normal barrier homeostasis, resulting in increased permeability and edema formation. Under normal conditions, the endothelium forms a tight barrier, actively regulating interstitial fluid homeostasis. In addition, genetic and environmental factors can also confer susceptibility to development of pulmonary hypertension. If the hypoxic stimulus persists for a prolonged period, contraction is accompanied by remodeling of the vasculature, resulting in pulmonary hypertension. Pulmonary vascular tone is also altered by hypoxia, which causes pulmonary vasoconstriction. Factors controlling pulmonary blood flow include vascular structure, gravity, mechanical effects of breathing, and the influence of neural and humoral factors. As compared with the systemic circulation, pulmonary arteries have thinner walls with much less vascular smooth muscle and a relative lack of basal tone. The pulmonary circulation accommodates the entire cardiac output, maintaining high blood flow at low intravascular arterial pressure. For example, it is the only organ with two circulations: the pulmonary circulation, the main function of which is gas exchange, and the bronchial circulation, a systemic vascular supply that provides oxygenated blood to the walls of the conducting airways, pulmonary arteries and veins. The circulation of the lung is unique both in volume and function.
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