ABSTRACT
The pulmonary vascular system is unique in that, unlike the vascular system of other organs, the entire cardiac output must pass through it. The normal pulmonary circulation offers remarkably little resistance to flow. The pulmonary vascular smooth muscle is unique compared to systemic vascular smooth muscle in that the most important stimulus for contraction is hypoxia. Pulmonary vascular remodeling is characterized by concentric thickening of the vessel and is an important pathologic cause of decreased vessel radius and increased pulmonary vascular resistance in all species. Pulmonary vascular occlusion decreases the total cross-sectional area of the pulmonary vessels and is another important cause of increased pulmonary vascular resistance in small animals. The pulmonary circulation undergoes passive distention and recruitment of pulmonary vessels in response to an increase in pulmonary blood flow. Conditions that elevate left atrial pressure cause a proportional increase in pulmonary artery pressure and pulmonary hypertension.
