The Normal Fetal and Neonatal Pulmonary Circulation

Abstract

In utero, the pulmonary circulation is characterized by high pulmonary vascular resistance (PVR) and low pulmonary blood flow, as the fetal lung receives less than 8% of combined ventricular output. At birth, the pulmonary circulation must suddenly undergo a striking vasodilation to allow for an eightfold to tenfold rise in blood flow for the lung to assume its postnatal role in gas exchange. This acute change in pulmonary vascular tone and reactivity is followed by progressive vascular growth, remodeling, and structural adaptations, as the lung circulation continues to grow and mature over time in response to postnatal stimuli. Normal lung vascular development is not only critical for successful adaptation at birth, but ongoing growth and remodeling remains essential throughout postnatal life. The ability of the lung to successfully achieve normal gas exchange requires the growth and maintenance of an intricate system of airways and vessels, including the establishment of a thin yet vast blood–gas interface. Insights into basic mechanisms that regulate lung growth and maturation continue to provide new understanding of lung diseases and their treatment, in newborns and adults alike. Vascular growth in the lung is not only important regarding the risk for pulmonary hypertension, but normal vascular growth and structure are absolutely necessary for establishing sufficient surface area for gas exchange. Developmental abnormalities of the pulmonary circulation contribute to the pathological and pathophysiological processes of diverse diseases, including persistent pulmonary hypertension of the newborn, lung hypoplasia, congenital diaphragmatic hernia, and congenital heart disease. This chapter provides a brief overview of structural and functional changes in the pulmonary circulation that contribute to PVR during fetal life and mechanisms that contribute to the dramatic changes in vascular tone at birth in the normal newborn.