Regulation of the pulmonary circulation in the perinatal period and in children.

In the fetus, the pulmonary circulation is actively maintained in a constricted state due to low oxygen environment and perhaps leukotrienes. Pulmonary blood flow represents about 8% of fetal cardiac output. Shortly after birth, pulmonary vascular resistance (PVR) and pulmonary arterial pressure fall rapidly, whereas pulmonary blood flow reaches systemic levels. The exact mechanisms by which the dramatic changes in PVR occur with the onset of ventilation at birth are extensively reviewed. They include: oxygen environment and release of vasoactive substances such as bradykinin, PGI2 or PGD2. Thus, regulation of the fetal and immediate postnatal pulmonary circulation reflects a balance between factors producing active pulmonary vasoconstriction and those producing vasodilatation.