Chapter 3:

Pulmonary Circulation and Lung Development

The big difference between fetal circulatory and nonfetal circulatory systems is that, since the fetus does not use the lungs for gas exchange, very little blood actually perfuses the pulmonary circulation. The fetus has mechanisms to bypass the lungs.

By 16 to 20 weeks of gestation, the process of pulmonary arterial branching has nearly been completed, the central pulmonary trunk in the fetus has elastic laminae and its walls have become thick. Prior to birth, blood bypasses the lungs in utero, with only about 10% of the cardiac output carried by the pulmonary circulation. With the majority of cardiac output being shunted past the lungs via the ductus arteriosus, fetal pulmonary vascular resistance (PVR) is very high, making flow through the ductus the path of least resistance.

Branching away from the main pulmonary artery, arterial elastic laminae decrease. Arteries with diameters of 2 mm down to approximately 200 microns undergo a transition to a more muscular type of vessel where there are changes in PVR. These are located adjacent to the terminal bronchioles. As the vessels get smaller, the amount of muscle gradually decreases and eventually disappears entirely in vessels adjacent to the alveoli.