| Chapter 11: | The Infant |   |
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At birth, expansion of the lungs causes PVR to fall dramatically. Ventilation helps to overcome fetal pulmonary vasoconstriction through several mechanisms. The first is a rise in Pa02 associated with filling the alveoli with air. This has a direct vasodilator effect on the capillaries. As more blood passes through the lungs and is exposed to air, vasoconstricting prostaglandins are inactivated. In the fetus,these prostaglandins are necessary to help maintain a high PVR and keep blood flowing through the ductus arteriosus.
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A rising Pa02 also increases circulating bradykinin levels. This helps constrict the ductus and force more blood through the pulmonary circulation. Mechanical expansion of the lungs stretches and straightens the capillaries which, in turn, decreases the resistance to flow. In utero, the capillaries are very kinked and the path is tortuous. This keeps the PVR high. Stretching the alveoli and surrounding tissue correct this.