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The successful transition from intrauterine to extrauterine life is dependent upon significant physiologic changes that occur at birth. For the transition to be successful, the following changes must occur.

Alveolar Fluid Clearance

During pregnancy, lung growth and development occurs through the accumulation of alveolar fluid. During late gestation and labor, catecholamine and other hormones are increased, resulting into active resorption of fluid from alveoli, which is enhanced further by increased oxygen tension (PaO2) at birth. A minor mechanism of alveolar fluid clearance is the squeeze of the infant chest wall during vaginal delivery; the thorax is compressed as it traverses the birth canal. This expresses some fluid from the upper airways. Most of what remains is absorbed by the pulmonary capillaries and lymphatics.

Lung Expansion

After birth, the air movement into the lungs expands the alveolar air spaces resulting in stimulation of surfactant release, which reduces alveolar surface tension and prevents alveolar collapse.

Establishment of Regular Breathing and Gas Exchange

The mechanical expansion of the lungs at first breath and the rise in alveolar PO2 lead to a rapid decrease in pulmonary vascular resistance and an increase in pulmonary blood flow. A number of factors are involved in the initial stimulus to respiration. The most important is the fall in PO2 and the rise in PCO2 that follows the cessation of umbilical circulation. Tactile, thermal, and proprioceptive inputs also play significant roles.

Circulatory Changes

Changes in Pressure

Fetal circulation is characterized by relatively high right ventricular and pulmonary artery pressures. These are maintained by elevated pulmonary arteriolar resistance and by the presence of a large ductus arteriosus. The ductus equalizes pressures between the pulmonary artery and aorta and directs most of the right ventricular output into the systemic circulation. Systemic pressures are decreased by the presence of the umbilical circulation, which acts as a low-pressure shunt.

After clamping the umbilical cord and removal of the placenta and its low vascular resistance, the systemic vascular resistance increases; meanwhile, lung expansion after the first effective breaths results in a drop of pulmonary vascular resistance and pulmonary artery pressure. This will result in elevation of left ventricle pressure above that of the right ventricle.

Closure of Fetal Vascular Channels

Foramen Ovale

The increase in pulmonary venous return to the left atrium leads to a rise in left atrial pressure. As the left atrial pressure increases and the right atrial pressure falls, right-to-left shunting across the foramen ovale decreases. Eventually, the left atrial pressure exceeds the right atrial pressure. This compresses the valve of the foramen ovale and produces functional closure of the interatrial ...

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